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Physics
HSC Course
Stage 6
Motors and generators
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Acknowledgments
This publication is copyright Learning Materials Production, Open Training and Education Network –
Distance Education, NSW Department of Education and Training, however it may contain material from
other sources which is not owned by Learning Materials Production. Learning Materials Production
would like to acknowledge the following people and organisations whose material has been used.
• Extracts from Physics Stage 6 Syllabus © Board of Studies NSW, amended November 2002. The
most up-to-date version can be found on the Board's website at
http://www.boardofstudies.nsw.edu.au/syllabus_hsc/syllabus2000_listp.html#p
All reasonable efforts have been made to obtain copyright permissions. All claims will be settled in
good faith.
Writer:
Richard Morante
Editor:
Julie Haeusler
Illustrator:
Thomas Brown
Consultants:
Malcolm Mors (General Manager, Sithe Energies Australia Pty Ltd)
Graeme Gurr (Manager Operations, Sithe Energies Australia Pty Ltd)
Garry Smith (Business Development Manager/Network, TransGrid)
Copyright in this material is reserved to the Crown in the right of the State of New South Wales.
Reproduction or transmittal in whole, or in part, other than in accordance with provisions of the
Copyright Act, is prohibited without the written authority of Learning Materials Production.
© Learning Materials Production, Open Training and Education Network – Distance Education,
NSW Department of Education and Training, 2001. Revised January 2003, 51 Wentworth Rd.
Strathfield NSW 2135.
Contents
Module overview ...................................................................... iii
Indicative time ..................................................................................... iv
Resources ........................................................................................... iv
Icons ................................................................................................... vi
Glossary ............................................................................................. vii
Part 1: The incredible moving charge .................................1–49
Part 2: Induction ..................................................................1–31
Part 3: Powering up ............................................................1–38
Part 4: Transmission ..........................................................1–18
Part 5: Transformers ..........................................................1–32
Part 6: Motors and other electrical applications ..................1–28
Student evaluation of the module
Introduction
i
ii
Motors and generators
Module overview
Michael Faraday was a giant of science. His achievements in chemistry
and physics set him apart from any of his peers. His refusal to patent any
of his inventions or concepts marks him as a true giant seeking the
progress of all humankind. Faraday showed the world the generator and
hinted at the possibility of a useful electric motor.
Faraday’s gentleness and generosity of thought was not shared by the
popularisers of the new, useful energy source, electricity.
These scientists were also businessmen mostly from the New World like
Thomas Alva Edison and George Westinghouse. They saw the practical
possibilities of bringing electricity to all and proceeded to do just that.
Remember as you work through this module that electricity is a relatively
new technology yet it probably is the energy source that most influences
your way of life.
Introduction
iii
Indicative time
This module is designed to cover the activities and learning required in
the syllabus module Motors and generators. These materials, including
readings, activities, tape and exercises should take the average student
around 30 hours to complete.
There are six separate parts in the module. Each part should take around
five hours to complete although some parts may take a little longer
whereas other parts may require less time for some students.
Resources
Some of the syllabus practical activities required in this module will
require attendance at a practical session with your teacher. This is due to
the specialised nature of some of the equipment.
To enhance the learning in this module it is recommended that you have
access to a computer and the Internet.
For Part 1 you will need:
•
a 40 cm length of single strand insulated copper wire
•
a toilet roll centre
•
an AA cell, a A cell and a D cell battery
•
a bar magnet
•
electrical insulating tape
•
two large silver metal paperclips
•
a small electric motor
•
a multimeter.
For Part 2 you will need:
iv
•
a 10 m length of insulated copper wire
•
a multimeter
•
a 1 m length of single strand copper wire
•
a bar magnet
•
insulating tape
•
a broom handle
•
an aluminium soft drink can
Motors and generators
•
a pair of scissors
•
a pair of bar magnets or a single large strong horseshoe magnet
•
three silver metal paper clips
•
Blutak® or sticky tape
•
a ballpoint pen.
For Part 3 you will need:
•
a multimeter
•
a computer with a digital oscilloscope program
•
an old set of bud earphones you can dismantle and use as input leads
in the computer. Note these will be destroyed so don’t use good
ones.
•
access to the Internet
•
a small DC electric motor from a toy
•
two small electric leads and alligator clips
•
a multimeter or a galvanometer and a millivoltmeter.
For Part 4 you will need:
•
access to a cassette tape recorder or alternatively a computer with the
Internet.
For Part 5 you will need:
•
no special equipment is needed for this part.
For Part 6 you will need:
Introduction
•
a new sharpened lead pencil
•
a small test tube able to fit the pencil or the body of a syringe
•
a bar magnet
•
sticky tape
•
Blutak®
•
an empty aluminium soft drink can
•
a pair of scissors
•
around 20 cm of light cotton thread.
v
Icons
The following icons are used within this module. The meaning of each
icon is written beside it.
The hand icon means there is an activity for you to do.
It may be an experiment or you may make something.
You need to use a computer for this activity.
Discuss ideas with someone else. You could speak with
family or friends or anyone else who is available. Perhaps
you could telephone someone?
Listen to an audio file.
There is a safety issue that you need to consider.
There are suggested answers for the following questions
at the end of the part.
There is an exercise at the end of the part for you to
complete.
vi
Motors and generators
Glossary
The following words, listed here with their meanings, are found in the
learning material in this module. They appear bolded the first time they
occur in the learning material.
alternating current
The commonly available electric power supplied
by an AC generator. This is mains current and is
distributed in single– or three–phase forms.
AC current changes its direction of flow 50 times
per second for single–phase power.
AC motor
A motor operating on AC current. There are two
general types: induction and synchronous.
armature
The portion of the magnetic structure of a DC or
universal motor that rotates.
capacitor
Capacitors have the purpose of storage of electric
charge, combined with the ability to release this
charge quickly. Unlike a battery, a capacitor does
not create electricity.
A device that, when connected in an
alternating–current circuit, causes the current to
lead the voltage in time phase. The peak of the
current wave is reached ahead of the peak of the
voltage wave. This is the result of the successive
storage and discharge of electric energy.
Capacitors are used in single–phase motors to
start the rotor turning, or in three–phase motors
for power factor correction.
Introduction
capacitor motor
A single–phase induction motor with a main
winding arranged for direct connection to the
power source, and an auxiliary winding
connected in series with a capacitor.
coil (stator or armature)
The electrical conductors wound into the core
slot. They are electrically insulated from the iron
core. Coils are connected into circuits or
windings, that carry independent current. It is the
coils that carry and produce the magnetic field
when the current passes through them.
commutator
A cylindrical ring mounted on the armature shaft
consisting of a number of copper segments
arranged around the shaft that are insulated from
it and each other. The motor brushes ride on the
periphery of the commutator and electrically
connect the armature coils to the power source in
sequences as the commutator rotates.
vii
viii
constant speed motor
A DC motor that changes speed only slightly
from a no–load to a full–load condition.
These are synchronous type motors for AC
motors.
core
The iron portion of the stator and rotor. This is
usually made up of cylindrical laminated steel or
iron plates. The stator and rotor cores are to
enable the rotor to turn within the stator.
DC (direct current)
A current that flows only in one direction in an
electric circuit. It may be continuous or
discontinuous and it may be constant or varying.
DC motor
A motor using either generated or rectified DC
electricity. A DC motor is often used when
controlled variable–speed operation is required.
eddy current
Localised currents induced in an iron or steel core
by alternating magnetic flux. These currents
translate into energy losses in the form of heat.
Their minimisation is an important factor in
lamination design in both motors and generators.
efficiency
The efficiency of a motor is the ratio of electrical
input to mechanical output.
electromotive force
(emf)
A synonym for voltage that is usually restricted to
generated voltage.
flux
The magnetic field established around a current
carrying conductor or permanent magnet. The
density of the flux lines is a measure of the
strength of the magnetic field.
full–load torque
That torque of a motor necessary to produce its
rated horsepower at full–load speed.
inductance
The feature shown by an electric circuit by which
varying current produces a varying magnetic field
that causes voltages in the same circuit or in a
nearby circuit.
induction motor
An induction motor is an alternating current
motor where the primary winding on one member
(usually the stator) is connected to the power
source. A secondary winding or a squirrel cage
secondary winding on the other member (usually
the rotor) carries the induced current. There is no
physical connection to the secondary winding; its
current is induced.
inductance
The characteristic of an electric circuit by which a
varying current in it produces a varying magnetic
field which causes voltages in the same circuit or
in a nearby circuit.
Motors and generators
insulator
A material that tends to resist the flow of electric
current.
In a motor the insulation serves two basic
functions: it separates the various electrical
components from one another; and it protects
itself and the electrical components from attack
by contaminants such as dust and oil.
inverter
An electronic device that converts fixed
frequency and fixed voltages to variable
frequency and voltage.
An inverter enables the user to adjust the speed of
an AC motor.
laminations
The steel or iron portion of the rotor and stator
cores made up of a series of thin laminations
(sheets) stacked and fastened together by cleats,
rivets or welds. Laminations are used instead of a
solid piece to reduce eddy current losses.
magnetic polarity
North or south poles.
phase
Indicates the space relationships of windings and
changing values of the recurring cycles of AC
voltages and currents. Due to the positioning (or
the phase relationship) of the windings, the
various voltages and currents will not be similar
in all aspects at any given instant.
The most common power supplies are either
single– or three–phase (with 120 electrical
degrees between the three–phases).
poles
In an AC motor, poles refers to the number of
magnetic poles in the stator winding. The
number of poles determines the motor's
revolution per minute capacity.
In a DC motor, poles refers to the number of
magnetic poles in the motor. Poles create the
magnetic field in which the armature operates
(speed is not determined by the number of poles).
polyphase motor
Two– or three–phase induction motors that have
their windings, one for each phase, evenly
divided by the same number of degrees.
Reversal of the two–phase motor is accomplished
by reversing the current through either winding.
Reversal of a three–phase motor is accomplished
by interchanging any two of its connections to the
line.
Polyphase motors are used where a three–phase
power supply is available.
Introduction
ix
polyphase motor (cont.)
These motors are limited primarily to industrial
applications although they may be used in
airconditioning units around the home.
The starting and reversing torque characteristics
of these motors are exceptionally good. This is
due to the different windings being identical and,
unlike other motors, the currents are balanced.
They have an ideal phase relation, which results
in a true rotating magnetic field over the full
range of operation.
resistor
a material that restricts the flow of electricity is
packaged in an insulating material. Resistors do
not stop electrical flow, they reduce it to a
determined value.
rotating magnetic field
The magnetic force created by the stator once
power is applied to it that causes the rotor to turn.
rotor
The rotating member of an induction motor.
This is usually made up of stacked iron or steel
laminations.
squirrel cage
The rotor of an induction motor. A shaft runs
through the center and a squirrel cage made in
most cases of aluminum, holds the laminations
together, and act as a conductor for the induced
magnetic field.
The squirrel cage is made by casting molten
aluminum into the slots cut into each lamination.
starting current
Amount of current drawn at the instant a motor is
switched on. In most cases this current is much
higher than the current required for running the
motor at speed.
starting torque
The torque or twisting force delivered by a motor
at the instant it is switched on.
stator
That part of an AC induction motor's magnetic
structure that does not rotate. It usually contains
the primary winding.
The stator is made up of laminations with a large
hole in the center in which the rotor can turn.
There are slots in the stator in which the windings
for the coils are inserted.
transformer
A series of ‘windings’ (coiled wire) for
transforming source electricity to the type or
voltage that is required for an appliance or to a
suitable state for transmission.
Transformers are used to increase or decrease the
voltage in a conductor.
x
Motors and generators
Transformer (cont.)
Transformers either step up or step down the
voltage being transmitted.
Most transformers are two coils of wire wound
around a common iron core. One is the primary
(input) coil and the other is the secondary
(output) coil. The ratio of windings in the
primary coil to the windings in the secondary coil
determines the ratio of the input to the output
voltage.
torque
Introduction
Turning force delivered by a motor.
xi
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Physics
HSC Course
Stage 6
Motors and generators
Part 1: The incredible moving charge
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Contents
Introduction ............................................................................... 2
Charges..................................................................................... 4
Electric current and magnetism ...........................................................4
The force between conducting wires...................................................8
The motor effect .................................................................................13
The electric motor.................................................................... 19
The Thomas Davenport motor...........................................................19
The turning power of a motor.............................................................25
Is a loudspeaker really a motor? .......................................................37
Summary................................................................................. 40
Suggested answers................................................................. 43
Exercises–Part 1 ..................................................................... 47
Part 1: The incredible moving charge
1
Introduction
When Hans Christian Oerstead first demonstrated that a current can
produce a magnetic field the world was changed forever. An entire new
technology was made available because of this phenomenon.
That technology spawned the new electrical age. It enabled the
development of the electric generator and the electric motor. Today you
would not be able to describe your daily activities without mentioning
the use of electrical devices. These devices are credited with having
revolutionised work in industry and on the home front.
Electric motors are the drivers of the modern home. This part looks at
the principles that describe the operation of the electric motor.
In Part 1 you will be given opportunities to learn to:
•
2
discuss the effect, on the magnitude of the force on a
current–carrying conductor, of variations in:
–
the strength of the magnetic field in which it is located
–
the magnitude of the current in the conductor
–
the length of the conductor in the external magnetic field
–
the angle between the direction of the external magnetic field
and the direction of the length of the conductor
•
describe qualitatively and quantitatively the force between long
F
II
parallel current–carrying conductors: = k 1 2
l
d
•
define torque as the turning moment of a force using: t = Fd
•
identify that the motor effect is due to the force acting on a
current–carrying conductor in a magnetic field
•
describe the forces experienced by a current–carrying loop in a
magnetic field and describe the net result of the forces
•
describe the main features of a DC electric motor and the role of
each feature
•
identify that the required magnetic field in DC motors can be
produced either by current–carrying coils or permanent magnets.
Motors and generators
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In Part 1 you will be given opportunities to:
•
solve problems using:
F
II
=k 1 2
l
d
•
perform a first–hand investigation to demonstrate the motor effect
•
solve problems and analyse information about the force on
current–carrying conductors in magnetic fields using: F = BIl sin q
•
solve problems and analyse information about simple motors using:
•
t = nBIAcosq
identify data sources, gather and process information to qualitatively
describe the application of the motor effect in:
–
the galvanometer
–
the loudspeaker.
Extract from Physics Stage 6 Syllabus © Board of Studies NSW, amended
November 2002. The most up-to-date version can be found on the Board's
website at
http://www.boardofstudies.nsw.edu.au/syllabus_hsc/syllabus2000_listp.html#p
Part 1: The incredible moving charge
3
Charges
Electric current and magnetism
You should recall from the module Electrical energy in the home that
Hans Christian Oersted demonstrated in 1820 that a current in a
conducting wire produces a magnetic field. The magnetic field forms a
circular pattern as shown in the figure following.
I
Flux produced by a DC current flowing in a conductor. This pattern can be
produced by sprinkling iron filings on a sheet of card through which a
conducting wire passes. Note if the current flowing in the conductor is an AC
current, the flux direction will change with the frequency of the AC current. The
pattern will therefore not emerge when iron filings are sprinkled on the card.
The magnetic flux direction is easily determined by the ‘right hand
conductor rule’ as shown in the figure below.
4
Motors and generators
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flux
convential
current
The ‘right hand conductor rule’. The finger tips point in the direction of the flux
around the coil. Remember that conventional current suggests the charge flow
is from the positive terminal to the negative terminal.
To show the direction of a current in the cross section of a conductor a
dot is used to symbolise the current flowing toward the reader or coming
out of the page while a cross is used to symbolise the current flowing
along the conductor away from the observer, or into the page.
This convention comes from the concept of the arrow flying towards you
or away from you. This convention is demonstrated in the figures below.
X
A cross in a circle shows the current moving away from you.
A dot in the circle shows the current moving toward you.
Part 1: The incredible moving charge
5
The magnitude of the flux density (B) at a point around a single long
current carrying wire is dependent on the medium surrounding the wire
and is proportional to the current in the wire (I). It is inversely
proportional to the perpendicular distance of the point from the wire (d).
I
d
where k is a constant = 2 ¥ 10 -7 Tm A -1 .
B= k
Do return exercise 1.1 now.
In the module Electrical energy in the home you learned that if two current
carrying wires are parallel and the current in both is in the same direction
that attraction occurs between the two wires due to the magnetic fields
produced by the current flow in the wires.
If the direction of the current flow in the two wires is opposite then the
conducting wires will be pushed apart by the magnetic fields produced by
the wires.
X
repulsion
repulsion
.
attraction
attraction
X
attraction
X
attraction
Flux around DC current carrying wires next to one another.
6
Motors and generators
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Why do the wires attract or repel?
Look at the figure below showing two wires with current travelling in the
same direction.
The crosses and dots on the diagram represent the direction of magnetic
flux surrounding each wire. Note that the flux directions are opposite.
This is equivalent to the flux from a north pole and a south pole.
The wires will therefore attract.
X
X
X
X
X
X
X
X
X
X
X
X
Flux surrounding current carrying wires (arrows) where the current is flowing in
the same direction.
Confirm that if the two current carrying wires shown in plan below are
carrying electric current in opposite directions that the flux between the
wires is in the same direction and will therefore result in repulsion of the
two wires from each other.
Part 1: The incredible moving charge
7
Check your answer.
The force between conducting wires
If two long parallel wires of length l, separated by a distance d are
conducting currents I1 and I2 the magnetic fields produced by each
respective conductor affect the other conductor. The direction of that
force (attraction or repulsion) depends on whether the current is flowing
in the same direction or in opposite directions. This is shown in the
figures following.
Force
two conductors with
current travelling in
the same direction
point where
no field exists
Force
The attractive force that occurs when the current carrying wires carry currents
travelling in the one direction. Note the point where no field exists.
8
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Force
two conductors with
current travelling in
opposite directions
all field lines have
the same direction
Force
The force that is produced when current carrying wires have currents operating
in the opposite directions.
The magnitude of the force F, is dependent on the separation distance
of the two wires, d. The greater the separation distance the weaker
the force.
The size of the force is dependent on the size of the respective currents in
the current carrying wires I1 and I2. This is because the strength of the
magnetic fields (B) surrounding each wire due to the current flow is
proportional to the number of charges flowing in the conductor (I = q/t).
The force is also proportional to the length of the conductor in the
magnetic field. The longer the conductor, the more charges flowing
within it at a specific current. Therefore, there are more lines of force.
The magnitude of the force is also dependent upon the ability of the
magnetic field to permeate the material between the wires. That ability is
different for different materials.
To overcome that unknown factor a constant (k = 2 ¥ 10–7 TmA–1) is
introduced to signify the magnetic field is in air or a vacuum. If the
wires are in any other medium then a different k would apply.
The force between the current carrying wires is therefore:
F=
kI1I 2 l
d
Part 1: The incredible moving charge
9
But, B = k
I
d
Therefore, F = BIl .
You should recall this equation was referred to in the preliminary module
Electrical energy in the home.
The force (F) on a current carrying conductor at an angle q to a magnetic
field (B) is F = BIl sinq . The angle q is shown in the figure below.
The directions of F, B and I can be determined using the right hand palm
rule. These operate at 90° to one another.
If the current carrying conductor is at some angle other that 90° to the
magnetic flux lines then the resulting force is reduced by a factor of sinq
from its maximum value.
F is perpendicular to B
F
flux lines leave the N-pole
and enter the S-pole
B
current direction
is the angle between the
conductor and the magnetic
field lines
Note: F is a maximum when F, B and are all perpendicular
When the current carrying wire is at an angle other than 90° to the
magnetic field direction you need to find the component of its length that
is equivalent to a wire at 90° to the magnetic field direction. That is the
sin q component of the wire length hence the equation becomes
F = BIl sinq .
Sample problem
1
10
Find the magnitude and direction of the magnetic force between two
long parallel conducting wires of length 2 m that are 0.1 m apart if
both carry a current of 5 A when:
Motors and generators
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a) the currents are in the same direction
_____________________________________________________
_____________________________________________________
b) the currents are in opposite directions.
_____________________________________________________
_____________________________________________________
_____________________________________________________
Solution
1
a) Since the currents are in opposite directions the force is an
attractive one.
kI1I 2 l
d
2 ¥ 10 -7 ¥ 5 ¥ 5 ¥ 2
=
0.1
= 0.0001 N
F=
b) Since the current is in the opposite direction the force is a
repulsive one. The magnitude of the force is determined from
the equation below. It has the same magnitude as above but this
time is repulsive.
kI1I 2 l
d
2 ¥ 10 -7 ¥ 5 ¥ 5 ¥ 2
=
0.1
= 0.0001 N
F=
Problem
2
A DC power line 250 m long is oriented NE–SW in the Earth’s
-5
magnetic field that has a strength of 1.5 ¥ 10 T in the north–south
direction (Hint: The North Pole is a south pole and the South Pole is
a north pole). If the current carried by the powerline is 150 A
flowing from the NE calculate the magnitude and direction of the
force exerted on the powerline due to the interaction of the current
carrying wire and the magnetic field.
_________________________________________________________
_________________________________________________________
_________________________________________________________
Part 1: The incredible moving charge
11
Solution
F = BIl sin q
where
B = 1.5 ¥ 10 -5 T
l = 250 m
I = 150 A
q = 45∞ (from reference to the points of the compass)
F = BIl sin q
F = 1.5 ¥ 10 -5 ¥ 150 ¥ 250 ¥ sin45
F = 0.4 N
From Fleming’s left hand rule where the direction of the current is
indicated by the tall finger (current component at 90° to B is to the
west) , the direction of the magnetic field by the index finger (from
South to North) and the direction of the force by the thumb when the
two fingers and thumb are arranged at 90° to each other the force is
directed downwards.
Calculate the magnetic force between two long parallel conducting wires of
length 100 m that are 1 m apart if both carry a current of 200 A when:
a)
the currents are in the same direction
______________________________________________________
______________________________________________________
b) the currents are in opposite directions.
______________________________________________________
______________________________________________________
Check your answer.
Do return exercise 1.2 now.
12
Motors and generators
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The motor effect
When a conductor carries a current,
a magnetic field exists about the
conductor. If a magnet is brought
near the current carrying conductor
then a repulsive or attractive force
will occur.
conducting wire
carrying no current –
no deflection occurs
If a conductor not carrying a
current is placed within a magnetic
field (between a pair of bar
magnets) as shown in the figure
opposite there is no effect on the
conductor.
Conductor carrying no current in a
magnetic field.
If the conductor has a current flowing out of the page as shown in the
figure below then the resultant magnetic field produced by that current in
the conductor will interact with the field from the permanent magnet and
repulsion will occur in the direction as indicated.
current carrying wire
F
Repulsion of a current carrying wire in a magnetic field when the current is
coming out of the page.
If the direction of the current in the conductor is reversed then the
direction of repulsion will be reversed.
Part 1: The incredible moving charge
13
conductor carrying
current into the page
F
Repulsion of a conductor carrying a current into the page when the conductor is
in a magnetic field.
The direction of the force on the conductor is determined by the direction of the
external magnetic field compared to the direction of the magnetic field produced
by the current flowing in the wire.
If the current is coming towards you out of the page the magnetic field
surrounding the conductor is anti–clockwise (from the right hand
conductor rule). The interactions of the fields from the conductor and the
permanent magnets is such that the field is strengthened on one side of
the conductor and weakened on the other. This should come as no
surprise as you should recall that magnetic flux is a vector quantity and
vectors can be added.
1
14
Draw a diagram to show the forces on the conducting wire that occur
when the direction of the magnetic field from the permanent magnets is
moved through 90∞ clockwise as shown in the figure below.
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2
Draw a diagram to show the forces on the conducting wire that
would occur when the magnetic field from the permanent magnets
are moved through 180∞ clockwise as shown in the figure below.
3
Draw a diagram to show the forces on the conducting wire that
would occur when the magnetic field from the permanent magnets
are moved through 270∞ clockwise as shown in the figure below.
4
How would you describe the effect of variations in the direction of
the external magnetic field and the direction of the length of the
conductor?
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Part 1: The incredible moving charge
15
5
If the magnets were rotated through a full 360∞ rapidly what would
be the effect on a free moving length of the conducting wire?
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______________________________________________________
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Check your answers.
Applications of the motor effect
The uses for the motor effect other than using it for a motor were
developed early in the exploration of the technology of electricity.
Among these uses was the development of a meter to measure how much
electric current was being carried by a conductor. That device was called
a galvanometer.
In 1820, Johann Salomo Christoph Schweigger, a German physicist,
constructed the first simple galvanometer. William Sturgeon invented
the first suspended coil galvanometer in 1836.
In a typical galvanometer such as you would find in a science lab for
students studying physics, a current is passed through a coil in a
magnetic field as shown in the figure below. As a consequence of the
magnetic field generated in the coil interacting with the permanent
magnets, the coil experiences a turning force or torque.
To enhance the effect of the magnetic field generated in the coil a soft
iron core cylinder is placed in the centre of the coil. That greatly
enhances the effect of the magnetic field in the coil.
The torque in the coil is proportional to the current flowing though the
coil. The coil's movement is opposed by a coil restoring spring hence the
amount of deflection of a needle pointer attached to the coil is
proportional to the size of the current passing through the coil.
The magnets in the meter are curved so as to ensure that the field is
perpendicular to the coil no matter what the orientation of the coil as it
rotates. That provides a constant force as the coil spins.
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Motors and generators
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6
5
4
3
2
1
0
soft iron core
1
2
3
4
5
6
Magnet surface is
curved. This ensures
the force is constant.
Magnetic force for
the curved magnet
is perpendicular to
the magnet surface
reversing the current
reverses the direction
of the pointer
current I
Restoring spring returns the
spring loaded pointer to 0
when no current flows
In most galvanometers the current can be set to flow in either direction
through the meter. This means the pointer is set to show the deflection to
the left or right of a zero mark. This also shows the direction of current
flow.
A galvanometer of the type used in school laboratories.
Part 1: The incredible moving charge
17
Explain why a galvanometer is suited to measuring a DC current but is not
useful for measuring a high frequency AC current. Consider the makeup of
the galvanometer and its mode of operation in your answer.
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_________________________________________________________
_________________________________________________________
_________________________________________________________
Check your answer.
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Motors and generators
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The electric motor
The origin of the first electric motor is controversial. The development
of the device has been credited to many of the workers in electricity
including the great pioneer of electromagnetic experiments, Michael
Faraday. Although many of the ‘electric motors’ cited as the first could
be deemed as utilising the motor effect they were not able to be used to
do any real work. The first motor with the potential to do work was
probably that of Thomas Davenport.
Use the internet or any other reference source such as an encyclopedia to
research the first electric motor. Write down the names of the people you
find are credited with the invention of the first electric motor and the year of
their invention. Note also any description of their device and how it works.
Decide for yourself who you believe invented the first true electric motor.
Your opinion following your research will probably be as valid as the one
that follows.
The Thomas Davenport motor
Thomas Davenport invented the electric motor and the commutator and
brushes in 1833. A model of his motor is in the Smithsonian Institute in
Washington. Davenport’s motor used the force generated by two
electromagnets interacting to do work. This was an unusually early
application of the electromagnet which was a device only recently
discovered at the time.
The electricity source for the electromagnets Davenport used in his motor
was a galvanic battery of the type developed by Volta. You learned
about galvanic cells in the preliminary module Electrical energy in the
home. The battery Davenport used to provide the DC current to his
electromagnets consisted of a bucket of weak acid for an electrolyte with
concentric cylinders of different metals for electrodes. These electrodes
were wired so they could provide the electric current to the
electromagnets used in the motor.
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19
Davenport mounted one electromagnet on a wheel that was free to turn.
This was his motor’s rotor or moving part. The other electromagnet was
fixed to a stationary wooden frame. This was his motor’s stationary part
or stator. The force generated by the interaction of the two magnetic
fields between the fixed and stationary electromagnets caused the magnet
attached to the free moving wheel to turn half a revolution. By reversing
the wires to one of the magnets, hence reversing its polarity (making
north south and south north) Davenport found that he could get the rotor
to complete another half–turn.
Davenport then devised a brush and commutator set up to enable the
motor to continue to rotate without the need to switch the wires and
reverse the polarity of the rotating magnet constantly. The commutator
was made by attaching fixed current carrying wires from the frame to a
segmented conductor that supplied current to the rotor–mounted
electromagnet. This provided an automatic reversal of the polarity of the
rotor–mounted magnet twice per rotation, resulting in continuous
rotation.
Davenport actually tried to patent his motor but was refused a patent
because he was the first person to have ever attempted to patent an
electrical device. The patent office didn’t know how to handle his
application.
To see a site that describes the work of Thomas Davenport and the story
behind his invention of the ‘first’ electric motor see a site on the physics
website page at: http://www.lmpc.edu.au/science.
An electric motor is a machine that converts electrical energy into
mechanical energy. When an electric current is passed through a wire
loop that is in a magnetic field whether that field is produced by a
permanent magnet or by an electromagnet, the loop will rotate because of
a force created between the magnetic field in the current carrying coil
and that of the magnet. That rotating motion is transmitted to a shaft,
providing useful mechanical work. The trick is to make the motor
continuously rotate to make it useful. Without the commutator in a DC
motor that doesn’t happen because the turning force or torque on the coil
would be opposite after the coil had rotated 180∞. This would stop the
motor after rotating a half turn.
A simple electric motor is shown in the figure following. Note the
commutator (in this case the commutator is a ring that is cut in two called
a split ring) that rotates against the brushes so the direction of the current
feed into the coil changes as the coil and commutator rotate. Note the
polarity of the brushes on the figure is fixed.
20
Motors and generators
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armature
force
B
~
N
force
S
brush
(fixed)
axis of
rotation
brush
split ring
commutator
Commercial commutators are made of more that one pair of plates
because normally motors have more than one pair of magnetic poles
(a north and a south) on the rotor. Each pair of magnetic poles requires a
pair of conducting plates on the commutator. The diagram below shows
a sketch of a commercially available split ring commutator in a motor.
multiple poles on the motor
brushes
bearing to allow free
rotation of the motor
soft iron core
split ring commutator
insulated copper
coil windings
a cutaway of the external coil
A universal AC/DC motor. This type of motor can operate using either AC or
DC electric currents. (Photo: Thomas Brown.)
The partners of the commutator in a DC motor are the brushes.
The brushes literally brush against the commutator and deliver electric
current to the commutator conducting plates. Most brushes in larger
motors are made of compressed carbon.
Part 1: The incredible moving charge
21
The term brush is a hangover from when the brushes were literally
brushes made from metal similar to a wire brush. The idea of using
carbon blocks as brushes was first proposed by a Mr Van Depoele, who
was working at the Thompson Houston Electric Company in the 1880s.
Metal brushes made from brass touching rotating commutators wore out
themselves or the commutators very rapidly. Van Depoele suggested
carbon blocks with a large surface area instead of the brushes. It was
ridiculed but when tried was found to work. Carbon brushes are now the
most common way to get the current to the commutator. They wear well
and actually tend to polish the commutator improving the contact as they
run because the brushes remove any oxide build up on the commutator
surface. The carbon brush was patented in the 1890’s.
The first motor was a DC motor and required the invention of a commutator
before it could work. Why is it necessary to have a commutator to have a
DC motor?
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_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Check your answer.
To see a website that outlines the story of the invention of the carbon brush
see a page on the physics website page at: http://www.lmpc.edu.au/science
Making a DC motor
To do this activity you will need:
22
•
around 40 cm of single strand insulated copper wire
•
a toilet roll centre
•
a D–cell battery
•
a bar magnet
•
electrical insulating tape
•
two large silver metal paperclips.
Motors and generators
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Procedure:
1
Leaving 3–4 cm of wire loose at each end, wrap the wire around a
toilet roll centre several times to make a tight coil, then pull the toilet
roll centre out from the coil.
Wrap the wire tightly around the toilet roll centre.
2
Loop the loose ends of the wire around the coil to hold it together.
When doing this leave around 2–3 cm of wire sticking out. This
wire sticking out will form the axle of your motor.
3
Remove 1–2 cm of the insulation from half of the circumference of
the wire sticking out and arrange your coil as shown in the figure
below.
remove plastic insulation
from top half of the wire
Note: the arms must be evenly
balanced with the “arms” positioned
directly across from each other.
Loop the remaining wire around the middle
to keep the coil tight. Remove the plastic
insulation from the ends of the wire.
4
Place the plastic cup upside down on the table
5
Position the D cell battery on top it horizontally and tape it to the cup
with insulation tape.
Part 1: The incredible moving charge
23
6
Bend two metal paper clips as shown in the figure below and tape
one to each of the battery's terminals so they will form a cradle for
your coil.
tape over the
end of battery
Lay the battery on the cup, tape it
down, and tape the two paper clips
into place on the ends of the battery.
7
Tape a bar magnet to the top of the battery perpendicularly to it,
lying flat.
8
Carefully lay the coil across the paper clip supports, making sure the
stripped ends of the coil are touching the clips.
the coil should be 5–10 mm
from the magnet – adjust the
paper clips if necessary
Place the magnet on the top of the
battery, and give the coil a spin.
9
If all is well and you have a well balanced coil, the coil should start
to oscillate. Give it a little push to start it rotating. It should then
rotate on its own.
10 After you have made the observations below try changing the shape
of your coil by squashing it into a square or rectangle. Check
whether it changes the power of the motor by changing its speed of
revolution when operating.
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Motors and generators
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Observations:
1
What happens to the coil when you put it on the cradle?
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2
How long can you get the coil to turn before it stops?
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3
Look closely at the contact points of the turning coil. What do you
see?
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4
Stop the coil after you have had it working. Turn the coil slowly by
hand while it is in contact with the cradle. Can you feel a difference
in the level of magnetic attraction and repulsion of the coil? What is
the angle between the coil and the magnet when the attraction is a
maximum and a minimum.
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5
When you change the shape of the coil you are changing the area of
the coil. When you bend your coil into a shape other than a circle
that shape has a smaller area. Does a change in the area of the coil
make a difference to the turning power of the motor.
_____________________________________________________
Check your answers.
Draw a labelled diagram of your motor on your paper. Label on your
diagram the following bits:
•
an armature or rotor
•
a split ring commutator and brushes
Return your labelled diagram with the exercises for this part.
Do return exercise 1.3 now.
Part 1: The incredible moving charge
25
The turning power of a motor
The turning power of a motor is generally how the torque (t) of a motor
is described. The torque of a motor is defined as the turning moment of
the force (F) supplied by the coil. The torque is therefore defined as:
t = F¥d
where d is the distance at which the force is supplied acting in a
clockwise or anticlockwise direction about a centre of rotation.
This relationship tells you that the effect of a force applied at a distance
to the turning axis is greater than the effect of a force applied close to a
turning axis. To put this in an everyday perspective, try to push open a
door using the handle. You should find it easy. Now try to push open the
same door by pushing on the hinge. You should find it considerably
more difficult. To open the door you need to apply a much greater force.
This is because the torque required to open the door is fixed but you have
changed the distance from the turning axis or hinge.
Electric motors are exactly the same. The torque the motor supplies is
dependent upon the distance of the coils from the turning axis and the
size of the force applied to the coil.
d
d
Consider the following situation where a single rectangular current
carrying coil is positioned horizontally within a horizontal magnetic
field. The coil is free to rotate about its centre.
axis of rotation
When the current is flowing in the coil the sides of the square coil that
are parallel with the magnetic flux lines have no force acting on them.
This is because the angle between the magnetic field and the current flow
is 0∞. From F = BIl sinq if q is zero then F = 0 N. For the sides of the
26
Motors and generators
coil at 90∞ to the magnetic flux lines, from the equation F = BIl sinq , if q
is 90∞ then F = BIl.
The direction of the force on each side of the coil is opposite because of
the opposite direction of the current flow. This can be confirmed using
the right hand palm rule.
The direction of both forces acting on the arms of the coil though, will be
up or down giving the coil a turning force or torque.
The torque due to the force on each side of the coil is t = F ¥ d where d
is the distance of the side perpendicular to the magnetic field and parallel
to the axis of rotation.
The torque on each side of the square coil is therefore:
t = BIld
but since l ¥ 2d is really the area (A) of the rectangular coil therefore the total
torque on the coil due to both sides is
t = BIA
Motors are rarely (if ever) constructed with rectangular or square coils
but the relationship t = BIA still applies.
The torque of the coil is related to the area of the coil. Also motors are
never a single turn of a coil. That means the coil of an electric motor coil
has many turns. The effect of this is that the torque of the motor is
multiplied by the number of turns in the coil.
The torque of a motor is therefore t = nBIA .
The torque at any point in the rotation of the coil can be determined from
the relationship t = nBIAcos q .
When the coils are at 0° to the magnetic field as shown in the figure
below the torque is at a maximum because cos 0° = 1. When the coil has
rotated a quarter turn it is at 90° to the magnetic field the torque is a
minimum because cos 90° = 0.
To overcome this problem the pole surface of magnets in real motors are
curved and almost meet. Since the magnetic flux lines are perpendicular
to the magnet’s surface this has the effect of maximising the rotation
interval over which the coils are at 0∞ to the magnetic field.
Part 1: The incredible moving charge
27
armature
B
~
N
S
brush
(fixed)
axis of
rotation
brush
split ring
commutator
rotates
A DC motor.
Sample problems
1
What is the size of the torque for a rectangular coil of 50 loops each
of dimension 0.1 m by 0.5 m where the current flow in the coil is 5
A and the coil is horizontal in a magnetic field of 1 T?
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2
Determine the direction of rotation of the coil in the DC motor
shown in the figure below.
A
C
N
B
D
S
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28
Motors and generators
Solutions
1
t = nBIAcos q
= 50 ¥ 1 T ¥ 5 A ¥ (0.1 m ¥ 0.5 m ) ¥ cos 0 °
= 12.5 Nm
The torque on the coil is 12.5 Nm.
Note that the units Nm are derived directly from the relationship
t = F ¥ d where F is in newtons and d is in m.
2
Use the right hand palm rule to determine the direction of the force
applied to coil side AB and to coil side CD. This shows that the
force resulting from the interaction of the magnetic field from the
current in the coil with the permanent magnet’s field will result in a
force upward acting on side AB and a force downward acting on side
CD. The result will be a clockwise rotation of the coil.
Do return exercise 1.4 now.
Examining a DC motor
In an electric motor the stationary parts are called the stator. The moving
part of the motor assembly that carries the coil is called the rotor, or
armature.
It is easy to control the speed of DC motors by varying the magnetic field
from the current carrying coil. A higher current makes the motor spin faster.
This is important because these motors are used where motor speed control
is necessary. Applications include portable cordless power tools and toys
such as slot cars.
To do this activity you will need:
•
a small electric motor
•
a bar magnet or a compass
•
a D cell battery
•
an AA cell battery
•
a AAA cell battery
•
two connecting wires.
•
a multimeter.
Locate a simple DC electric motor such as you would find in a small
battery powered toy. A can type one is best. You will need to
disassemble the motor. This means you will need to take the cover off.
That may result in the motor becoming no longer serviceable.
Part 1: The incredible moving charge
29
A simple electric motor has the following parts:
•
an armature or rotor
•
a split ring commutator
•
brushes
•
a centre of rotation. This is an axle.
•
a magnet to supply the magnetic field
•
a DC power supply such as a battery.
You will need to try to identify the mechanical arrangement on your
simple motor that correspond to each of these parts. When you take apart
a small electric motor you will find that it contains:
•
two small permanent magnets
•
a commutator
•
two brushes
•
an armature made of three electromagnets made by winding wire
around a piece of laminated iron metal. To increase the field of the
electromagnet the axle of the motor is generally made of soft iron.
You should recall from your work on electromagnets in the
preliminary module Electrical energy in the home that the soft iron
core increases the strength of the magnetic field in an electromagnet
when a current flows in a surrounding coil.
forward switch
reverse switch
A small motor from a remote controlled toy. Note the control that switches the
motors rotation direction. The switches enabling the motor to go forward and
reverse simply change the direction of the DC current through the motor.
(Photo: Thomas Brown)
30
Motors and generators
Procedure
These instructions assume your motor is a can motor like the one shown
in the photo on the previous page.
1
Observe your motor from the outside. You should be able to
identify the steel chassis of the motor, the plastic or nylon cap held
in place by screws or metal tabs and two electrical leads.
2
Measure the current supplied by your AAA cell battery with the
multimeter by touching the contact probes to both ends of the
battery. Record the current.
Hook your motor up to the AAA cell battery. It should run but
notice the maximum revolution speed and direction of revolution of
the axle. You may be able to note the sound (pitch) the motor makes
while running or you may connect a paper disc to the end of the
motor to enable you to estimate the motor revolution speed.
3
Reverse the connections of the leads to the battery and observe what
happens to the axle rotation direction. Record your observations
below.
4
Measure the current supplied by your AA–cell battery with the
multimeter. Record the current. Hook the motor up to the AA–cell
battery. Does the speed of revolution of the motor change? Record
your observations below.
5
Measure the current supplied by your D–cell battery with the
multimeter. Record the current. Hook up the motor to the D–cell
battery. Does the speed of revolution of the motor change? Record
your observations below.
6
Bend back the metal tabs or remove the screws that hold the plastic
end cap in place and remove the end cap. Note that once this is done
the motor may not be able to be reassembled to work properly again!
tabs to hold the nylon
casing to the metal case
Small motor of the type found in toys. (Photo: Thomas Brown)
7
Look inside the end cap. Locate the motor’s pair of brushes. They
are connected to the exterior terminals and are designed to touch the
motor’s commutator as it spins. Their purpose is to complete the
electric circuit to the coil on the armature.
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31
The brushes can’t be solidly connected to the armature that must be
allowed to spin freely. Instead the circuit is completed by the
brushes literally brushing on the commutator.
Inside the cap of the small motor showing the two brushes hanging down.
(Photo: Thomas Brown)
8
Locate the commutator. This is the end of the axle that is the centre
of the armature. Pull the armature out of the steel chassis after first
removing the small gear from the end of the gear shaft. You may
need pliers to get the small nylon gear off the shaft.
small gear
gear shaft
(Photo: Thomas Brown.)
The commutator should be recognisable as a set of curved copper
plates fixed to the axle. Careful examination should reveal it has at
least two and probably three splits. This is called the split ring
commutator.
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Motors and generators
rotor
coil contact is connected
to one segment of the
splitring commutator
stator
The rotor removed from the stator of the small motor. Note the insulated
copper wire making up the coil. This wire although it appears to be bare copper
wire is actually insulated with either a clear plastic lacquer such as polyurethane
or a thin coating of transparent plastic. If the insulation fails the effectiveness of
the motor is diminished because there are effectively less coils.
(Photo: Thomas Brown.)
insulated copper coil
pole 2
wear lines from the brushes
contacting the commutator
pole 1
split ring
commutator
coil contacts
laminated iron
electromagnet
pole 3
The armature of the motor. Note this is a three pole motor. The motor has
three poles to increase its efficiency. The armature components are shown.
(Photo: Thomas Brown)
9
Look at the armature. This consists of a set of electromagnets (3)
made from soft iron plates with a coil of thin copper wire wrapped
around them. The ends of the wire coils are soldered to terminals.
Each terminal is then connected to one of the separate split plates
that make up the commutator.
10 Inside the metal motor chassis is usually two semicircular permanent
magnets. They are usually held in place in the chassis by crimped
impressions or pressed slits so they cannot move when the motor is
working.
Part 1: The incredible moving charge
33
semicircular
permanent
magnets
Small motor metal case with two semicircular magnets. The crimping on
the outside of the motor holds the magnets in place.
(Photo: Thomas Brown.)
Use a marker to mark your magnets with an arrow up toward the
opening so you can tell their orientation after you remove them from
the chassis.
Carefully pull the magnets out of the motor chassis without breaking
them. Check the polarity of the magnets. That is whether a north
pole was opposite a south or north pole by seeing whether the
magnets attempt to repel or attract when held in their original
relative positions. Record your observations.
Optional activity: Try to reassemble your small motor.
After re–assembly try to get the motor to work!
Observations
1
The current from the AAA–cell battery made the motor spin
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2
The current from the AA–cell battery made the motor spin
_____________________________________________________
3
The current from the D–cell battery made the motor spin
_____________________________________________________
4
The battery that caused the motor to spin the fastest was the
_____________________________________________________
5
The polarity of the permanent magnets was with the (opposite/same)
poles facing each other.
Check your answers.
To see a site that describes the components of a simple electric motor see:
http://www.lmpc.edu.au/science
34
Motors and generators
1
In a simple direct current (DC) motor, a device known as a split ring
commutator switches the direction of the current each half rotation to
maintain the same direction of motion of the shaft. How does the split
ring in the commutator ensure that the direction of the current in the
coils is always such that it can ensure one way rotation of the armature.
(Hint: Look at the direction of the current in the coil if the ring wasn’t
split.)
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
2
Look at the motor you have disassembled. Identify the passage of
the electric current through the motor when it is operating. Follow
this and describe that pathway with a flow chart as it passes through
the different components of the motor.
3
If the permanent magnets in your motor were replaced with more
powerful ceramic magnets what effect do you think that would have
on the speed of revolution of your motor? Explain your answer.
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
Check your answers.
Solving problems and analysing information about
simple motors
1
What do each of the symbols mean in the equation F =
kI1I 2 l
?
d
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_____________________________________________________
2
What does the equation F =
kI1I 2 l
describe?
d
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_____________________________________________________
Part 1: The incredible moving charge
35
3
Could you use F =
kI1I 2 l
to determine something about the
d
operation of a simple motor? If so what could you use it for?
______________________________________________________
______________________________________________________
4
What do each of the symbols mean in the equation t = nBIAcos q ?
______________________________________________________
______________________________________________________
______________________________________________________
5
What does the equation t = nBIAcos q describe?
______________________________________________________
______________________________________________________
6
Could you use t = nBIAcos q to determine something about the
operation of a simple motor? If so what could you use it for?
______________________________________________________
______________________________________________________
7
The simple motor with a square coil as figured below has ten coil
turns and a cross sectional area of 0.01 m2. The strength of the
magnetic field is 0.3 T. The current flowing in the motor is 2 A.
The limbs AB and CD are each 0.1 m long.
C
B
D
axis of rotation
A
a) Calculate the force on limb AB.
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__________________________________________________
__________________________________________________
__________________________________________________
36
Motors and generators
b) Calculate the maximum torque on the coil.
_________________________________________________
_________________________________________________
_________________________________________________
c) What is the torque on the coil when the plane of the coil makes
an angle of 30° to the magnetic field?
_________________________________________________
_________________________________________________
_________________________________________________
d) What is the torque on the coil when the coil is at 90° to the
magnetic field?
_________________________________________________
_________________________________________________
_________________________________________________
Check your answers.
Do return exercise 1.5 now.
Is a loudspeaker really a motor?
Most loud speakers consist of a circular permanent magnet surrounding a
freely moving coil slipped over the end of a cylindrical south pole of a
magnet. The outside cylinder is the circular north pole of the magnet.
The coil is attached to a cone shaped diaphragm.
Looking at a speaker
You may have an old set of bud earphones laying around that are no longer
working. If that is the case you may be able to carefully pull the speaker
apart to view the parts.
Do not do this with a good set of earphones. You will not be able to
repair the speaker after disassembly. The photograph below shows the
parts that make up a bud earphone speaker. The parts are arranged from
left to right as they would appear if you disassembled a bud earphone
speaker. This type of speaker is very simple in its construction yet
extremely effective.
Part 1: The incredible moving charge
37
The speakers are very cheap because of mass production. Even these
speakers work on exactly the same principles as larger loudspeakers.
Their components are simply miniaturised.
bud earphone case
magnet
copper washer
backing for the speaker where an AC
current flows into to combine with the coil
to make an electromagnet
copper coil
clear diaphragm that vibrates to convert
the electrical signal into sound
The components of a bud earphone speaker. (Photo: Thomas Brown.)
The clear diaphragm is the part of the speaker that is caused to vibrate by
the coil and ring magnet to produce the sound from the electrical input.
In the diagram of a loudspeaker following the circular magnet is partially
cut away so you can see how it operates.
Alternating electric current, generated by a microphone, radio, or another
source, flows through the coil of the speaker.
The current, alternating at the same frequency as the sound waves that
generated it, induces an alternating magnetic polarity field in the coil.
As the polarity of the magnetic field of the coil switches direction so
does the polarity of the magnetic field it generates. It is therefore
alternatively attracted to and repelled by the permanent magnet that has a
constant polarity. Because the coil is attached to the cone this causes the
cone shaped diaphragm to vibrate. This compresses or decompresses the
air in front of the cone to reproduce the sounds of the original source.
To see a site that demonstrates how a speaker works using a Java applet see
sites on the physics websites page at: http://www.lmpc.edu.au/science
38
Motors and generators
cylindical permanent magnet
The fluctuating
magnetic field in the
coil produced due to
the fluctuating current
causes the coil and
cone to vibrate in and
out in response to
current fluctuations
producing sound.
coil attached
to a fibre cone
N
coil
S
N
circuit supplying a fluctuating
AC current “sound” signal
A loud speaker.
Consider how the loudspeaker works. How is the loudspeaker utilising the
motor effect? Describe the action of the motor effect in operation in the
loudspeaker.
_________________________________________________________
_________________________________________________________
Check your answer.
Part 1: The incredible moving charge
39
Summary
Complete these statements to prepare a summary of this part of the
module.
•
What affect does each of the following have on the magnitude of the
force on a current carrying conductor of variations in:
–
the strength of the magnetic field in which it is located
__________________________________________________
__________________________________________________
__________________________________________________
–
the magnitude of the current in the conductor
__________________________________________________
__________________________________________________
__________________________________________________
–
the length of the conductor in the external magnetic field
__________________________________________________
__________________________________________________
__________________________________________________
–
the angle between the direction of the external magnetic field
and the conductor.
__________________________________________________
__________________________________________________
__________________________________________________
•
Describe what all the symbols mean in the equation F = k
I1 I2 l
.
d
______________________________________________________
______________________________________________________
40
Motors and generators
•
What would you use the equation F = k
I1 I2 l
for?
d
_________________________________________________
_________________________________________________
_________________________________________________
•
What is the rule that enables you to determine the force direction that
results as a result of the interaction of a positive charged particle
moving in a fixed magnetic field?
_____________________________________________________
_____________________________________________________
_____________________________________________________
•
What is meant by the term torque?
_____________________________________________________
_____________________________________________________
_____________________________________________________
•
What is a commutator and what does it do?
_____________________________________________________
_____________________________________________________
_____________________________________________________
•
Explain how a DC electric motor works.
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
Part 1: The incredible moving charge
41
42
Motors and generators
Suggested answers
Why do the wires attract or repel?
Using the right hand coil conductor rule for each wire shows you
that the flux directions for each wire where they are close together is
the same. Repulsion will therefore result.
X
1
X
X
X
X
X
X
X
X
X
X
X
The force between conducting wires
a)
The magnitude of the force whether attractive or repulsive is the
same. Because the currents are in the same direction the force will
be an attractive one.
kI1I 2 l
d
2 ¥ 10 -7 ¥ 200 ¥ 200 ¥ 100
=
1
= 0.8 N
F=
Note that these large currents produce very small repulsive or
attractive forces.
b) The magnitude of the force will be 0.8 N and because the current are
in opposite directions the force will be repulsive.
Part 1: The incredible moving charge
43
The motor effect
1
F
2
F
3
F
4
The effect of the magnetic field is a maximum when the direction of
the current carrying conductor is perpendicular to the magnetic field.
As the angle shifts from perpendicular toward parallel with the
magnetic field the effect reduces to zero. At intermediate angles, q,
the effect is reduced by cosq.
5
The wire would be pushed in a circular motion by the interaction of
the magnetic fields.
Applications of the motor effect
The galvanometer is able to measure the current flowing in both
directions. It is an example of the motor effect in operation where a
current flowing in one direction in a coil produces a magnetic field which
interacts with a permanent magnetic field to produce attraction or
repulsion of the free moving coil and hence movement of an attached
pointer.
44
Motors and generators
As a result an AC current would cause the indicating needle of the
galvanometer to fluctuate back and forth 50 times a second if measuring
a 50 Hz AC current as the direction of current flow changed.
Accurate readings would therefore not be possible with a galvanometer.
The Thomas Davenport motor
Without the commutator the most the motor can turn is through a half
turn. The direction of current would then need to be switched to enable
the motor to complete its rotation.
Making a DC motor
1
The coil should rock back and forth or may even begin to rotate.
2
The better the balance of the coil the longer it should spin. A
minute or two is good.
3
You should see small sparks.
4
The magnetic attraction should be at a maximum when the coil is at
90° to the magnetic field. It should be at a minimum when the coil is
parallel to the magnetic field.
5
The greater the area of the coil, the greater its turning power should
be.
Examining a DC motor
AAA battery is around 250 mA.
AA battery is around 300 mA
D cell is about 500 mA.
The battery that caused the motor to spin the fastest was the D cell
battery.
The polarity of the permanent magnets is with the opposite poles
facing each other.
1
The current flowing in the coil is flowing in one direction only.
The commutator ensures that the contact between the brushes and
the commutator plates is such to make sure the rotor appears to have
a current flowing only in one direction.
2
positive battery terminal > positive contact on the motor > brush >
commutator > coil >commutator > negative brush > contact on the
motor > negative terminal of the battery.
3
Since F = BIL the increased strength of the magnetic field would
result in a larger force that should result in a greater torque on the
motor that should result in a more rapid rotation.
Part 1: The incredible moving charge
45
Solving problems and analysing information about
simple motors
1
F is force, k is a constant, I1 is a current flowing in a wire, I2 is a
current flowing in a wire, l is the length of the wires, d is the
distance of separation of the wires.
2
The force set up between two long DC current carrying wires.
3
Nothing could be determined about the operation of simple motors
directly. F = BIl can be derived from this equation. That equation
can be used to determine the torque of a simple motor.
4
t is the turning force on the motor coil, n is the number of turns in
the coil, B is the magnetic field from the stator, I is the current
flowing in the coil, A is the area of the coil, cos q is the angle
between the coil and the magnetic field.
5
The torque or turning force of the rotor in relation to the stator.
It is the turning force supplied by the motor.
6
You could use it to determine the torque of the motor.
7
a)
F = BIl
= 0.3 ¥ 2 ¥ 0.1
= 0.06 N
(b
Total
t = nBIAcos q
= 10 ¥ 0.3 ¥ 2 ¥ 0.01 ¥ cos 0 °
= 0.06 Nm
(c
d)
t = nBIAcos q
= 10 ¥ 0.3 ¥ 2 ¥ 0.01 ¥ cos 30 °
= 0.052 Nm
t = nBIAcos q
= 10 ¥ 0.3 ¥ 2 ¥ 0.01 ¥ cos 90 °
= 0 Nm
Is a loud speaker really a motor?
The magnetic field generated by the coil in the speaker carrying a current
is attracted and repelled by the permanent magnet in an example of the
motor effect in action. Because the current in the coil is constantly
reversing the coil in the speaker is switching magnetic polarity in tune
with the current direction switching.
46
Motors and generators
Exercises – Part 1
Exercises 1.1 to 1.6
Name: _________________________________
Exercise 1.1
What is the strength of a magnetic field surrounding a wire carrying a
current of 2 A at a distance of 0.1 m?
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Exercise 1.2
Determine the force on two 10 m long parallel current carrying wires
separated by 0.2 m if each has a current of 5 A flowing in them. Assume
the currents flowing in the wires are flowing in opposite directions.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Part 1: The incredible moving charge
47
Exercise 1.3
If two 10 m long parallel current carrying wires separated by a distance
of 0.1 m are experiencing a repulsive force of 1.7 N determine the size of
the currents flowing in the wires if I1 is equal to I2.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Exercise 1.4
Describe the function of the commutator and brushes in the DC electric
motor.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Exercise 1.5
What is the size of the torque for a rectangular coil of 1000 loops each of
dimension 0.05 m by 0.1 m where the DC current flow in the coil is 10 A
and the coil is horizontal in a magnetic field of 10 T?
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
48
Motors and generators
Exercise 1.6
Determine the torque of a DC motor where the cross–sectional area of
the coil is 0.1 m2. The coil has 100 turns. The current flowing through
the coil is 2 A. The magnetic field is 0.2 T.
_________________________________________________________
_________________________________________________________
_________________________________________________________
Part 1: The incredible moving charge
49
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Physics
HSC Course
Stage 6
Motors and generators
Part 2: Induction
2
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0
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Contents
Introduction ............................................................................... 2
Michael Faraday........................................................................ 4
Generating electricity................................................................. 5
Magnetic field strength.......................................................................10
Lenz’s law ...........................................................................................12
Eddy currents .......................................................................... 15
Eddy current and motors....................................................................17
Electromagnetic braking ....................................................................20
Using induction for heating...................................................... 22
Summary................................................................................. 24
Suggested answers................................................................. 27
Exercises–Part 2 ..................................................................... 29
Part 2: Induction
1
Introduction
‘My theory of electrolysis; my ideas on electromagnetism and fields
of force I shall leave to others to ponder and refine. I give forth the
bold idea of antimaterialism; breaking the unanimous hold of
millennia on matter that says its consideration must be either material
or spiritual; giving freedom to thought.’
Michael Faraday (1791–1867) was perhaps the greatest experimental
scientist of his time.
In this part you will have opportunities to learn to:
•
outline Michael Faraday’s discovery of the generation of an electric
current by a moving magnet
•
define magnetic field strength B as magnetic flux density
•
describe the concept of magnetic flux in terms of magnetic flux
density and surface area
•
describe generated potential difference as the rate of change of
magnetic flux through a circuit
•
account for Lenz’s Law in terms of conservation of energy and relate
it to the production of back emf in motors
•
explain that, in electric motors, back emf opposes the supply emf
•
explain the production of eddy currents in terms of Lenz’s law
At the end of Part 1, you will have had an opportunity to:
2
•
perform an investigation to model the generation of an electric
current by moving a magnet in a coil or a coil near a magnet
•
plan, chose equipment or resources for, and perform a first–hand
investigation to predict and verify the effect on a generated electric
current when:
–
the distance between the coil and magnet is varied
–
the strength of the magnet is varied
–
the relative motion between the coil and the magnet is varied
Motors and generators
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•
gather, analyse and present information to explain how the principle of
induction is used in cooktops in electric ranges
•
gather secondary information to identify how eddy currents have
been utilised in electromagnetic braking.
Extract from Physics Stage 6 Syllabus © Board of Studies NSW, amended
November 2002. The most up-to-date version can be found on the Board's
website at
http://www.boardofstudies.nsw.edu.au/syllabus_hsc/syllabus2000_listp.html#p
Part 2: Induction
3
Michael Faraday
Michael Faraday (1791–1867) is the claimed father of the electric
generator and the developer of the theory that resulted in the invention of
the electric motor.
In September 1831 Faraday discovered magneto–electric induction; that
is the production of an electric current by purely magnetic means. To do
this, Faraday attached two wires through a sliding contact to a copper
disc. Faraday then rotated the disc between the poles of a horseshoe
magnet. This had the effect of moving an electric circuit in a magnetic
field. This generated a continuous direct current. Faraday had invented
the first electric generator.
The mechanism by which the generator worked was embodied in
Faradays Laws:
•
When the magnetic flux threading a circuit is changing an emf is
induced in the circuit.
•
The magnitude of the induced emf in a coil is directly proportional to
the time rate of change of the magnetic flux.
In 1832 Faraday did a series of experiments that proved that the
electricity induced from a magnet and copper disc, voltaic electricity
produced by a battery and static electricity were all related phenomena.
Do Exercise 2.1 to 2.2 now.
To see sites that discuss the life and amazing works of Michael Faraday see
pages on the physics websites page at: http://www.lmpc.edu.au/science.
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Motors and generators
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Generating electricity
You are probably familiar with the bike generator. This device uses the
rotation of the cycle wheel to turn the coil of a small generator fixed
between two magnets to generate enough electricity to run the lamp on a
bicycle. If you are not familiar with such a device you can probably find
someone who is familiar with the way a bike generator operates.
One reason they are rarer today than they were in the past is that you
must be pedaling at a certain rate to get the lights on the bike to have
good illumination.
Discuss how a bike generator works with someone who is familiar with one.
Ask about how the speed of the bike affected the illumination of the bike
lamps powered by the generator.
They will tell you that the faster the bike travelled the brighter were the
lights. This was because the faster you turned the coil in the magnetic
field the greater the current the generator produced.
A similar device to a bike generator works to produce the electricity to
recharge the battery and operate the electrical system in the family car as
you drive along. That device is called an alternator.
An alternator from a car. Note the fan
blades attached just behind the pulley
wheel. These force cooling air into the
alternator to stop it from getting too hot
while running. This helps to keep the
solid state circuitry in the alternator
functioning and prevents the stator coils
burning out. Note also the laminated
steel plates making up the central part
of the case. The coils where the AC
current is generated are wrapped
around these plates as an efficiency
measure because it cuts down on eddy
or back currents that would otherwise
be a problem and reduce efficiency of
the alternator if the steel core of the
winding was solid.
Part 2: Induction
5
The photo above shows an alternator. Notice the pulley wheel on the
front of the alternator. A belt attached to this wheel and to another
turned by the engine enables the rotor containing a series of
electromagnets to turn so that the alternator can induce an AC current in
the windings in the case (stator) of the alternator. That current is then fed
through a voltage regulator and rectifier to produce electricity that is DC
within the allowable voltage range to run the electrical systems and
recharge the battery of the car.
It was only after the invention of a solid state voltage regulator and
rectifier that alternators became popular in cars. That happened in the
1960s. Prior to that time the generation of electricity to supply the
electrical power needs of cars was from a generator.
laminated plates at the centre
of the stator coil windings
fan
The holes in the case are where the
coils are wrapped around the laminations
in the stator (case). It is in this internal coil
that forms part of the case where the AC
current is generated. AIr flow through the
holes cools the coils.
pulley
output terminal
An alternator from a car showing a different view where the steel plate
laminations forming part of the stator core windings are more easily seen.
(Photo: Thomas Brown.)
Find a local autoelectrician or someone who knows about how alternators
work. Discuss with them how the alternator works. They may be able to
show you the internal structure and components of an alternator they may be
repairing.
6
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Generating an electric current
Perhaps the simplest method of generating an electric current is to use a
length of wire connected to your multimeter and the Earth’s magnetic field
to generate an electric current. Find a friend to assist you with this activity.
To do this activity you will need:
•
a length of insulated thin copper wire around 10 m long
•
a friend to help you
•
a multimeter
•
insulating tape.
Procedure
1
Go outside away from all metal objects with around 10 m of copper
wire and a multimeter.
2
Connect the wire ends to the terminals of your multimeter. You may
need to tape the wire in place in contact with the terminals of the
multimeter probes.
3
Set the multimeter to read mA. The 200 mA setting (or the lowest
current setting available) is best.
4
Determine which way is north with a compass or from the direction
of sunset and sunrise being east–west.
5
Stand facing east–west and swing the wire up and down while
looking at the reading on the multimeter. You should see the
reading rise and fall.
The multimeter is able to detect an electric current flowing in the circuit
you have made because you are generating an electric current in the
circuit by the action of your wire cutting across the Earth’s magnetic
field. The current alternates because the wire moves up and down in the
Earth’s magnetic field.
If you stood with your wire facing north–south and swung it up and down do
you think you would get the same sized current generated in the circuit?
Explain your answer.
_________________________________________________________
_________________________________________________________
_________________________________________________________
Check your answer.
Part 2: Induction
7
In the previous activity the electric current was generated by moving the
conductor in a magnetic field. The reverse situation of moving a
magnetic field with respect to a conductor that is stationary will also
generate an electric current.
Making an electric generator
To do this activity you will need the following equipment.
•
a digital multimeter or a galvanometer
•
a 1 m length of single strand copper wire
•
a broom handle
•
insulating tape
•
a bar magnet.
Procedure
8
1
Make the wire into a coil by wrapping the wire around the broom
handle to make a tight coil but do not overlap the turns of the coil.
2
Connect the ends of your coil to the probes of the digital multimeter.
You may have to attach the wire ends with either alligator clips or
with insulating tape.
3
Set the multimeter to read 200mA or the lowest reading possible on
your meter. If using a galvanometer try to use one that measures
mA.
4
Stand the bar magnet on its end with the N–pole end facing up. This
means the magnetic flux from the magnet is fixed but the coil is free
to move.
5
Move the coil over the top of the magnet rapidly in one direction.
Note the reading that you get on your meter and whether the reading
is negative or positive. (The negative or positive reading on the
meter just gives you the sense of direction of the current generated in
the coil.) Move the coil more slowly over the magnet and note what
happens to the reading on the meter.
6
Move the coil in the opposite direction rapidly. Note the reading
that you get on your meter and whether the reading is negative or
positive.
7
Turn your magnet around so that the S–pole end is facing up and
repeat steps 5 and 6. Record your observations.
8
Keep the coil still and move the N–pole of the bar magnet into the
coil in a single direction. Look at the meter and note what happens
as you do this.
Motors and generators
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in
9
Move the magnet out of the coil in the opposite direction. Note what
happens on the meter when you do this.
out
10 Move the magnet back and forth over your coil with different
speeds. Note whether moving the magnet rapidly or slowly causes
the higher reading on the meter.
11 Move the magnet rapidly along your coil at a distance of 1 cm
(approximately) from the coil and record the size of the current
generated in your coils circuit. Then move the magnet further away
from the coil to a distance of around 5 cm and repeat the activity
noting the size of the current generated in the coil. Note any
difference in the size and polarity of the current generated.
Observations
An electric current is generated in a coil when:
•
The direction of the current is (dependent/independent) on the direction
of the movement of the coil with respect to a fixed magnetic flux.
•
The direction of the electric current is (dependent/independent) of the
polarity of the magnetic field.
•
The faster the lines of magnetic flux cut the coil the (greater/smaller)
the size of the current generated in the coil.
•
The size of the current generated in a coil is (dependent/independent) on
the strength of the magnetic field (closeness of the magnet) cutting
across it.
Check your answers.
Part 2: Induction
9
Magnetic field strength
You know that some magnets are stronger than others. That is they have
a stronger magnetic field. Identifying how strong a magnet really is
involves a little imagination. The idea is to imagine the magnetic field to
consist of a large number of lines that connect the north and south poles
of the magnet. These imaginary lines can be thought of as travelling out
of the north pole and into the south pole of the magnet. These lines are
called lines of magnetic flux.
The stronger the magnetic field surrounding an object the more magnetic
flux ( F ) lines per unit area (A) or the greater the flux density (B).
That is the flux = flux density ¥ area, or F = B ¥ A .
Consider the situation below where a conductor is moving in the
magnetic field between two bar magnets. In all the cases shown the
conductor is moving at the same speed. The magnetic field is
represented as lines of force leaving the north pole and entering the south
pole. If that conductor is a straight wire of length (l) then the conductor
will have a potential difference generated in it as it moves with some
velocity (v) through the magnetic field (B). The potential difference is
called the emf (e) and is measured in volts. The emf developed in such a
case is e = Blv .
A casual observation should reveal to you that the conductor will pass
through the greatest number of lines of flux in a set time if its motion is
at right angles to the flux lines. This is shown in the figure below.
flux lines
distance conductor
moves in time t
conducting wire
maximum number of flux lines
cut in a a fixed time
In the diagram following the conductor is moving so that in the same
time (remember all conductors are moving at the same velocity) it will
only pass through about 70% of the flux of the conductor above.
10
Motors and generators
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distance conductor
moves in time t
less flux lines cut in the same time
conducting wire
In the case of the conductor below the wire is travelling parallel to the
field. It therefore passes through no flux lines.
distance conductor
moves in time t
conductor moving parallel to the lines of flux cut
no flux lines therefore no current is generated
According to Faradays Law of electromagnetic induction, the emf (e)
induced in a conductor is the amount of flux cut (∆F) ÷ the time taken
(∆t). In other words e =
DF
. Where the circuit involves a coil, hence
Dt
resulting in multiple loops of the circuit cutting the flux, the emf
generated is in proportion to the number of coil turns (N) that cut the
flux. That is the emf generated is e = N
DF
. It is therefore easy to see
Dt
why the coil in real generators must have many turns.
Consider the three situations described in the diagrams above.
a)
Which of the three situations above would produce the maximum
emf in the conductor? Explain your reasoning.
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_____________________________________________________
_____________________________________________________
Part 2: Induction
11
b) Which of the three situations above would produce zero emf?
Explain your reasoning.
______________________________________________________
______________________________________________________
______________________________________________________
c)
How would the magnitude of the emf generated in the situation
shown in the middle situation compare to the emf generated in the
situation shown in the figure on the bottom of the page?
______________________________________________________
______________________________________________________
______________________________________________________
Check your answers.
Do Exercise 2.3 now.
Lenz’s law
Lenz’s law was first proposed by Heinrich Lenz (1804–1864).
This law says:
‘If an induced current flows, its direction is always such that it will
oppose the change in flux that produced it.’
Consider the example below where a current is produced by inserting a
magnet into a coil.
The coil is attached in a circuit containing a galvanometer that can detect
small current flows and their direction.
If the north pole of a bar magnet is inserted into the coil as shown in the
figure following, the current induced in the coil will be such that it
produces a north pole opposing the insertion of the bar magnet.
12
Motors and generators
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in
The magnetic field associated with the current generated in the coil by the
movement of the bar magnet is in the opposite direction to the magnetic field
that generated the current. Pushing the bar magnet against that field does work
and provided the energy that drives the induced current.
The north pole produced by the current induced to flow in the coil
opposing the entry of the bar magnet into the coil can be confirmed using
the right hand solenoid rule you learned about in Part 5 of the
preliminary module Electrical energy in the home. That rule says:
‘If a solenoid (coil) is grasped in the right hand, with the fingers
pointing in the direction of the current, the thumb will point in the
direction of the magnetic field produced by the current flow in the
solenoid.’
The right hand rule.
In this case the current is flowing from right to left across the page from
the coil so the magnetic field produced by that current is as shown on
the figure.
If the magnet is pulled out of the coil as shown in the diagram following,
the current induced in the coil will be such that it produces a south pole
on the left hand side of the coil. That south pole attracts the north pole of
the bar magnet that is being withdrawn from the coil and pulls on the
north pole of the bar magnet opposing its motion out of the coil.
Part 2: Induction
13
out
The magnetic field due to the current generated in the coil is in the opposite
direction to the magnetic field from the bar magnet. This attraction means work
must be done to pull the magnet out of the coil.
Lenz’s law follows from the Law of conservation of energy. That law
says energy cannot be created nor destroyed but can simply change form.
Energy must therefore be transferred to the coil to produce the induced
current flow (electrical energy). It is therefore necessary that work must
be done against the north pole being inserted into the coil and against the
north pole being withdrawn from the coil to generate the emf in the coil.
The movement of a magnetic flux with respect to a coil generates the
electric current you use everyday in appliances and to produce light.
That generated electrical energy can obviously do work and be converted
into other forms of energy.
The induced emf and the size of the current produced in a generator is
increased when:
•
the magnet is moved faster
•
there are more turns on the coil
•
a stronger magnet is used.
Consider each of these three factors listed immediately above and by
referring to the equation e = N
DF
explain why each these factors affect the
Dt
size of the induced emf in a coil.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Check your answer.
Do Exercise 2.4 now.
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Eddy currents
Observing the production of eddy currents
To do this activity you will need the following equipment:
•
an aluminium soft drink can
•
a pair of scissors
•
a pair of bar magnets or a single horseshoe magnet
•
three paper clips
•
some Blutak® or some sticky tape.
Procedure
Take care when doing this activity not to cut yourself with any sharp edges
you produce. Throw all scrap metal into the garbage or recycling to avoid
risk to yourself or others.
1
Use the scissors to cut a triangle of aluminium metal with
approximate dimensions 10 cm high by 8 cm wide from the side of
the aluminium can. Flatten out the triangle.
2
Unfold one of the paper clips so that the paper clip is straight.
Then place a bend in the middle. Bend both the ends down slightly.
3
Punch a hole in the sharpest angle of your aluminium triangle that is
big enough to pass your unfolded paper clip through and assemble
the gear as shown in the figure below with the triangle free to swing
like a pendulum.
4
Swing the triangle and time how long it takes to come to rest.
Part 2: Induction
15
Blutak®
aluminium can
triangle that is able
to swing freely
The aluminium triangle and paperclip set up. The Blutak® insulates the
system. (Photo: Thomas Brown.)
5
Start the triangle swinging again with a similar swing to you used in
Step 4 but this time place two bar magnets with opposite poles
adjacent the swinging aluminium triangle. Time how long it takes
for the triangle to come to rest.
6
Repeat Step 5 a number of times.
Triangle swinging freely within a magnetic field.
(Photo: Thomas Brown. Hands: Ric Morante)
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Observations
Did you notice that the triangle stops swinging a lot faster when the
magnets are adjacent the metal triangle?
_________________________________________________________
The aluminium triangle stops faster when in the magnetic field (when the
opposite poles of the magnet are adjacent it.) This is because the
movement of the aluminium triangle (conductor) in the magnetic field
produces an electric current in the aluminium triangle. The direction of
that current is such that the magnetic field it produces is in the opposite
direction to the magnetic field from the permanent magnets that produced
the current. Because the magnetic fields are opposite they attract.
That attraction causes the aluminium triangle to stop swinging a
lot faster.
Eddy current and motors
When an electric motor is turning the coil of the armature is turning with
respect to a magnetic field in the stator. The relative movement means
that an emf is generated in the coil of the armature because it is cutting a
flux. By Lenz’s law the direction of that induced emf is opposing the
emf causing the motion of the armature. The current produced by the
generated emf in the motor is called the eddy current. The direction of
the motor generated emf must therefore be such that it opposes the
supply emf that produces the motion in the motor. This emf is called the
back emf.
Any electrical conductor when present in a varying magnetic field, will
have a current induced in it. The electric motor demonstrates a wanted
effect of this in the form of the rotational energy that is produced in the
rotor, and unwanted effects that manifest themselves as eddy currents or
a back emf that reduces the efficiency of the motor.
When a DC electric motor is first started the coil is stationary so the
current flowing through the armature coil is high. Once the armature
begins to spin, eddy currents operating in the opposite direction to the
supply current are initiated. The faster the motor spins the larger these
currents become. Since these eddy currents are in the opposite direction
to the supply current they effectively reduce the effective supply
potential difference. The supply current flow through the motor is
therefore reduced once the motor is in motion. To protect a motor at start
up from the risk of burnout due to it drawing too much current a resistor
is placed into the circuit. This reduces current flowing through the coils.
The resistor is often designed as part of a centrifugal device so that as the
motor achieves a rate of revolution the resistor is removed from the
Part 2: Induction
17
circuit. That is because it is not longer required because the back emf at
higher revolutions of the motor is sufficient to protect the motor coils
from burnout.
Improving motor efficiency
In an electric motor with a solid iron armature, the eddy current produced
in the armature damps the efficiency of the motor. This is because the
opposing eddy current causes the resistance in core of the armature that is
usually made from soft iron. The armature heats up wasting energy that
might otherwise go into producing a rotational force.
One strategy to minimise this problem is to make the motor armature
from a laminated stack of thin soft iron sheets. Each sheet is insulated
from the adjacent sheets by a thin oxide film on the surface of each
lamination. By having the laminations the build up of eddy currents is
severely disrupted and cannot occur along the full length of the armature.
This means the back emf induced in the motor is lowered to a much less
significant effect.
rotation direction
induced back emf
solid armature
laminated armature
(prevents build-up of
back emf)
A solid and a laminated armature. Note the windings of copper wire around the
armatures are not shown. The laminated armature is far more efficient because
of the disrupted eddy currents not forming a sizeable back emf. Each plate in
the laminated armature is insulated from the plates adjacent.
Do Exercise 2.5 now.
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Stopping the eddy currents
Use the same set up as for the activity Observing the production of eddy
currents but this time cut the aluminium triangle so that the base of the
triangle is now shaped similarly to the photograph below. Repeat the
activity using the serrated triangle shape.
Aluminium triangle with serrations cut into it. (Photo: Thomas Brown.)
Observations
Does the magnetic field cause the serrated aluminium triangle to stop
swinging more quickly?
_________________________________________________________
You should have found the serrated triangle shape doesn’t stop swinging
noticeably more quickly when the triangle is swinging through the
magnetic field.
Why didn’t the serrated triangle slow down more rapidly in the magnetic
field? (Hint: Think about the laminated armature in the motor.)
_________________________________________________________
_________________________________________________________
Check your answer.
Part 2: Induction
19
Electromagnetic braking
Eddy currents are often used for electromagnetic braking. The idea is
simple and follows on from the activities you have done above.
Passing a conducting plate or wheel through a magnetic field will
produce eddy currents in the conductor.
These eddy currents induced in the conductor produce a magnetic field
that is of the opposite polarity to that of the magnetic field causing the
eddy current. The attraction between the induced magnetism from the
eddy currents and the magnetism causing the eddy currents slows the
moving object.
Examples of using eddy currents for electromagnetic braking include:
•
Eddy currents are used for magnetic braking in some freefall
amusement park rides. A copper plate is attached to the ride capsule.
As the capsule falls it passes between strong magnets near the
bottom of the ride. The magnetic field produced by the eddy
currents induced in the copper plate produce a magnetic field that is
of opposite polarity and therefore attracted to the original magnetic
field. This is able to slow the ride down.
•
Eddy currents can be used as a braking force in rapid transit train
cars. Electromagnets on the train near the metal conductive rails are
turned on. This creates eddy currents in the rails that produce an
induced magnetic field that is of opposite polarity to the magnetic
field from the train. These opposing polarity magnetic fields slow
the train.
Because the magnitude of the induced eddy currents is a function of
the speed of the train the strength of the induced current in the rails
and its accompanying magnetic flux is reduced as the train slows.
That is as the train slows, the braking force is proportionally
reduced. This produces a smooth stop.
Maintenance costs in such a system are reduced because there are no
brake pads or discs to wear out.
Modelling electromagnetic braking
To do this activity you will need :
• three paper clips
• some Blutak®
• an empty aluminium soft drink can
• two bar magnets
• a pair of scissors
• the plastic inner ink reservoir tube from a ball point pen.
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Procedure
1
Cut the bottom from the soft drink can and stamp on it to make a
flat disc.
2
Punch a hole in the centre of the disc big enough to fit the plastic pen
tube through as shown in the photograph following.
3
Set up your equipment as shown in the photograph.
4
Spin your pen and disc and time how long it takes to come to a stop.
5
Place two bar magnets with opposite poles facing on either side of
the disc and spin it again as shown in the figure below. Time how
long it takes the disc to come to a stop while it is spinning in the
magnetic field. While spinning in the magnetic field an eddy current
is being generated in the disc.
The experimental set up. (Photo: Thomas Brown)
Observations
1
Did you notice the disc stopped spinning more rapidly when the disc
was in the magnetic field compared to when it was not in the magnetic
field?
_____________________________________________________
2
Explain your observations in terms of what you know of eddy currents
generated when a conductor moves relative to a magnetic field.
_____________________________________________________
_____________________________________________________
_____________________________________________________
Check your answers.
Do Exercise 2.6 now.
Part 2: Induction
21
Using induction for heating
One of the most novel uses for induction is the induction cook top.
Each cooking area on the cook top has one or more coils made of
ferromagnetic material under it. When an alternating current is passed
through these coils, a magnetic field fluctuating with the same frequency
is produced. A ferro–magnetic–based pan placed over the coils has an
electric current induced in it that is rapidly oscillating. The internal
resistance of the pan to this induced AC current results in heat being
produced in the pan. That heat is dissipated to the food in the pan and
does the cooking. Essentially the element of this type of cook top is
the pan.
The induction cook top only works when used with magnetic–based
pans, made from materials such as iron and steel that will allow an
induced current to flow within them.
A simple way to find out if a pan is induction cook top compatible is to
use the ‘magnet test.’ If a magnet will stick to the surface of the pan, that
pan is suitable for use with an induction cook top.
iron based
(ferromagnetic
pot/pan)
ceramic top
(cold)
oscillating
magnetic field
resistance to the
induced AC current
causes heating
oscillating electric
current is produced
in the pot
induction coil carrying
an AC current
to AC power supply
The principle of induction cooking. Note that as the AC current flows in the
induction coil it waxes and wanes between a maximum value and minimum
value of zero as well as in direction. The size of the current determines the
strength of the magnetic field so the magnetic field strength waxes and wanes
also.
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Induction cook tops are ceramic. Because the only heat produced is
generated due to resistance of the induced current in the metal in the pan
or cooking utensil, the surface of the cook top is cool when nothing is
placed on it. This eliminates the possibility of accidental burns and the
risk of accidental fire.
Because the cooking utensil itself is the device that does the heating, less
heat is dissipated to the surroundings than would be the case with other
methods of cooking. This typically makes induction heating the most
efficient way to heat and cook food. Claimed efficiencies of the energy
conversion from electrical energy to heat energy useful for cooking are
of the order of 90%.
To see pages that describe induction cook tops see pages on the Physics
website page at: http://www.lmpc.edu.au/science
Do Exercise 2.7 now.
Part 2: Induction
23
Summary
Complete the following statements to prepare a summary of this part.
•
Michael Faraday discovered that an electric current can be generated
by:
______________________________________________________
______________________________________________________
•
An experiment, example or activity that demonstrates each of
Faraday’s Laws of electromagnetism is:
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
•
A definition for electromagnetic flux is:
______________________________________________________
______________________________________________________
______________________________________________________
•
Lenz’s law says:
______________________________________________________
______________________________________________________
______________________________________________________
•
Back emf makes a motor less efficient in its operation because:
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
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•
Eddy currents can be used to produce electromagnetic braking by:
_____________________________________________________
_____________________________________________________
_____________________________________________________
•
A motor may require a resistor in the circuit to its coils to prevent
burnout at start up but not once the motor is running because:
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
•
Induction cook tops are cool to the touch but are able to cause a
heating effect in cookware because:
_____________________________________________________
_____________________________________________________
_____________________________________________________
Part 2: Induction
25
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Suggested answers
Generating electricity
No the current would be much less because you would be cutting less
flux as the Earth’s magnetic field runs north–south.
Making a generator
•
The direction of the current is dependent on the direction of the
movement of the coil with respect to a fixed magnetic flux.
•
The direction of the current is dependent on the direction of the
polarity of the magnetic field.
•
The faster the lines of magnetic flux cut the coil the greater the size
of the current generated in the coil.
•
The size of the current generated in a coil is dependent on the
strength of the magnetic field cutting across it.
Magnetic field strength
a)
The highest value of emf should occur where the conductor passed
through the flux lines at right angles. This conductor passes through
the maximum number of flux lines in the minimum time. This
situation is the one shown on the left side diagram.
b) The right hand side diagram conductor would have no emf generated
in it because there is no flux cut by this conductor.
c)
In this case the emf must be less in the middle diagram conductor
than in the left side diagram conductor. This is because in the same
time only 70% of the flux lines are cut by the middle diagram
conductor. From this it is apparent that since e =
DF
that the emf
Dt
in the middle conductor will be 70 % as large as that in the left side
diagram conductor.
Part 2: Induction
27
Lenz’s law
If the magnetic is moved faster more flux is cut by the coil in the same
amount of time.
More turns on the coil means that a greater area amount of flux is
involved in the generation of the emf as F = BA .
Similarly a larger B means a bigger flux because F = BA .
Stopping the eddy currents
The serrations break up the current therefore instead of the eddy current
establishing itself throughout the entire triangle it is only established in a
small portion. A smaller current means a smaller magnetic field is
established by that current.
Modelling electromagnetic braking
The disc does stop more rapidly when it is in the magnetic field.
The eddy currents are produced as the magnetic field is cut by the
spinning disc. The eddy currents are such that they produce a magnetic
field of opposite polarity to that of the magnets. That means they attract
the disc effectively producing a slowing force.
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Exercises – Part 2
Exercises 2.1 to 2.7
Name: _________________________________
Exercise 2.1
What was Michael Faraday’s experiment that demonstrated that an
electric current could be generated by a moving magnet?
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Exercise 2.2
Use a diagram to aid you to explain what magnetic flux is.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Part 2: Induction
29
Exercise 2.3
Explain how you could increase the current output of a simple
experiment with a fixed size coil and a fixed size magnet when the coil is
in a set circuit.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Exercise 2.4
Explain how Lenz’s law accounts for the production of back emf in an
electric motor.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Exercise 2.5
How can Lenz’s law be applied to the production of an eddy current?
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Exercise 2.6
How does electromagnetic braking work?
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
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Exercise 2.7
Explain why copper bottom cookware would not work on an induction
cook top yet is among the most efficient means of cooking on a normal
gas or electric range?
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Don’t forget to submit a labelled diagram of the electric motor you built in
the activity from Part 1 with this set of exercises.
Part 2: Induction
31
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Physics
HSC Course
Stage 6
Motors and generators
Part 3: Powering up
2
0
0
In
r2
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b S
o
t
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ng DM E
i
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ra E N
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rp A M
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Contents
Introduction ............................................................................... 2
Generators ................................................................................ 4
The DC generator...................................................................... 5
DC CRO traces ..................................................................................10
Comparing a motor to a generator ....................................................11
The AC generator.................................................................... 17
Producing an AC current....................................................................19
Why is the electricity supply AC? ......................................................20
Generating electricity commercially ..................................................23
Summary................................................................................. 31
Suggested answers................................................................. 33
Exercises–Part 3 ..................................................................... 35
Part 3: Powering up
1
Introduction
Electricity runs modern life. The use of electricity in all communication
devices such as radios, television, phones and computers is an indication
of the ubiquitous nature of electricity. There is rarely an aspect of daily
life that excludes the use of electricity. The household labour saving
devices such as the washing machine and the vacuum cleaner require
electricity. A day without electricity because of power generation
breakdown or undercapacity is the stuff of headlines. Governments are
held to account when the power fails.
The extensive use of electricity means that the generation of electricity is
a big business operation. Locations such as California and the Silicon
Valley in particular have massive requirements for uninterrupted
electricity supply. The humble AC generator is the device used for the
supply of most if not all of this requirement.
For all the talk of photovoltaic (solar) cells, the reality is commercial
quantities of electricity come from the generator. The energy to turn the
generator comes from a variety of sources but it is still the generator that
makes the electrical supply you know and depend on possible.
During the course of your learning in this part you will have
opportunities to learn to:
2
•
describe the main components of a generator
•
compare the structure and function of a generator to an electric
motor
•
describe the differences AC and DC generators
•
discuss the energy losses that occur as energy is fed through
transmission lines from the generator to the consumer
•
assess the effects of the development of AC and DC generators on
society and the environment.
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At the end of Part 3, you will have had an opportunity to:
•
plan, chose equipment or resources for, and perform a first–hand
investigation to demonstrate the production of an alternating current
•
gather secondary information to discuss advantages / disadvantages
of AC and DC generators and relate these to their use.
Extract from Physics Stage 6 Syllabus © Board of Studies NSW, amended
November 2002. The most up–to–date version can be found on the Board's
website at
http://www.boardofstudies.nsw.edu.au/syllabus_hsc/syllabus2000_listp.html#p
Part 3: Powering up
3
Generators
The generator is based on the principle of electromagnetic induction that
was discovered by Michael Faraday in 1831. Faraday discovered that if
an electric conductor is moved through a magnetic field an electric
current will be induced in the conductor. The mechanical energy of the
moving conductor is converted into the electric energy of the current that
flows in the conductor.
In Australia almost all electricity supply to homes and industry comes
from generators. The generators used in industry are almost all designed
to produce AC current to meet the demands of home and industry.
Special applications such as large motors require the generation of
DC current.
The configuration of the current collection devices, either slip rings and
brushes for the AC generator or split ring commutator and brushes for the
DC generator will determine the type of current generated.
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The DC generator
The figure following and on the next page shows the arrangement for a
simple single coil DC generator. The coil turning in the fixed magnetic
field has a current induced in it. The direction of the induced electric
current is a constant and is determined by the direction of rotation of
the coil.
The graph in the diagram shows the changing magnitude of the induced
current in the coil as it rotates through the magnetic field.
loop possessing
potential energy
B
A in sequence
0
galvanometer
clockwise rotation
of loop
B
B in sequence
0
B
C in sequence
0
Part 3: Powering up
5
B
D in sequence
0
B
E in sequence
Induced
current
0
B
0
A
D
C
E
Time
Notice how the induced current is at a maximum when the moving coil is
parallel to the magnetic field and at a minimum when the moving coil is
at 90∞ to the magnetic field.
Most schools and science education institutions have a hand generator
similar to the one shown in the photograph below. Often these hand
generators can be set to generate either AC or DC current.
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bar magnets
S S
N N
small lightglobe
pulley connected to the coil
S S
insulated copper wire coil
A hand generator. (Photos: Thomas Brown. Hands: Ric Morante)
bar magnets
split ring
commutator
slip rings
coil
The hand generator showing the slip rings enabling generation of AC current
and the split ring commutator enabling generation of DC current. Note the split
in the commutator is just visible on the edge behind the contact. This split is
opposite another split on the commutator ring that divides the commutator ring
in two segments. (Photo: Ric Morante.)
The figure on the next page shows a CRO trace that was produced from a
simple hand cranked generator switched to produce DC current.
Part 3: Powering up
7
0.2 V/Div
5 ms/ Div
A CRO trace from a simple DC generator. Note how the trace is dominated by
an upward signal in one direction only and is somewhat like an AC generator
trace with the bottom half of the waveform chopped off (see the next page for
this). The noise in the signal is due to the irregular current generation in the
simple coil.
0.2 V/Div
5 ms/ Div
A DC trace produced with a simple hand generator with the coil rotated slightly
faster in the opposite direction.
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The CRO trace below was made when the hand generator was switched
to produce AC current and cranked slowly.
An AC current trace on the CRO produced with a hand generator. The wave
form is symmetrical about its baseline in this case and forms a square wave.
A CRO trace of an AC current generated by rotating the coil a little more slowly.
Do return exercise 3.1 now.
Part 3: Powering up
9
DC CRO traces
The image of the CRO output current of the DC generator has some
significant differences from the DC current output of batteries.
The DC output from a battery is a constant current (at least while the
chemical reaction producing the current in the galvanic cell or battery
can be sustained).
The CRO output signal from a battery is shown in the figure following.
voltage
baseline
13:35:04
0.25 V/Div
0.106 ms/Div
DC current trace from a battery or galvanic cell. The trace from the galvanic
cell is the upper horizontal line. The lower horizontal line is the zero base
against which to measure the voltage produced. Note the voltage output does
not fluctuate at all and is a constant 0.26 V. Note if the leads are reversed in
their connection to the battery terminals the same sized voltage is produced but
this time it is below the base line indicating that the current is flowing in the
opposite direction.
Look at the CRO output from the galvanic cell and those from the DC
generator shown previously in this part. You will see significant
differences.
Propose a reason why the DC output from the generator is not a constant
current as with the DC current from the battery but rather fluctuates between
a maximum positive value and zero.
(Hint: Consider the equation f = BA cosq .)
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_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Check your answer.
Comparing a motor to a generator
Look at the figures below that show a DC generator and a DC motor.
armature
B
~
N
S
brush
(fixed)
axis of
rotation
brush
split ring
commutator
A DC motor.
B
A DC generator.
Part 3: Powering up
11
1
Identify the main (labeled) components of the motor and the
components generator by listing them in the table below. Describe the
function of each of the components along side their name.
Motor part
2
Function
Generator part
Function
Does the generator have any bits that the motor does not or vice
versa?
______________________________________________________
Check your answers.
It probably doesn’t surprise you to learn that the DC motor and generator
are in fact the same thing. The motor can act as a generator and a
generator can be adapted fairly simply to act as a motor.
Do return exercise 3.2 now.
Making a simple DC generator from a motor
To do this activity you will need:
•
a simple DC electric motor from a toy
•
two leads and alligator clips
•
a multimeter set to read current or a galvanometer and
millivoltmeter.
The emf (e) produced by a conductor in a magnetic field is determined
by the strength of the magnetic field (B), the speed of the conductor
moving in the magnetic field (v) and the length of conductor in the
magnetic field (L). This relationship is described in the equation
e = BLv . Note that the emf is not really a force but is rather a voltage.
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Procedure
1
Connect the leads of the meter to the input terminals of the electric
motor.
2
Turn the armature of the DC electric motor at a reasonably slow pace
in one direction only and observe the meter. Record the maximum
reading.
3
Reverse the direction in which you turned the armature of the motor
and observe the reading on the meter. Record the maximum reading.
4
Now turn the armature shaft a little faster and observe the maximum
reading you can obtain on the meter.
5
Switch multimeter to read millivolts or connect the motor to a
millivoltmeter and repeat steps 2 to 4.
Small motor connected to the multimeter. The experiment is done by rotating
the shaft on the small motor. (Photo: Thomas Brown.)
Part 3: Powering up
13
Observations
1
Was the motor acting as a generator? How did you know?
_____________________________________________________
_____________________________________________________
2
What difference did the speed with which the armature was turned make
to the size of the current generated?
______________________________________________________
3
What difference does turning the armature faster make to the emf
(or voltage) produced by the generator?
______________________________________________________
4
Explain why the current is increased as the rate of turning the motor
armature changes.
______________________________________________________
______________________________________________________
5
Explain why the potential difference or emf of the motor is greater
when the input of kinetic energy to the motor is greater. (Hint:
Think about the Law of conservation of energy.)
______________________________________________________
______________________________________________________
______________________________________________________
6
Describe the operation of a DC generator as it produces electricity of
a varied sized voltage and current.
______________________________________________________
______________________________________________________
Check your answers.
Using a motor as a generator
To do this activity you will need:
14
•
access to a CRO or a CRO simulation (digital oscilloscope)
program
•
an old set of bud earphones appropriately prepared to enable input
of any signal to the CRO
•
a small electric motor from a toy.
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Procedure
wiring from cut off bud earphone acts as input
device into the CRO or digital oscilloscope program
input jack
remaining bud earphone can
act as a microphone for digital
oscilloscope program input
The connections required to the small motor. The input jack can be connected
to the microphone input of the computer if using a digital oscilloscope computer
program. Note that the two wires required to complete the input circuit come
from only one of the bud earphone leads. (Photo: Thomas Brown.)
1
Connect the separate leads from one bud earphone to the electric
motor and the CRO input.
2
Turn the motor over and look for an output signal on the screen of
the CRO.
Observations
You should see an output that is similar to the one shown below.
This output is not a particularly clean signal. This is because the signal is
produced by three separate coils wrapped around the poles of the motor.
This means that in the process of generating the electricity contributions
are made from three separate tiny generators in the motor.
The result is that the signal although representing a DC current tends to
have a flat bottom and peaks of generation. This is probably because
although the motor can act as a generator its primary purpose and design
is to act as a motor.
Part 3: Powering up
15
9:29:26
0.1 V/Div
100 ms/Div
The CRO output signal from a small toy motor used as a generator. The
straight line represents the baseline signal of the CRO.
In the motor electrical energy is converted to mechanical energy.
When the same motor is used as a generator the mechanical energy
inputted to turn the shaft of the motor is converted into electrical energy.
Train motors are used as generators
The fact that electric motors can be used as generators was not lost on the
designers of the interurban rail network in NSW. Trains on the
interurban electric network in NSW are powered by DC electric motors.
The supply from the overhead electric cables is 1500 V DC.
The system persists with the use of DC motors even though significant
cost savings in terms of energy use could occur if the system switched to
AC motors. The reason for this is the cost of infrastructure replacement
prohibits the changeover.
One advantage that the DC system does have though is the ability of a
DC motor to act like a generator. Trains climbing hills (gaining potential
energy) draw power from the grid while trains coming down the same
hills (losing potential energy) can use their motors as generators.
The electric power generated is fed back into the grid to provide power to
the other trains climbing the hills. This energy saving partly offsets the
relative inefficiency of the DC motors compared to their AC equivalents.
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Motors and generators
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The AC generator
Most commercial generators are AC generators. These generators use
slip rings as a mechanism of collecting the electric current rather than the
split ring commutator and brushes arrangement used in the DC generator.
The operation of the AC generator is otherwise essentially the same as
that for the DC generator. AC generators have a number of advantages
over DC generators. They are:
•
cheaper and simpler in construction with less parts to wear
•
more reliable than equivalently power rated DC generators partly
because the DC current is generated in the rotor. That current is then
drawn from the windings through a commutator and a brush pair.
The many segments of the commutator (required to give DC output)
are more likely to suffer wear and short out. By contrast, the
alternator generates an AC current in the stator (the non–rotating
windings) and is rectified using diodes if DC is required. It is much
easier to draw the current through a fixed connection in the stator
rather than through a commutator.
•
the rotor is used to create the 'field magnetisation' that causes the
generation of AC current in the stator when the rotor is spun. To do
this the rotor only needs to have two continuous conductive bands or
'slip rings' to conduct the electric field current from the brushes to
the rotor. The slip rings are continuous. They do not wear the
brushes as fast as the commutators in a generator.
•
there is no possibility of creating a short between segments in an
alternator because the slip rings are already continuous. There is
with a commutator in a generator because for the commutator to
work it must be divided into segments.
•
alternators are more compact than generators of equivalent output
because their design is more efficient and there is less likelihood of
overheating occurring.
In addition to all the above factors, most electrical equipment is designed
to use AC because AC motors are cheaper, simpler in design and more
reliable than their DC counterparts and hence require less maintenance.
Part 3: Powering up
17
The figure below shows the mechanism of generation of an AC current
from a single coil generator. The figure below shows the position of the
coil at each point A–E in a single cycle of AC. The turning of the coil in
a magnetic field produces a current in both sides of the coil which add.
clockwise rotation
of loop
A in sequence
B
B
0
B
t
= max
=0
galvanometer
B in sequence
B
B
0
B
t
=0
= max
C in sequence
B
B
0
B
t
= max
=0
D in sequence
B
B
0
B
t
=0
= max
E in sequence
B
B
0
B
Induced
current
t
= max
=0
B
A
0
E
C
D
Time
An AC generator in operation. The full cycle of rotation of the coil is required to
produce one cycle of AC current. Since the component of the velocity
perpendicular to the magnetic filed changes sinusoidally with the rotation, the
generated voltage is AC.
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Take a highlighter or coloured pencil and draw over one side of the square
coil that is perpendicular to the magnetic field. Trace that side in its passage
through a full AC cycle. You should see that the full AC cycle (sinusoidal
curve) requires the coloured side of the coil to turn through 360∞.
Do Exercise 3.3 now.
Producing an AC current
You are required to plan and perform an experiment to demonstrate the
production of an alternating current. An alternating current has the
following characteristics.
•
The current is cyclic fluctuating between zero and a maximum value
with a regular change in direction.
•
The current flowing through a galvanometer will cause the needle
fluctuate back and forth as the current direction changes.
•
The CRO trace of an AC wave is a sine wave shape like the one
shown in the figure below.
An AC output trace on the CRO. This output trace was produced using a
transformer to dramatically step down the voltage.
Part 3: Powering up
19
Do not under any circumstances attempt to record an AC signal from mains
power with a CRO or CRO simulator program. Electricity can kill.
An alternating current can be produced by a magnet rotating in a coil or the
regular insertion and retraction of a bar magnet into a coil.
The principle of how this works is shown in the figure of a simple
experiment to generate an AC current shown below.
Simple alternator – the turning
magnet generates a AC current in
the alternator’s stationary winding
Alternating current – as the armature,
or rotor, rotates the current is
continually reversed
simple coil
galvanometer
bar magnet on a pencil
stationary windings in
which current is generated
You need to plan and perform an experiment that demonstrates the
production of an alternating current. You will need to submit your
experimental procedure, equipment list and observations to confirm the
production of an AC current with your return exercises for Part 6 of this
module.
Why is the electricity supply AC?
It is not possible to store electrical energy in the quantities demanded by
society. Transmission of electricity is therefore essential since the
resources to generate electricity (hydro power, coal, gas, wind and solar
energy) are fixed. They can’t move easily or economically, so the
electrical energy moves to the consumer. In addition even if a power
plant could be built in a particular place there needs to be sufficient
customers demanding that electricity to make the running of the power
plant worthwhile. Because this is rarely the case immediately adjacent to
a power generation plant the transmission of excess power is the only
viable option.
In 1991, about 7% of electricity generated in the US was lost between
generation facilities and end use.
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Reliable estimates in Australia suggest that with our sparse population
the losses in Australia may be even higher. If these transmission and
distribution system losses are reduced, less electricity needs to be
generated to meet consumer demand.
This could reduce greenhouse gas emissions since the majority of
electricity is generated in Australia by thermal power stations that burn
coal for their energy source.
The preferred type of electricity to be transmitted over significant
distances is AC. This is in large part because of the capacity of AC
electricity to be transmitted at high voltages and low currents. It can then
be easily converted into a more usable form by a transformer.
DC current cannot be easily transformed.
Power losses are very significant over longer distances. The relationship
that describes power consumption is P = I 2 R (or P = VI). The ability of
a network to be able to transmit power at high voltages and relatively low
currents is therefore critical (because the I is squared). This is
increasingly important when the generators producing the power are long
distances from the consumers of the power.
The best way to understand the reason for the significant advantages of
high voltage AC transmission is to look at a comparison of power losses
that occur per kilometre during AC transmission at a variety of voltages.
To do this consider the example below of an imaginary transmission of
electricity from a power station generating 1000 W of power through a
transmission line with a resistance of 1 Ω km–1. The table considers
three transmission voltages, 50 V, 100 V and 1000 V. The effective
power delivery in each of these cases is calculated.
Transmission
voltage
50 V
100 V
1000 V
Generator power
1000 W
1000 W
1000 W
Current in conductor
(from I = P/V)
1000 W / 50 V
=20 A
1000 W / 100 V
= 10 A
1000 W / 1000 V
= 1A
Transmission power
loss km–1
(from P = I2R)
(20 A)2 ¥ 1 W
= 400 W
(10 A)2 ¥ 1 W
= 100 W
(1 A)2 ¥ 1 W
=1 W
Power available after
losses
(1000–400) W
=600 W
(1000–100) W
= 900 W
(1000–1) W
= 999 W
Part 3: Powering up
21
Which of the transmission modes is the least effective in the delivery of the
electrical energy generated to the consumer? Explain your answer.
_________________________________________________________
_________________________________________________________
Check your answer.
Do Exercise 3.4 now.
All conductors operating at the normal temperatures transmission lines
operate at are resistive to some degree to the passage of an electric current.
The standard copper wire used in electrical transmission is resistive to a
level of around 8 Ωkm–1.
As you can imagine the transmission of electrical energy over many
hundreds of kilometres is therefore a huge drain on the electrical
transmission system. Minimising that drain is vital. The easiest way to
do that when using conventional transmission equipment is therefore to
transmit the power at low current / high voltage values. The only current
that can be transformed efficiently to that state is AC. As an additional
way to save energy and reduce power losses it is possible to build the
power stations near to where demand occurs.
The Sithe Energies Australia Pty Ltd® Smithfield Energy Facility® gas thermal
power station and steam turbine generation facility at Smithfield in Western
Sydney. The three high stacks are from the gas turbine generators. The four
large tank like structures on the left of the photo are the condenser towers.
(Photo: Ric Morante.)
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Motors and generators
This approach has been taken at Smithfield NSW where a power station
has been built to supply nearby houses and industry with electrical
energy. This power station burns natural gas so has clean emissions.
It is located within 1km of domestic dwellings. Alternatively to reduce
energy losses in transmission industry may be built near the power
station. This has occurred with electricity hungry industries such as
aluminium smelting in the Hunter Valley and in Tasmania.
Make a list of five of the possible strategies that could be employed to
reduce the energy losses that occur when electrical energy is fed through the
transmission lines.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Check your answers.
Generating electricity commercially
Electricity is generated commercially at large power stations throughout
NSW. Most of these facilities are thermal power stations that use as their
energy source the burning of coal.
These electricity generation plants supply around 90% of the 11 000 MW
or so of power required by NSW. Supplementary power is supplied by
the Snowy Mountains Hydroelectric Scheme power plants and small
alternative energy generators such as wind turbines, solar energy and the
state of the art Sithe Energies Australia® Smithfield Energy Facility.
The figure below shows an overview diagram of the entire Sithe Energies
Australia® Smithfield Energy Facility power station.
Part 3: Powering up
23
natural gas
high pressure steam
combustor
air compressor
to supply oxygen
Gas turbine generator 1
turbine
low pressure steam
intermediate pressure
electricity to utility
generator
36 MW
steam
turbine
heat recovery
steam generator
natural gas
generator
60 MW
Steam turbine generator
combustor
Gas turbine generator 2
raw water
low pressure steam
to customer
demineraliser
air compressor
to supply oxygen
turbine
generator
36 MW
heat recovery
steam generator
cooling
pump
natural gas
condenser
combustor
air compressor
to supply oxygen
Gas turbine generator 3
turbine
low pressure steam
to customer
generator
36 MW
feed water
heat recovery
steam generator
boiler feed pump
natural gas supply
Sithe Energies Australia® Smithfield Energy Facility.
gas turbine stacks
boiler units
transformers
Gas turbine stacks at the Sithe Energies® Smithfield Energy Facility. Note the
proximity of the transformers to the stacks and the 38 MW generators located
immediately to the rear of the stacks. These are 33 kV transformers that feed
electricity directly In to the supply at the Guildford Substation via three
transmission lines. Exhaust heat from the gas turbine generators is used to
make steam for use in the steam turbine generator. (Photo: Ric Morante.)
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Motors and generators
Transmission line between the Smithfield Energy Facility and the Guildford
substation. (Photo: Ric Morante)
The generator for each gas turbine is entirely enclosed in a steel case with
access doors for maintenance. (Photo: Ric Morante.)
The Smithfield energy facility is located in western Sydney within only
hundreds of metres from residential dwellings and industry.
The generating plant generates 160 MW of electricity that is fed into the
electricity grid through the Guildford Substation located around 500 m
from the plant. The substation is protected from lightning strikes by a
network of lightning rods on purpose built towers around and through
the substation.
Part 3: Powering up
25
lightning rods
Guildford substation
transformers
Guildford substation. (Photo: Ric Morante.)
The Smithfield Energy Facility sells steam produced as a result of
burning gas in its three gas turbine generators to the nearby Visy®
industries paper recycling plant as one of its products. The remainder of
the heat energy produced and steam generated is used to provide
high pressure steam to run a high pressure steam turbine that drives a
65 MW water/air cooled steam generator. The electricity from that
generator is also fed into the Guildford Substation via an underground
electrical cable.
steam turbine generator
transformer
The steam turbine generator and its accompanying transformer used to step up
the voltage of the output electricity to 33 kV before the electricity is fed into the
electricity grid. (Photo: Ric Morante.)
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Motors and generators
Back of the steam turbine generator. (Photo: Ric Morante.)
insulating jacket around the steam
turbine to reduce heat loss
Steam turbine showing the insulating jacket. (Photo: Ric Morante.)
Part 3: Powering up
27
The steam pressure in the steam turbine drops from 9.8 MPa to under
vacuum at–95 kPa to ensure the maximum energy utilisation.
After passing through the steam turbine the steam is condensed and
passed through air /water cooling towers by a circulatory system driven
by large pumps.
The water used in the boilers and steam turbine is demineralised and
re–circulated in a closed pipe system to prevent scale build up in the
boilers which could cause a safety hazard.
Water and fans are used in the cooling towers to cool the steam turbine
water down before it is re–circulated through the boiler system attached
to the gas turbine generators where it is heated to make high pressure
steam again.
The water that can be seen passing out of the cooling tower structure as
steam is water drawn from the mains water system and is used for
cooling only.
The cooling towers are made from wood to enhance efficient cooling.
Air is drawn from the opening at the base of the cooling tower structure
by huge fans and circulates up through the tower. At the same time a
rain of water runs over the hot steam pipes to cool the steam down and
continually condense the demineralised water for the boilers.
generator
demineralised water tank
Boiler water tank containing demineralised water for use in the closed circuit
steam turbine generator system. (Photo: Ric Morante.)
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Motors and generators
boiler units
Boiler units over the gas turbines heat the demineralised water with exhaust
heat from the burning of the gas in the turbines. The high pressure steam from
the boilers is then circulated to the steam turbine generator system.
(Photo: Ric Morante.)
steam emission
inlet pipes
wooden frame structure
Cooling towers showing steam inlet pipes. The base of the structure is open to
allow cooling air to be drawn up the cooling tower structure. Although there are
4 towers the use of one tower would be sufficient to cool down the steam from
the plant. As an efficiency measure two cooling towers are generally operated
at any one time. (Photo: Ric Morante.)
None of the steam seen coming from the cooling towers has been directly
involved in the production of the electricity.
This steam is produced cooling the high pressure steam that has been
involved in the turning of turbines.
Part 3: Powering up
29
The water is pumped around the system using large motors and pumps.
These motors are of the order of 450 kW.
450 kW pump motor
pump
A 450 kW motor drives the pumps at the Sithe Energy Facility.
(Photo: Ric Morante.)
Complete Exercises 3.5.to 3.8.
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Motors and generators
Summary
•
The main components of a generator are:
_____________________________________________________
_____________________________________________________
•
What is the difference between a DC motor and a DC generator?
_____________________________________________________
_____________________________________________________
•
Why are there energy losses that occur as energy is fed through
transmission lines from the generator to the consumer?
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
Part 3: Powering up
31
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Motors and generators
Suggested answers
Making a galvanic cell
1
The observational evidence for the unidirectional current produced
by a galvanic cell is that the readout on the meter will either be
positive only or negative only.
2
The readout on the meter is steady. (The current is therefore steady.)
Comparing the DC CRO traces
As the coil rotates through 360° the value of cosq fluctuates between 0
and 1. This means the current out put also fluctuates between a
maximum and a minimum value.
Comparing a motor to a generator
Motor part
Function
Generator part
Function
split ring
commutator
ensures the
torque on the coil
is in one direction
split ring
commutator
ensures the current
from the coil is in one
direction only
brushes
connects the
exterior circuit to
the coil
brushes
connects the coil to the
exterior circuit
coil
where current
induces a
magnetic field
coil
where a current is
induced by cutting a
magnetic field
permanent
magnet
oppose the
magnetic field
induced into the
coil to produce
motion
permanent
magnet
induces the electric
current in the moving
coil
Part 3: Powering up
33
Making a simple DC generator from a motor
1
Yes it was acting as a generator. As the armature (rotor) was turned
a current was observed to cause the meter needle to move or a
reading to be observed.
2
A larger current was observed when the armature was turned faster.
3
A faster rate of turning the armature caused a greater voltage or emf.
4
A greater turning force is required to turn the armature faster. N, B
and A are constant therefore rearranging t = nBIA to make I the
subject results in I =
t
. Therefore as the torque is increased the
nBA
current must also become greater.
5
emf = BLv therefore as the velocity of rotation is increased the emf
will be greater.
6
The speed of armature rotation is increased to increase the current
and voltage and decreased to reduce the current and voltage.
Why is the electricity supply AC
Transmission at 1000 V is the most efficient.
Transmit at high voltages, use highly conductive metals as transmission
wire, build power stations close to where demand is or conversely build
high demand industry near to power stations.
34
Motors and generators
Exercises – Part 3
Exercises 3.1 to 3.8
Name: _________________________________
Exercise 3.1
What are the main components of a DC generator and what is the
function of each of these components?
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Exercise 3.2
It has been stated that a generator is essentially an electric motor
operating in reverse. Comment on the validity of that statement making
sure you explain fully all points you make.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Part 3: Powering up
35
Exercise 3.3
Look at the figure of the simple generator shown below. Would this
generator produce AC or DC electricity? Explain your reason for your
choice.
brushes
split ring
commutator
coil
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Exercise 3.4
Show through calculations why an electric current from a 1 MW power
station can be more efficiently transmitted as a 100 000 V transmission
than as a 240 V transmission.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
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Motors and generators
Exercise 3.5
Assess how the development of the AC and DC generator impacted on
society?
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Exercise 3.6
Outline how the development of the AC generator and large scale
thermal power plants has impacted on the environment?
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Exercise 3.7
Discuss the relative impact of the following types of power station on the
environment: thermal coal burning power station, hydroelectric power
station, wind generated power.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Part 3: Powering up
37
Exercise 3.8
Identify the relative advantages of the AC generator over the DC
generator for general use?
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
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Physics
HSC Course
Stage 6
Motors and generators
Part 4: Transmission
2
0
0
In
r2
e
b S
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t
c NT
O
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t
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Gill Sans Bold
Contents
Introduction ............................................................................... 2
Edison versus Westinghouse .................................................... 3
Transmission............................................................................. 5
History of transmission.........................................................................6
Transmission in NSW...........................................................................7
Transmission lines................................................................................8
Summary................................................................................. 14
Suggested answers................................................................. 15
Exercises–Part 4 ..................................................................... 17
Part 4: Transmission
1
Introduction
Electricity is generated at specific locations in NSW. The transmission
of that electricity to where it is consumed is an enterprise that requires
huge infrastructure investment.
The energy losses from transmission are significant and must be kept to a
minimum.
The monitoring of the grid to ensure regular and reliable energy supply is
essential.
You are probably familiar with the sight of high voltage transmission
lines stretching across the country side. These arteries carry the energy
that powers modern society. They appear so common these days that
they hardly rate a mention and blend into the natural landscape yet they
are an example of the high technology modern society demands to supply
its thirst for energy in the useable form you know as electricity.
At the end of Part 4, you will have had an opportunity to:
•
analyse secondary information on the competition between
Westinghouse and Edison to supply electricity to cities
•
gather and analyse information to identify how transmission lines
are:
–
insulated from supporting structures
–
protected from lightning strikes.
Extract from Physics Stage 6 Syllabus © Board of Studies NSW, amended
November 2002. The most up–to–date version can be found on the Board's
website at
http://www.boardofstudies.nsw.edu.au/syllabus_hsc/syllabus2000_listp.html#p
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Edison versus Westinghouse
In the early days of the electricity industry the competition to supply
electricity to cities was intense. It was really a battle of DC versus AC.
Thomas Edison proposed that DC was the superior system for the
transmission and generation of electricity. George Westinghouse and his
partners proposed that electricity should be generated as AC and
transmitted as AC. The industry was split. Both the opponents in this
battle had the respective advantages to tout for their system.
Edison claimed DC was better because DC was safer for the consumer.
If you received a shock from the domestic supply of DC by gripping the
wire you could simply open the hand and remove it. The same cannot be
said of an equivalent voltage AC shock that would tend to paralyse the
hand and force you too continue to hold the wire. Edison was said to
have made the following statement about AC electricity;
"Just as certain as death, [George] Westinghouse will kill a customer
within six months after he puts in a system of any size."
Quote from Blow, Michael. Men of Science and Invention. New York,
American Heritage Publishing Co., Inc., 1961. p. 95.
Edison had a vision of the establishment of DC generators all over cities
transmitting the electricity short distances to where it was required in
small local networks. In many instances the idea was that each building
would have its own electricity generator. Under these conditions the
transmission energy loss problem due to electrical resistance when using
a DC system was not such a problem. DC when used under these
conditions was at an advantage in the efficiency of generation at the time.
If you didn’t have to transmit the electricity too far it was a good system.
Edison had some other advantages over Westinghouse in the early
competition for domination of the electricity industry. These were
largely due to his head start. His Pearl Street power station near Wall
Street New York was up and running in 1882, and the banker’s backing
was more easily obtained for small local stations initially. Power stations
cost money to build. Large power station as such as those preferred by
Part 4: Transmission
3
the generators of AC such as Westinghouse required a large capital
investment on which it took a number of years to provide a return.
The small DC generators used locally were cheaper per unit, represented
less risk to the banks and financiers and had their product market
guaranteed. They didn’t need to look for customers at a distance.
The truth of the matter in the end was that AC won out over DC because
it was a better system. Its use was able to drive down the cost of
electricity supply. Part of the problem with the small DC generation
systems was that you needed to have the capacity to run the generator all
day but demand was only peaking at certain times. Your capacity had to
match the peak demand. The location of customers able to use the
electrical generating capacity outside of peak demand periods was easier
for large power generating systems with broad distribution networks.
More efficient use of expensive generating capacity by AC generators
meant that costs for electricity could be reduced. Cheaper electricity
meant more access to consumers as they switched on to the cleaner
alternative electrical energy from alternatives such as gas lighting.
The whole thing then became self–perpetuating.
Today electrical hot water is often off–peak. The electrical supply
companies sell cheap electricity to heat water in periods of low
electricity demand. That way their load capacity is more evenly utilised
and they do not have vastly excessive capacity under–utilised for much
of the day.
History now tells us now that the AC system of George Westinghouse
won the day. In many respects AC wasn’t as safe as the DC system but it
did have the advantage that it could be transmitted long distances
efficiently. The power plant could also be established close to the energy
source and the electricity moved into the city along transmission wires.
This was pivotal to the success of the Niagara Falls hydro–electric power
plant built by Westinghouse in 1895. This decentralisation meant that
the power plant could be built on relatively cheap ground and that the
generation and accompanying pollution (if not a hydro electric station)
was on the outskirts of the cities or in the surrounding countryside.
To sum it up, the battle for domination of the electricity industry and
control of the lucrative city market was lost by Edison in the short term
but won by both Westinghouse and Edison’s General Electric
Corporation. The two companies started by these men still dominate the
generator industry today. It was an economic and political battle.
The winner was a technological society!
To see sites that describe some of the battle between Edison and
Westinghouse to provide electricity to cities see links on the physics
websites page at: http://www/lpmc.edu.au/science.
4
Motors and generators
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Transmission
A high voltage transmission tower. (Photo: Ric Morante. )
Part 4: Transmission
5
History of transmission
In 1876, the California Electric Company of San Francisco opened.
Its purpose was the selling of electricity. The company’s market was
small in terms of the number of customers served but it was still the first
electrical transmission system.
In 1881, Lucien Gualard and John Gibbs patented an AC transmission
system in England.
In 1882, Thomas Edison opened the Pearl Street Station in New York.
This system provided DC electricity to around one half of a square
kilometre.
In 1885, George Westinghouse and William Stanley developed the
transformer to a high level of efficiency.
In 1886, Stanley demonstrated the advantages of AC transmission.
In this demonstration he used a transformer to step up a generated
voltage to 3000 V for transmission. Then, after transmitting it around
1200 m he used a second transformer to step down the voltage to 500 V.
This was a much more highly efficient means of transmitting electricity
than with the competing DC system where the energy lost during
transmission was much greater.
In 1887, Nicola Tesla patented the polyphase (three–phase) AC system
of electricity generation and the motors that could use AC electricity.
In 1890, the first commercial AC transmission power line was in use
between Willamette Falls and Portland, Oregon. This was a distance
of 21 km.
From this time on, AC transmission began to dominate the electricity
transmission business.
The acceptance of the use of the AC electricity system as the dominant
means of transmitting electrical energy was not smooth. The battle has
been termed the ‘transmission wars’.
The proponents of the DC system were the Edison Electric Company.
The proponents of the AC system were Tesla and his business partner
George Westinghouse. The battle for dominance was not clean. It had
all the intrigue of an adventure story.
To access information that give insights into this battle of the currents go to
links on the physics websites page at: http://www.lmpc.edu.au/science.
6
Motors and generators
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A power plant produces power that has to be transmitted to a city 12 km
away. If they transmit the electricity at 100 000 V, and the transmission
wire has a resistance of 1 Ωkm–1, how much power is lost in transmitting
20 MW (megawatts) of electricity to the city?
_________________________________________________________
_________________________________________________________
_________________________________________________________
Check your answer.
Do Exercises 4.1 and 4.2 now.
Transmission in NSW
The population base in NSW and hence the greatest demand for electric
power is in the eastern coastal part of the state. The transmission system
in NSW links the main power stations and those of the Snowy Mountains
hydroelectric scheme through more than 15000 km of network.
This network also extends into the power stations that feed into Victoria
and form part of the national grid.
The transmission system has developed as the demand for electricity has
increased. In 1950 the only high voltage transmission line in operation in
NSW was a 132 kV line linking power stations at Port Kembla near
Wollongong and Burrinjuck Dam near Yass. The 132 kV network was
expanded as demand increased for power and large decentralised power
stations were built closer to energy supplies of coal in NSW.
In 1959 a 330 kV line was built from the new Snowy Mountains
Hydroelectric Scheme to Yass. This network of 330 kV lines has
expanded to provide electricity from power stations scattered around
NSW to the major centres of demand.
In 1979 far western NSW towns such as Broken Hill were connected into
the 220 kV power grid of Victoria.
In 1984 a 500 kV line was established between Eraring Power Station on
Lake Macquarie and Kemps Creek substation in south–western Sydney.
A second 500 kV line was commissioned in 1986 between Bayswater
and Mount Piper power stations.
The longer the distances travelled by the electricity between generation
and use, and the greater the demand for electricity the higher the voltages
required for efficient transmission become.
Part 4: Transmission
7
A 330 kV line can carry ten times the electrical energy of a 132 kV line.
As demand for electricity rises in the urban centres higher voltage lines
must be constructed.
Listen to the tape called Substations and transformers or alternatively
download and listen to the tape on steaming audio from the physics website
page at: http://www.lmpc.edu.au/science.
Transmission lines
You are familiar with transmission lines. Most neighbourhoods still have
transmission lines above ground though there is a tendency in newer
suburbs to place the electrical power lines below the ground.
The electricity lines you are probably most familiar with are not really
transmission lines at all–they are distribution lines. These distribution
lines carry electricity from a substation to your home.
A distribution line attached to a power pole is shown in the photograph
below. These lines often carry electricity at around 22 000 to 35 000 V.
Electricity is transformed to domestic supply voltage at 240 V.
shield conductor
earth from the shield
conductor leading to
the ground
A power distribution pole. (Photo: Ric Morante.)
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Transmission lines are the giant towers carrying high voltages (of the
order of 300 000 to 500 000 V) across country from power stations to the
substations that supply electricity to consumers. A set of transmission
lines on a pole is shown in the figure following.
A high voltage transmission pole. This particular pole is carrying 33 kV.
(Photo: Ric Morante.)
1
Look at the photo above. What is the transmission tower made from?
_____________________________________________________
Like most transmission towers the tower shown in the photograph above is
made from steel. This material is conductive.
As a consequence, this presents a problem for the transmission tower
during electrical storms. Notice that the transmission tower is the highest
thing in the area. In fact, the area around high voltage transmission lines
is a buffer zone where construction is not allowed. The higher the
voltage carried by the transmission lines the larger the buffer zone.
The tower is therefore a high conductor exposed to lightning strike.
2
Look again at the photo of the transmission tower. Is there any
evidence of active lightning protection such as the installation of a
lightning rod system around the tower?
_____________________________________________________
Most transmission towers have a similar arrangement to that shown in
the photograph on the previous page.
Part 4: Transmission
9
Each of the three separate bundled wires shown on each side of the tower
in the photo above carry a single phase of the three phase AC
transmission. Commercial generators produce three–phase current
because that is the most efficient generator design.
Each of these wire pairings that carries a single phase of the electric
current is separated from the tower by a set of insulators bundled together
known as an insulator chain. The insulator chain looks like a set of
plates stacked on top of each other. These plates are usually a rubber or
ceramic material.
ceramic insulators that
can be connected to
form an insulator chain
note the metal hooks are not
in contact with other metal
parts to ensure insulation
A variety of ceramic transmission line insulators. There is no direct contact
between the metal parts of the insulators. (Photo: Ric Morante.)
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Chain insulators for high voltage transmission lines. These insulators are
rubber with a fibre glass core running through them. These particular insulator
chains are around 2 m in length. The insulator chains are shaped like stacked
saucers to ensure that dust build up on the insulator doesn’t occur and increase
the conductivity and to ensure a long pathway for the electricity in case of spark
discharge or arcing. (Photo: Ric Morante.)
An electricity substation. Note the extensive use of insulator chains where the
transmission lines come into contact near metal supports. Their plate like
shapes are designed to prevent electricity arcing over them.
(Photo: Ric Morante.)
The photograph below shows a high voltage transmission tower with the
lightning protection features labelled. The top pair of wires on the
transmission tower are called the shield conductors. Notice that they are
Part 4: Transmission
11
the highest pair of wires and that they do not have an insulator chain.
The shield conductors are connected directly to the metal transmission
tower. They act like lightning conductors to prevent the current
transmitting layers from being struck by lightning. The shield
conductors protect the transmission wires beneath them from lightning
strike.
shield conductor
pair of electrical conductors
phase 1
conductor
insulation chain
phase 2
conductor
phase 3
conductor
High voltage transmission tower. (Photo: Ric Morante.)
3
Identify any structures you can see on the photo that may be
designed to protect the transmission lines in event of a lightning
strike hitting the tower?
______________________________________________________
______________________________________________________
4
How do you think the structures you identified might work?
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
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5
Lightning strikes occur most frequently on objects that are relatively
high above the ground compared to their surroundings. One strategy
to protect transmission lines from lightning strike is to place the
transmission lines underground. Why do you think this form of
protection isn’t used more often?
_____________________________________________________
_____________________________________________________
_____________________________________________________
Check your answers.
Transmission lines have a certain amount of passive protection from
lightning strikes. The towers themselves can act as conductors to take
any excess charge to the ground. To facilitate the discharge of the excess
current from the lightning strike, the towers are well earthed with a large
surface area of metal buried to enable the rapid dissipation of the charge
into the ground. In other words, the base of the tower has low resistance.
In addition, the tower is isolated from the adjoining towers by a
minimum distance of around 150 to 200 m. This means that should one
tower be struck any adjacent tower should suffer no effects of the
lightning strike due to the significant distance between towers allowing
the current to dissipate before reaching the adjacent towers.
The top wires (shield conductors) are connected to the transmission
towers directly. Should the shield conductors be struck by lightning, the
lightning will be conducted through the towers and earth wires attached
to the tower to earth.
Part 4: Transmission
13
Summary
The following questions are designed to make you think about the
learning in this part. Complete your answers to form a summary of the
learning you should have done in this part.
•
Why are there energy losses that occur as energy is fed through
transmission lines from the generator to the consumer?
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
•
What are some of the claimed physiological effects on humans living
near high voltage power lines?
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
•
Transmission lines are protected from lightning strikes by:
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
14
Motors and generators
Suggested answers
History of transmission
If the power plant is to deliver 20 x 106 W of power at 100,000 V, then
the current required is:
I = Pdelivered/V = 20000000 W ÷ 100000 V = 200 A.
With a resistance of 2 ohms, this means that the power lost is:
Plost = I2R ¥ d = (200 A)2(1 Ωkm–1) ¥ 12 km = 480 000 W.
Transmission lines
1
The transmission tower is made from some metal. It is probably
steel.
2
None that is visible.
3
The two shield conductors and the insulator chains.
4
The shield conductors are the highest wires on the towers and span
the gap between the towers. Since lightning tends to strike the
higher points first, then that point will be struck first and will carry
the charge through the tower to earth. This protects the wires below
from lightning strike.
The insulator chain isolates the conducting transmission cable from
the tower. The plate like insulators are designed to stop any flashing
of the charge when the lightning strikes occurring from the tower to
the transmission lines.
5
The cost of making an underground system for the transmission of
electricity is much higher initially. After construction the
maintenance of an underground system is also much more
expensive.
Part 4: Transmission
15
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Motors and generators
Exercises – Part 4
Exercises 4.1 to 4.3
Name: _________________________________
Exercise 4.1
Explain why AC electricity eventually became the preferred mechanism
to produce and distribute electricity on a commercial scale.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Exercise 4.2
In terms of the history of the development of the electricity industry,
identify the developments in technology that led to electricity eventually
being transmitted as AC current.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Part 4: Transmission
17
Exercise 4.3
Look at the power pole shown in the photograph below. Label clearly on
the photograph in the indicated areas 1–4 the names of the features that
insulate the transmission lines from the pole and protect the transmission
lines from lightning strike. Explain how all of these features work.
2
1
3
4
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
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Physics
HSC Course
Stage 6
Motors and generators
Part 5: Transformers
2
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0
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b S
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Contents
Introduction ............................................................................... 2
Transformers............................................................................. 4
Voltage transformations and energy ...................................................7
Transformers in power stations ...........................................................9
Electricity transmission............................................................ 11
Transformers at substations.................................................... 15
Transformers in the home ...................................................... 17
Types of transformers ............................................................. 24
Summary................................................................................. 26
Suggested answers................................................................. 27
Exercises–Part 5 ..................................................................... 29
Part 5: Transformers
1
Introduction
The decision to use AC current as the supply current was based largely
on the more efficient transmission of high voltage, low amperage current.
The transformer allowed the conversion of the electricity to be
transmitted at the power station to the most efficient current and voltage
for transmission and then at the substation to the most efficient state for
distribution. Without the transformer the electrical age would never have
taken off. Electricity to every home would have remained an elusive
goal of powerful industrialists and not the reality that most of us
enjoy now.
During the course of your learning in Part 5 you will have opportunities
to learn to:
•
describe the purpose of transformers in electrical circuits
•
compare step–up and step–down transformers
•
identify the relationship between the ratio of the number of turns in
the primary and secondary coils and the ratio of primary to
secondary voltage
•
explain why voltage transformations are related to conservation of
energy
•
explain the role of transformers in electricity sub–stations
•
discuss why some electrical appliances in the home that are
connected to the mains domestic power supply use
a transformer
•
discuss the impact of the development of transformers on society.
In Part 5 you will be given opportunities to:
•
perform an investigation to model the structure of a transformer to
demonstrate how secondary voltage is produced
•
solve problems and analyse information about transformers using:
v
n
=
v
n
2
p
p
s
s
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•
gather, analyse and use available evidence to discuss how difficulties
of heating caused by eddy currents in transformers may be overcome
•
gather and analyse information and use available evidence to assess
the need for transformers in the transfer of electrical energy from a
power station to its point of use.
Extract from Physics Stage 6 Syllabus © Board of Studies NSW, amended
November 2002. The most up–to–date version can be found on the Board's
website at
http://www.boardofstudies.nsw.edu.au/syllabus_hsc/syllabus2000_listp.html#p
Part 5: Transformers
3
Transformers
Transformers are used to alter the voltage in AC circuits. A transformer
usually consists of two coils of wire wound on the same iron core.
The primary coil is the input coil of the transformer; the secondary coil is
the output coil.
The operation of transformers is based on the principal of mutual
inductance. An alternating current in the primary coil induces an
alternating current to flow in the secondary coil. That AC current
induced in the secondary coil is out of phase with the AC current in
the primary coil but has the same frequency as the current in the
primary coil.
primary coil
secondary coil
input voltage
V1 n1
=
V2 n2
V1
V2
output voltage
iron core
A transformer showing the main components.
Mutual induction causes voltage to be induced in the secondary coil.
If the output voltage of a transformer is greater than the input voltage, it
is called a step–up transformer. If the output voltage of a transformer is
less than the input voltage it is called a step–down transformer.
If the voltage induced in the secondary coil is lower than the voltage
induced in the primary coil then the secondary coil winding has less turns
than the primary coil winding.
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primary winding
Vp
Np
secondary winding
Ns
Vs
A step–down transformer
If the voltage induced in the secondary coil is higher than the voltage
induced in the primary coil then the secondary coil has more turns than
the primary coil.
primary winding
Vp
Np
secondary winding
Ns
Vs
A step up transformer.
The emf induced in the secondary coil is related to the emf in the primary
coil by the relationship:
emf across primary coil
number of turns in the primary coil
=
emf across secondary coil number of turns in the secondary coil
This relationship is expressed as
Vp
Vs
=
np
ns
where Vp is the voltage in the primary coil, Vs is the voltage in the
secondary coil, np is the number of turns in the primary coil and ns is the
number of turns in the secondary coil.
From this relationship it becomes apparent that the voltage output from a
transformer can be controlled very precisely by determining the ratio of
turns in the primary to secondary coils required to produce the desired
output voltage from a known input voltage. This voltage control is
critical in many of the applications of transformers.
The applications of transformers include many AC domestic electrical
appliances such as computer monitors, televisions, radios and battery
chargers.
Part 5: Transformers
5
The transformer is used wherever the alternating voltages required to
operate a device are different from the voltage supply. In the case of a
television the voltages required to accelerate the electron beam toward
the screen is much higher than the 240 V supply. This higher voltage is
produced by a step up transformer. In the case of a motor vehicle battery
charger the required voltage is of the order of 12 V to 15 V for effective
charging. This is much lower than the 240 V supply.
1
Explain the purpose of a transformer in an electrical circuit such as a
television.
_____________________________________________________
_____________________________________________________
_____________________________________________________
2
How does the transformer cause voltages to be stepped up or down?
______________________________________________________
______________________________________________________
______________________________________________________
3
A transformer is required to step up the primary voltage of 240 V to
1920 V in an appliance. If the primary coil in the transformer has
100 turns how many turns does the secondary coil need to have to
supply the required output voltage?
______________________________________________________
______________________________________________________
______________________________________________________
Check your answers.
Modelling a transformer
To do this you will need access to the following equipment:
6
•
a length of iron rod or a large bolt
•
2 lengths around 2 m of enamelled copper wire
•
two light globes and holders
•
a connecting leads with an alligator clip
•
a low voltage AC source such as a laboratory power pack. A setting
of 6 V should be used as a maximum. This may represent C on the
transformer dial.
•
a roll of electrical insulation tape.
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Procedure:
1
Cover the bolt in insulation tape.
2
Wind one length of wire onto the bolt to form your primary coil.
Take care not to overloop any of the windings of your coil.
Leave around 20 cm of wire at both end of your coil.
3
Cover your primary coil with insulation tape.
4
Wind on a second coil over the tape but make the second coil have
less loops or windings than the first. This is your secondary coil.
Leave around 30 cm of wire hanging out at both ends of your coil.
5
Connect your secondary coil to your light globes using the holder.
6
Connect your primary coil to the other light globe with one end of
your coil wire and then to the AC power source with a connecting
lead.
7
Connect the other end of your coil lead to the AC source to complete
the circuit to the coil and light globe.
8
Make sure the wires from the primary and secondary coils are not in
contact.
9
Switch on the AC source for about a second. You should see the
light globe in the primary coil circuit light up. You should also see
the secondary coil light globe not connected to the power source
light up (though not as brightly as the first).
Make sure that you do not leave the power switched on at the transformer
for more than one or two seconds as this will cause the circuit breaker in the
power source to trip out.
The fact that the light globe in the secondary coil illuminates shows that
a secondary voltage is induced on the secondary coil.
Do Exercises 5.1 and 5.2 now.
Voltage transformations and energy
The electrical power entering a circuit can never exceed the electrical
power output from the circuit. Transformers satisfy the Law of
conservation of energy because the power entering equals the
power leaving.
Power is measured in watts (W) or Js–1. The amount of energy entering a
transformer equals the amount of energy leaving a transformer in the
same period of time.
Part 5: Transformers
7
From the module Electricity energy in the home you should recall that
P = VI . For a transformer, I1 V1= I2 V2.
An increase in voltage output from a secondary coil in a transformer is
therefore accompanied by a corresponding reduction in the output
current. Similarly, a decrease in the output voltage from a secondary coil
in a transformer is accompanied by a corresponding increase in the
output current.
This is yet another statement amounting to the Law of conservation
of energy.
Transformer efficiency
A transformer has no moving parts. Because of this, it is an
exceptionally efficient machine. In real terms, this means that means an
efficiency of around 99% is not uncommon.
The efficiency of a machine is determined from the equation:
Efficiency =
energy output
¥ 100 %
energy input
The loss of energy that does occur in the transformer is eventually
converted into heat. The big problem with this is that transformers often
have large energy throughputs. The heat energy produced is thus an
extremely large amount. The increased heat that builds up in a
transformer presents another problem to the efficiency of the transformer.
If the transformer gets hot the resistance of the wiring in the coils
increases. If the electrical resistance of the coils increases then the
passage of electricity produces more heat and the transformer gets even
hotter. The emphasis then must be to keep the transformer cool.
The strategies that have been developed to keep the transformers cool
include:
8
•
adding heat sink blades to the transformer to increase the rate of heat
dissipation to the environment through a larger surface area
•
making the transformer case out of a black material so that the heat
produced internally is efficiently absorbed by the case and
re–radiated to the environment efficiently. Most small transformer
rectifier units found around the home are black
•
concrete pad mounted transformers at ground level have ventilated
cases. They may also have an internal fan designed to cool the
transformer.
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•
filling the transformer with a non–conducting liquid that transports
the heat produced in coils away from the coils efficiently.
The liquids used in the transformer transfer heat to the transformer
case and cooling slats to be dissipated to the environment. To
improve the circulation of cooling oil, the transformer may have
pumps fitted rather than rely solely on convection
•
large transformer units are always kept in the open or in well
ventilated areas to maximise air flow around them.
Large transformers have the air flow over heat radiators increased
with the use of fans. These fans are often thermostatically controlled
and cut in at a specified temperature that is usually around 50∞C.
Do Exercise 5.3 to 5.5 now.
Transformers in power stations
The greatest contribution of the transformer to modern society has been
the ease and efficiency with which the transformer has enabled the
transmission of electrical energy from the power station to the consumer.
This has come about because it is more efficient to transmit electricity at
high voltages and low currents. As such, it is important that the electric
current generated at relatively low voltage at the power station be
stepped up to higher voltages for transmission.
Voltages from Wallerawang (1000 MW of power produced) and
Mt Piper (1320 MW of power produced) power stations are stepped up to
330 kV for transmission to domestic and commercial consumers at
efficient levels. The electricity must be stepped up to that voltage in
order to enable efficient distribution across Transgrid’s® extensive
electricity network.
1
Determine the current carried by Transgrid’s® transmission lines
leaving Wallerawang power station assuming transmission at 300 kV
and an output from the power station of 1000 MW.
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
Part 5: Transformers
9
2
The generation of electricity at the Mt Piper power station occurs
with two 660 MW generators. Each of these generators produces an
output of 23000 V. What current is produced by each of these
generators?
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
3
If the transmission of electricity from the power station at Mt Piper
is through 330 kV transmission lines what sort of transformer is
required at the power station?
______________________________________________________
4
If the primary coil in the situation from the question above had
100 turns of extremely thick copper bar how many turns would you
expect the secondary coil to have?
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
Check your answer.
10
Motors and generators
Electricity transmission
In NSW, most of the electricity consumed is generated by a network of
coal–fired power stations. Electricity cannot be effectively stored in the
quantities required by modern society, so the power stations operate
24 hours a day.
The generators in NSW are mostly 660 MW generators that produce
electricity at 23 000 V. These generators operate at speeds of 3000
revolutions per minute (rpm).
Like all AC generators these large generators have a rotor and a stator.
The rotor is really an electromagnet supplied with direct current from a
device known as a static exciter. The AC current is generated in the
windings in the stator as the rotor rotates.
After generation, the transmission of power begins with the electricity
being stepped up in terms of voltage and stepped down in terms of
current by transformers at transformer stations close to the point of
electricity generation.
When transmitting electricity over longer distances or in greater
quantities high voltage transmission is required.
Most of the transmission lines in NSW carry electricity from a power
station at 330 kV. However, it is possible to transmit at 500 kV on
selected transmission lines in NSW such as the transmission line
operating between Eraring Power Station on Lake Macquarie and
Kemp’s Creek substation (140 km). At Kemps Creek substation the
voltage is stepped down to 330 kV for re–transmission to other
substations around Sydney.
After transmission, electricity is stepped down at substations for
distribution to consumers. Initially, these step downs in voltage are
usually to 132 kV for distribution to regional electricity suppliers.
Distribution networks usually transmit electricity in smaller quantities or
over shorter distances often around tens of kilometres.
Part 5: Transformers
11
explosion vent
cooling fins
cooling fan
oil pumps
A 330 kV to 132 kV transformer at the Transgrid ® Sydney West Substation.
(Photo: Ric Morante.)
heavy insulation to prevent flashover
high input voltage
at low current
low output voltage
at high current
laminated iron
core to cut down
on eddy currents
primary coil
secondary coil
Inside the transformer case the structure is often similar to the figure above.
The heavily insulated input terminals are designed to prevent arcing of the
electric current from one terminal to another. The iron core is laminated to
reduce eddy currents with each lamination insulated from the others. The case
is filled with insulating oil.
At distribution substations, the electricity from the 132 kV transmission
lines are stepped down by transformers to 33 kV and further stepped
down to 11 kV for distribution to pad mounted or pole mounted
transformers. At the pad or pole mounted transformer, the voltage is
stepped down to 415 V for distribution to homes and factories.
12
Motors and generators
132-16 kV transformer
cooling fins
A small transformer at the Sydney West substation. (Photo: Ric Morante.)
cutaway of a pole transformer
explosion vent
oil tank
phase 1, 2, 3
primary coils
secondary coils
A cutaway of a pole mounted transformer from the Transgrid® training facility at
Sydney West substation. (Photo: Ric Morante.)
Part 5: Transformers
13
vents to enable heat dissipation
A pad mounted transformer unit at Strathfield.
(Artwork: Anon. Photo: Ric Morante.)
transformer
plates to
radiate heat
A pole mounted 11k V– 415 V transformer. Note the three separate inputs for
each phase of the three phases of electricity and the three outputs. The input
terminals are heavily insulated to prevent arcing across the terminals.
(Photo: Ric Morante.)
14
Motors and generators
Transformers at substations
The transformer has been described as the heart of the electricity
substation.
The role of the transformer is to ensure that, no matter what the incoming
voltage and current, the outgoing voltage and current are suitable for
feeding into the consumer electricity distribution network.
This invariably means that the transformer at the substation is involved in
the stepping down of an incoming voltage.
Substation transformers are rated according to their primary voltage input
compared to their secondary voltage output and their power carrying
capacity.
A typical substation transformer might be rated 132–33 kV and 18 MVA.
This means the primary or high voltage is 132 kV; the secondary or low
voltage is 33 kV; and the transformer has a power rating of 18 MVA or
18 MW.
Substation transformers consist of a core and coils. The whole assembly
is generally immersed in oil in a steel tank. The oil is non–conductive
and acts as an electrical insulator and a coolant. This assists with
keeping the core at reliable operating temperatures.
To assist with cooling, large transformers have fins for the oil to circulate
to dissipate heat. Some transformers have fans to force air across the
cooling fins; other transformers have pumps added to circulate the oil to
improve the transfer of heat away from the transformer coils.
In some situations, a utility may add water spray systems to spray the
transformer case with water on hot days and in cases where high load
conditions occur. However, no facility to do this is found at the Sydney
West substation.
Part 5: Transformers
15
cooling fins
fans to circulate
cooling air
pumps to circulate oil
The cooling system for a 330k V to 132k V transformer. (Photo: Ric Morante.)
How are the difficulties of heating overcome in large transformers such as
those found at substations?
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Check your answer.
Do Exercises 5.6 and 5.7 now.
16
Motors and generators
Transformers in the home
Many small electrical appliances used in the home have transformers or
transformer/rectifier units between them and the mains power supply.
You are probably familiar with many of these devices as the small black
boxes you plug into the power point as part of a large plug.
The purpose of these transformer units is to make sure the voltage supply
to the appliance is appropriate for it to work. For example, electric
doorbells connected into the mains power and other devices with
transformers, use power to step–down your home’s 240 V to the nine or
so volts they need.
Often small transformers are relatively inefficient. The energy they
dissipate as heat energy contributes to the wasted electrical energy in the
home. Because these devices are often attached to devices on standby,
power usage in the average household can add up to about
50 Wh–1, or about 450 kilowatt hours a year. At about 13 cents per
kilowatt hour that means the cost of keeping these standby devices
connected to small transformers and on standby is $60 per year of the
electricity bill.
Look around your home and identify as many transformers as you can in
your home. Identify the uses of the power supply.
In many cases, the transformer devices have their output voltage listed as
DC rather than AC. But, this is only because the transformer box contains a
separate circuit that converts AC current into DC current.
The table following lists the transformers found in a typical home today.
Circle any found in your home.
List any additional transformers that may be in your home in the empty
spaces in the table.
Part 5: Transformers
17
Device
Step–up or step–down transformer
Computer
multiple transformers with both types in
the one device
cordless telephone
step–down
arc welder
step–down
video camera charger
step–down
mobile phone
step–down
nicad battery charger
step–down
computer printer
step–down
electric razor
step–down
electric drill
step–down
electric sander
step–down
rechargeable torch
step–down
digital clock in the stove
step–down
digital clock in the microwave
step–down
microwave oven
step–down
electric screwdriver
step–down
smoke alarm
step–down
computer speaker system
step–down
slot car set
step–down
television
step–up and step–down
amplifier
step–down
microwave
step–up
inverter for converting DC to AC
from battery storage in isolated
areas
18
step–up
Part 5: Transformers
transformer
A computer monitor showing one of the transformers. (Photo: Ric Morante.)
Most transformers in the home are step–down transformers. That is, they
supply electricity to appliances that require low voltages such as small
electric motors and electronic devices.
heat sinks
transformers
The inside box of a personal computer with some of the transformers identified.
Note the thick aluminium bars acting as heat sinks to transfer heat generated in
the transformers. (Photo: James Stamell.)
Many of these transformers also act as rectifier units. That is, they
convert the AC current input into DC current. That conversion is done
with a separate circuit usually close to the transformer or within the same
black box.
Part 5: Transformers
19
Often the current output from step–down transformers is limited to
ensure that the current supplied to small motors or sensitive electronics
does not exceed the load limit for the electronics or small electric motor.
The step–up transformers in use in the home are generally those found in
televisions or computer monitors where the voltage needs to be stepped
up to around 25 kV to accelerate electrons toward the screen.
Looking at a transformer cube
The photographs following show a transformer cube similar to those you
would expect to find around your home powering electrical equipment.
Look at the photographs and answer the following questions based on your
knowledge of transformers.
vents
Transformer case cover after removal. (Photo: Ric Morante)
1
Are there any vents in the case of the transformer?
_____________________________________________________
_____________________________________________________
2
What do you think the vents are for?
______________________________________________________
______________________________________________________
20
Part 5: Transformers
A transformer rectifier cube with the cover removed. The output side of the
transformer is facing the front in this photo. The electrical circuit assembly
visible at the front is the rectifier unit and the heat sink.
(Photo: Ric Morante.)
3
Identify any bits of the transformer that are immediately visible and
label them on the photograph. (Hint : You could identify: the
laminated iron core, the primary and secondary coils in their
insulating cover; the aluminium heat sink; some capacitors; a diode;
the input and output leads from the primary and secondary coils.)
A transformer rectifier cube with the cover removed. Note the laminated
iron core around which the input and output coils are wound. The input
side of the transformer is on the right. This is because the voltage is
stepped down prior to being rectified. (Photo: Ric Morante.)
Look at the photo below of the primary and secondary coil.
This transformer is a step–down transformer.
Part 5: Transformers
21
primary coil
secondary coil
Transformer coils exposed after the insulation has been cut away.
This shows the input side with the input wiring connected to the primary
coil. (Photo: Ric Morante.)
4
Can you see a difference between the thickness of the wire in the
primary (input) and secondary (output) coil?
______________________________________________________
5
Thicker wires are used to carry larger currents. Does this explain the
nature of the primary and secondary coils in this transformer? Give
a reason for your answer.
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
Check your answers.
The circuit following the transformer part of the transformer rectifier unit
consists of diodes and capacitors. The diode converts the AC into a DC
current. The capacitors act to even out the current output so that the
CRO trace appears more like that from a battery or galvanic cell rather
than the output from a DC generator.
22
Part 5: Transformers
capacitor
diodes
The rectifier circuit inside the transformer used to convert the output
from AC to DC current. (Photo: Ric Morante.)
Do Exercises 5.8 now.
Part 5: Transformers
23
Types of transformers
dismantled current
transformer
primary coil
is a single loop
of thick copper
insulation tower
secondary coil
case
A current transformer as used in electricity substations. The large thick copper
coil shows the single primary coil loop capable of carrying currents on the order
of 2000 A. The secondary coil is probably on the order of 2000 turns. Such a
transformer would step–down a current of 2000 A in the primary to 1 A in the
secondary coil. When installed in the transformer this loop is inverted from its
present position. The insulation tower and secondary coil case is filled with oil
when this transformer is in service. (Photo: Ric Morante.)
24
Part 5: Transformers
Because transformers have many applications there are many different
types of transformers. These different designs enable them to do
different jobs although they all step up or step–down voltage.
In substations, the transformers such as the one shown previously are
called auto–transformers. Auto–transformers have one connection on the
primary and secondary coils in common. The idea is that both the
primary and secondary coils share a common iron core.
These transformers use less wire than those with two coils and so are
cheaper to make. Both step–up and step–down transformers are possible
with an auto–transformer setup.
Transformers can be classified as parallel transformers if their windings
are as shown in the figure below. Parallel transformers are compact.
If an iron core is used, they are able to transform very high voltages.
These transformers are used in electronics.
soft iron core
Input (AC)
output
A parallel transformer.
Serial wound transformers such as the one shown in the figure below
have the secondary coil completely insulated from the mains power.
These transformers are used in situations such as television picture tubes
where output voltages on the order of 25000 V may be required.
Input (AC)
output
A serial transformer
Do exercises 5.9 and 5.10 now.
Part 5: Transformers
25
Summary
•
In a circuit you would include a transformer if you needed to:
______________________________________________________
•
A step–down transformer cause the voltage output to:
•
A step–down transformer causes the current output to:
•
The relationship
Vp
Vs
=
np
ns
means:
______________________________________________________
______________________________________________________
•
The typical transformers involved in getting power from the power
station to the consumer are:
______________________________________________________
______________________________________________________
______________________________________________________
•
Heating as a problem is overcome in transformers by:
______________________________________________________
______________________________________________________
______________________________________________________
•
Small transformer rectifier units are common in most homes because
they:
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
26
Part 5: Transformers
Suggested answers
Transformers
1
A transformer is used where a voltage significantly different to the
supply voltage is required to operate an appliance or perform a
function within a circuit.
2
By a process of mutual induction. The oscillating current operating
in the primary coil induced a current with the same frequency in the
secondary coil. The voltage across the coil is determined by the
relative number of turns in the primary and secondary coils.
Vp
3
Vs
=
np
ns
240 100
=
1920 n s
1920
¥ 100
240
n s = 800
ns =
Transformers in power stations
P = VI
1000 MW = 300 kV ¥ I
1000 000 000 W
I=
300 000 V
I = 3333.3 A
1
P = VI
660 MW = 23000 V ¥ I
660 000 000 W
I=
23000 V
I = 28695.7 A
2
3
A step–up transformer.
Part 5: Transformers
27
4
Vp
Vs
=
np
ns
23000 V 100 A
=
300000 V
ns
n s = 100 ¥
300 000
23 000
n s = 1304
Transformers at substations
The transformers are oil filled with pumps to circulate the hot oil to
radiator fins and have cooling fans to circulate the air past the cooling
fins to overcome the difficulties in heating. The cooling fins may be
hosed with water in times of extreme demand or, in some cases, may be
inserted into a cooling pond of water.
Looking at a transformer cube
1
Yes, there are air vents on the top and bottom of the transformer
cube.
2
To allow air to circulate through the transformer and dissipate heat.
3
laminated iron core
heat sink
capacitor
diode
capacitor
secondary coil
The bits inside a small transformer. (Photo: Ric Morante.)
28
4
Yes, the input coil has many turns of fine wire. The output coil has
less turns of thicker wire.
5
Yes. As the voltage falls in the secondary coil the current must rise
because of the Law of conservation of energy.
Part 5: Transformers
Exercises –Part 5
Exercises 5.1 to 5.10
Name: _________________________________
Exercise 5.1
Draw and label a step–up and a step–down transformer in the space
below.
Exercise 5.2
A large transformer has a single loop primary coil made from thick
copper bar with a secondary coil of 2000 turns. If the input voltage is
2000 V what is the output voltage from the transformer?
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Part 5: Transformers
29
Exercise 5.3
A small transformer used to power a laptop computer has the following
characteristics printed on it:
•
Input AC 220 V, 50 Hz, 15 VA.
•
Output DC 5.0 V, 1.0 A.
Is this transformer efficient ? Support your answer with calculations.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Exercise 5.4
The transformer used to step–down the voltage from 11 000 V to 415 V
suitable for use in the home has 1100 turns in the secondary coil. How
many turns does it have in the primary coil? Assume the transformer is
perfectly efficient.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
30
Part 5: Transformers
Exercise 5.5
In a ideal transformer there is no heat produced. And if the input and
output currents is known, the output voltage for a particular input voltage
can be determined easily. Explain this by referring to the Law of
conservation of energy.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Exercise 5.6
It has been said that the transmission of electricity from power stations to
consumers would not be possible without the use of substations. What
happens at the substations and why are they needed?
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Exercise 5.7
Transformers at substations become hot. Why does that occur and how
is the problem of heat build up in transformers at the substation
overcome?
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Part 5: Transformers
31
Exercise 5.8
Explain why the output coil of a step–up transformer is generally made
from thinner wire than the input coil.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Exercise 5.9
The early days of electricity saw many arguments between the Edison
group and the Westinghouse group over whether supply should be
generated as AC or DC. The invention of the transformer by Tesla
virtually settled the argument. Explain why that was so.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Exercise 5.10
List the impacts the development of the transformer has had society.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
32
Part 5: Transformers
Gill Sans Bold
Physics
HSC Course
Stage 6
Motors and generators
Part 6: Motors and other electrical applications
2
0
0
In
r2
e
b S
o
t
c NT
O
ng DM E
i
t
ra E N
o
rp A M
o
c
Gill Sans Bold
Contents
Introduction ............................................................................... 2
The induction motor................................................................... 3
The squirrel cage induction motor .......................................................4
The single–phase inductor motor ........................................................6
Split phase induction motor .................................................................7
Using AC motors ................................................................................13
Applications of electricity ......................................................... 16
The electric light globe .......................................................................16
The toaster..........................................................................................18
The vacuum cleaner...........................................................................19
Summary................................................................................. 22
Suggested answers................................................................. 23
Exercises–Part 6 ..................................................................... 25
Part 6: Motors and other electrical applications
1
Introduction
The induction motor dominates the conversion of electrical energy into
mechanical energy in industry and the home. The motor in your washing
machine and refrigerator is almost certainly an induction motor.
The vacuum cleaner used to clean the house probably contains an
induction motor. In industry, the large motors used for pumps and to
drive heavy machinery are almost certainly induction motors.
The electrical train industry and huge vehicles to transport material
mined from the Earth are driven by AC induction motors in most cases.
The DC motor still has its uses. These are mostly in small and battery
powered equipment but the less maintenance hungry AC induction motor
is beginning to dominate the larger scale electromechanical world.
This part explores the principle of the AC induction motor, another gift
to humankind largely developed by the genius of Nichola Tesla.
During the course of your learning in this part you will have
opportunities to learn to:
•
describe the main features of an AC electric motor
At the end of Part 1, you will have had an opportunity to:
•
perform an investigation to demonstrate the principle of an AC
induction motor
•
gather, process and analyse information to identify some of the
energy transfers and transformations involving the conversion of
electrical energy into more useful forms in the home and industry.
Extract from Physics Stage 6 Syllabus © Board of Studies NSW, amended
November 2002. The most up–to–date version can be found on the Board's
website at
http://www.boardofstudies.nsw.edu.au/syllabus_hsc/syllabus2000_listp.html#p
2
Motors and generators
Gill Sans Bold
The induction motor
The most common use of induction is the AC induction motor.
Over 90% of all electric motors are AC induction motors. They are found
around the home in air conditioners, washers, dryers, fans, garden leaf
litter blowers, vacuum cleaners and most motorised kitchen appliances
that run on an AC supply.
The principle of the AC induction motor was invented in 1888 by
Nichola Tesla. Prior to then, only DC motors were known to exist.
The invention of the AC induction motor rapidly followed Tesla’s
discovery that a magnetic field could be made to rotate if two coils at
right angles are supplied with AC current 90° out of phase. That is the
current supplied to the motor was a two phase current. It was this
rotation feature that made possible the invention of the AC
induction motor.
The idea of the induction motor is based on Faraday’s laws. In the AC
induction motor, the current supply to the armature is by induction from
the magnetic field produced by a field current in a coil winding wrapped
around a soft iron core in the stator. The coil windings in the motor are
there to provide a path for the AC current to flow. It is this current flow
that in turn produces the magnetic field that will cause the rotor to rotate.
It is a fundamental principle of a winding that adjacent poles must be
wound to give opposite magnetic polarity.
The principle of an induction motor is best demonstrated with the
three–phase induction motor. A diagram of this type of motor is shown
in the figure of a three–phase induction motor shown following.
Each phase of electric current is 120° away from the other two.
This means that the magnetic field generated in the stator is effectively
rotating. That rotating magnetic field induces an electric current in the
rotor that is out of phase. Hence, with an opposite associated polarity
magnetic field that causes the rotor to be literally dragged around after
the rotating magnetic field in the stator.
Part 6: Motors and other electrical applications
3
three wires each deliver one phase of AC
rotor
1
2
3
2
magnetic flux
appears to rotate
1
2
3
1
3
A schematic of a three–phase motor.
The coils in the motor don’t actually have to be wound in different
directions before being placed into the stator. It does mean though, that
the winding must be connected so that when the current travels through
one pole in a clockwise direction, it must proceed through the next pole
in a counterclockwise direction to keep the motor turning.
The squirrel cage induction motor
Most electric motors are AC induction motors. Perhaps the simplest and
most reliable are the so–called three–phase squirrel cage motors.
In squirrel cage motors, the rotor winding consists of solid bars that are
joined at either end by a shorting ring. The term squirrel cage has come
about because the cage of the rotor resembles the rotating cylinder that
squirrels play with when in captivity.
The bars that make up the cage are
generally aluminium but they can
be copper or any other highly
conductive material. The use of
aluminium is a trade off between
conductivity and lightness.
Squirrel cage motors are often used
in heavy industrial applications
such as trains, cranes and large air
conditioning units.
4
The rotor cage of a squirrel cage motor.
Motors and generators
Gill Sans Bold
In the squirrel cage induction motor, the rotor turns because of rotation of
the magnetic field. The field coils are wound around soft iron cores to
produce a strong magnetic field on three sides of the stator in a triangular
relationship as shown in the figure below.
solid bars on rotor
A schematic of a squirrel cage motor. Note how the poles of opposite magnets
in the stator are different. Although presented here in this schematic figure as
bar magnets, the magnets are in reality electromagnets with an AC current
passing through them. Opposite stator poles are connected in the one circuit
receiving one phase of the three–phase current. That AC current constantly
changes the polarity of the electromagnets but maintains the opposing polarity
of opposite magnets in the stator. This creates an apparent rotating magnetic
field in the stator. That rotating magnetic field induces a current with opposing
magnetic polarity in the bars of the squirrel cage rotor. The rotor is then literally
dragged along chasing the rotating magnetic field in the stator.
Because the direction of the current in the each of the three field coils is
constantly changing, and the current on each phase is sequenced to
follow on from the previous coil, the polarity of the electric current
induced in the rotor is constantly changing. That constantly changing
magnetic field induces a current in the rotor that produces an opposing
polarity magnetic field. The effect is that the magnetic field from the
stator field coils and the rotor are rotating constantly.
The different current phases function in tandem in a manner that is
similar to pedalling on a bicycle with your feet strapped so you get the
pull and the push on opposite sides of the rotor. The time lag between
delivery of the different phases of the AC current act to create rotating
magnetic fields that interact with the induced currents in the armature to
cause rotation. The direction of three phase squirrel motors can be
reversed. Changing the sequence of delivery of the electricity phases to
the three field coils with a switching device does this.
Part 6: Motors and other electrical applications
5
The single–phase induction motor
Most AC motors around the home are one–phase induction motors.
They are called one–phase induction motors because the electricity that
powers them is one phase AC current.
There are a number of types of single–phase induction motors. All have
specific features designed to overcome the major flaw of the
single–phase induction motor, that is low or no starting torque.
There are several types of single–phase induction motors in use today.
Their modes of operation, once started, are identical. The difference is
the means of starting the motor. Once they are up to operating speed, all
single–phase induction motors operate in the same manner.
Unlike two– or three–phase induction motors, the stator field in the
single–phase motor does not rotate. It simply alternates polarity between
poles as the AC current changes direction.
Voltage is induced in the rotor as a result of induction, and a magnetic
field is therefore produced around the rotor. The magnetic field will
always be in opposition to the stator magnetic field according to Lenz's
law.
Because the force between the magnetic fields in the stator and the rotor
is across the rotor and through the pole pieces of the stator, there is no
rotary motion, just a push and/or pull along this line. The interaction
between the rotor and stator fields will therefore not produce rotation and
produces the situation shown in the figure below.
NStator, SStator = Stator field NRotor, SRotor = Rotor field
NStator
L2
NRotor
SRotor
Stationary
SStator
L1
A single–phase motor showing the interaction of the fields in the rotor and the
stator before the motor starts but when a current is applied.
When there is some other interaction, to produce an initial rotation of the
rotor with respect to the stator, rotation begins. Once started, the
interaction of the rotor and stator fields will cause the rotor to continue
to rotate.
6
Motors and generators
Gill Sans Bold
This situation is shown in the figure below. Note, the current direction in
the stator windings constantly changing means that as long as the rotor
rotation is able to keep up with the frequency of current polarity changes
from the AC current, the rotor will continue to rotate. Although the
magnetic field in the stator isn’t rotating in a true sense, its effect is the
same as the rotating field in a three–phase or two phase induction motor.
SRotor
NStator
SStator
NRotor
Rotating
L2
L1
A single–phase motor showing the interaction of the fields in the rotor and the
stator after the motor starts when a current is applied.
Split phase induction motors
In a split phase single–phase induction motor an inductance and a
resistance are used to displace the voltage so as to get an arrangement
similar to that obtainable with a two–phase motor for starting.
The starting torque is still low, but if the load requirement at start for the
motor is low, then the motor can start. Once going, a split phase
induction motor behaves similarly to a squirrel cage motor.
The capacitor motor is an example of a split phase induction motor.
The figure below shows a simplified capacitor motor.
main winding
AC single
phase supply
starting
winding
rotor
capacitor
A capacitor motor .
Part 6: Motors and other electrical applications
7
The starting winding is connected in parallel with the main winding and
is placed at right angles. An electrical phase difference between the
starting and main windings is created by connecting the starting winding
in series with a capacitor and starting switch. When the motor is first
switched on, the starting switch is closed. This places the capacitor in
series with the starting winding. The capacitor causes the current to lag
the line voltage by about 45° so the currents in the primary and starting
windings are 90° out of phase. So are the magnetic fields that are
generated by the currents in the primary and starter windings. The effect
is that the two windings act like a two–phase stator and produce a
rotating magnetic field to start the motor. When enough speed is
obtained by the rotor, a centrifugal switch cuts out the starting winding
from the circuit. The motor then runs as a plain single–phase induction
motor with the rotor chasing the magnetic field in the stator.
Since the starter winding in the capacitor motor is generally only a light
wire winding that does not carry a large current, the motor does not
develop sufficient torque to start when under a heavy load. Once stated
the motor will carry a reasonable load until its speed of rotation falls
below about 70% of its maximum.
Resistance start motors
Another type of split–phase induction motor is the resistance start motor.
This motor also has a starting winding as shown in the figure below as
well as a main winding.
The starting winding is switched in and out of the circuit just as it was in
the capacitor start motor generally using a centrifugal switch that cuts out
the motor when it reaches a certain rate of revolutions. The starting
winding is again located at right angles to the main winding as shown in
the figure below.
main winding
AC single
phase supply
starting
winding
rotor
resistor
A resistance motor circuit.
8
Motors and generators
Gill Sans Bold
An electrical phase shift between the currents in the starter and main
windings is obtained by making the impedance of the current in the two
windings unequal by use of the resistor.
The main winding has a high inductance and a low resistance.
Therefore, the current lags the voltage.
The starting winding is designed to have a low inductance and a high
resistance. The current therefore lags the voltage by a smaller amount
than in the main winding.
The magnetic fields in the two windings are therefore out of phase by the
same amount as the lag in voltages.
The ideal phase difference between the currents in the starter and main
windings and magnetic fields is 90° for the starting torque to be a
maximum but even a 30° phase difference will generate a rotating
magnetic field sufficient to start the motor when under light load.
When the motor comes up to speed, a speed controlled centrifugal switch
disconnects the starting winding from the circuit and the motor works as
an ordinary single phase induction motor.
This type of motor is commonly used in washing machines and light
applications such as refrigerators.
The shaded pole induction motor
The design of the shaded pole induction motor allows for the production
of a rotating magnetic field using pure induction.
For this motor to work, the field magnets have a special arrangement.
Only one of the field magnets is attached to the AC power supply.
That is the primary coil.
The current and consequent switching magnetic field produced by that
AC current in the primary coil induces a current in the second field
magnet (secondary coil). That current is out of phase with the current in
the in the primary coil hence the polarity of the magnetic fields produced
by these two field magnets are always opposite.
Since the current is oscillating in each field coil with the frequency of the
mains current, this creates a situation whereby the magnetic field in the
stator appears to rotate.
Part 6: Motors and other electrical applications
9
Both the primary coil and secondary coil act to induce currents that
produce magnetic fields in the rotor in opposition to the polarity of the
magnetic field produced by the current that produced them. The result is
the rotor turns.
The diagram below shows a simplified circuit for a shaded pole
induction motor.
second field magnet
(not connected to an AC supply)
rotor
current induced by the
current in the primary
field coil is out of phase
with the supply current
first field magnet called the primary coil
(connected to an AC supply)
A shaded pole induction motor. This type of motor works using a single phase
AC electric current.
The repulsion induction AC motor
The single–phase induction motor can act like a squirrel cage induction
motor if the rotor can be brought up to a speed where the rotor is rotating
at a speed approximately equal to the rotation of the magnetic field in
the stator.
The main problem with the single–phase induction motor is that the
motor has no starting torque so this necessitates novel solutions to supply
that starting torque.
One solution is the repulsion induction motor. This set up is used where
the starting torque is relatively heavy. In this motor, the starting torque is
supplied by repulsion that shifts over to induction once the rotor is up
to speed.
The repulsion induction AC motor has a commutator and brushes.
10
Motors and generators
Gill Sans Bold
When the motor is required to start the brushes are resting on the
commutator and they are, in turn, short circuited by a winding from the
stator. This short circuit provides a high current in the armature that is
sufficient to get the rotor turning because of repulsion. Once moving, the
brushes are lifted from the commutator by a centrifugal switch that
moves the brushes to a ring on the armature effectively short circuiting
the commutator segments together. The motor is then able to act as a
straight induction type motor until its speed drops.
The operation of this type of motor relies on the speed of rotation of the
rotor not dropping below a certain percentage of the speed of the stator
field. If the rotor speed drops below that critical level because of the
load on the motor, the motor stops.
What features do all induction motors have in common?
_________________________________________________________
_________________________________________________________
_________________________________________________________
Check your answer.
Modelling an induction motor
The equipment you will need to make your model induction motor.
(Photo: Ric Morante.)
Part 6: Motors and other electrical applications
11
To do this activity you will need:
•
a new sharpened lead pencil
•
a small test tube able to fit the pencil or the body of a syringe
•
a bar magnet
•
sticky tape
•
some Blutak®
•
an empty aluminium soft drink can
•
a pair of scissors
•
around 20 cm of light cotton thread.
Procedure
1
Cut the bottom of the soft drink can taking care not to cut yourself on
any sharp edges so that you end up with a round disc as shown in the
photograph following.
The setup for the induction motor. Shaking the syringe case will cause the
magnet to rotate. The rotating magnetic field will cause the aluminium soft drink
can bottom to begin to rotate in the same direction as the magnetic field.
(Photo: Ric Morante. Hand: Tim Reid.)
2
Stick the thread to the centre of the disc using Blutak® so that the
disc can hang from the thread horizontally.
Stick the free end of the thread to a table using some sticky tape so
that the disc is hanging freely.
12
Motors and generators
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You may need to balance your disc using small blobs of the Blutak
placed to even out the centre of gravity of the disc so that it will
hang horizontally.
3
Sticky tape your bar magnet to the end of your pencil so that it forms
a T with the pencil.
4
Place your pencil in the test tube or syringe body then hold it under
the suspended aluminium disc as shown in the photograph below.
5
Shake the test tube to make the pencil and magnet spin in one
direction. Observe what happens to the aluminium disc.
6
Shake the test tube to make the pencil and magnet spin in the
opposite direction. Observe what happens to the aluminium disc.
Observations
1
The rotating magnet caused the disc to rotate in the (same/opposite)
direction.
2
Explain what you have observed in terms of the mechanism by which
the induction motor works.
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Check your answers.
Using AC motors
One of the most common uses for AC motors is in small household
appliances and power tools. These devices run on 240 V electrical
supply and usually have small loads applied and hence require small
starting torques. Where the load increases beyond some specific limit the
appliance motor usually labours and actually stops running and may burn
out. Where motors have higher output requirements they usually draw
on three–phase electrical supply at 415 V.
There are two types of 240 V electrical motors used in these small
appliances.
Part 6: Motors and other electrical applications
13
Universal motors can run on either AC or DC current. These motors
have brushes and commutators and are used for portable tools like
routers, jigsaws and electric drills.
Single–phase induction motors run only on AC. These motors have no
brushes, and are usually found on stationary tools such as table saws,
drill presses, planers and jointers.
Both universal and induction motors only produce usable power when
slowed down by applied mechanical load. For induction motors, this
slowdown is called slip and represents the difference between the
unloaded motor spin rate (3000 revolutions per minute for a two pole
motor or 1500 revolutions per minute for a four pole motor) and the
loaded spin rate.
The greater the slip experienced by a motor, the greater the power output.
Induction motors are typically rated at 2850 revolutions per minute (two
pole motor) or 1450 revolutions per minute (four pole motor).
Universal motors do not have a synchronous speed, but have a
maximum no load speed. That speed depends upon the voltage applied
to the motor.
Why is the power output of 240 V AC
induction motors limited?
Most electric motors can put out a lot more maximum horsepower than
they can sustain continuously. There is a sound reason for that based on
the Law of conservation of energy.
By forcing more mechanical load on the motor, slowdown from the
synchronous speed is increased and so therefore is the motor’s output
horsepower. Electrically, the power input into the motor is volts ¥ amps.
By the Law of conservation of energy, mechanical output horsepower
must be balanced by the electrical input power.
Line voltage from mains supply is relatively constant at about 240 V
usually. This means that as a motor is placed under load the input
current must be increased to provide the increased power output.
The windings of electrical motors have some resistance so the higher
current demand as load and output power increases means more current
and hence electrical energy is dissipated in the motor windings producing
heat. The motor windings heat up in proportion to the square of the
motor current. Too high a current for too long can thus burn out the
motor windings if the motor is too overloaded.
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AC induction motor advantages
AC induction motors are by far the most common AC electric motor.
This is because they have a number of advantages over the alternative
motor types such as DC motors. Some of these are outlined below.
•
They are simpler to construct.
•
They require no mechanical contacts to work (such as brushes or
commutators). This greatly reduces the maintenance required to
ensure a long working life for the motor.
•
They are lighter than DC motors of equivalent power output.
•
Modern electronic switching devices allow AC motors to be
controlled effectively.
•
AC motors can be microprocessor controlled to a fine degree.
•
They are more robust and easier to maintain than DC motors.
•
The electricity supply is AC.
Complete Exercises 6.1 to 6.4 now.
Part 6: Motors and other electrical applications
15
Applications of electricity
Electrical energy is surely the energy of our times. The versatility of this
energy source is outstanding as the energy of choice.
Modern technologies such as the development of green power
concentrate on better, more environmentally friendly ways to generate
electricity.
The history of the development of electrical appliances has forced society
to depend upon electricity as the most versatile energy source known.
That versatility as an energy comes from the way electrical energy is
easily converted by appliances to forms required for a modern lifestyle.
The case studies below give insights into the early development of the
electrical energy industry. The inventions highlighted were really not
that long ago.
The electric light globe
The electric globe converts electrical energy to light.
On October 21, 1879 Thomas Edison crossed a threshold that made
electric light with the flick of a switch light an integral part of human life
all over the world.
Thomas Alva Edison found the answer to the most desired invention of
his time, the incandescent light. He started work on the problem of the
light globe after viewing an arc lamp.
Edison believed the key to the incandescent electric light lay in the wire
filament that the electricity travels through. Edison believed he could
solve the problem of the best filament quickly and so launched into a
series of structured experiments. His experiments with filament
materials lead him to the discovery of the elusive secret of the successful
manufacture of the incandescent light.
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The invention of the incandescent light globe was the product of around
twelve thousand experiments. More than six thousand plant materials
were tied as filament material!
The first experiments dealt with carbon. But, because carbon is easily
destroyed, these experiments with carbon were initially postponed.
After a year and two months of continuous experiment Edison found the
best filament for his light globe. He discovered the best and cheapest
filament material was cotton sewing thread carbonised by being burnt to
an ash and then sealed in a glass globe. The tube was evacuated of air
that could support further burning so when an electric current was passed
through the filament, the electrical energy was converted into light.
The first globe glowed for over two days.
Today, the incandescent light is a sealed glass bulb filled with the inert
gas argon. Argon does not support burning at all and the filament is
tungsten.
filament
The incandescent light globe changed
life forever by giving society light at
the flick of a switch in a safe non
polluting (at least on site) and non
burning form. When used for lighting,
the filament wire is sealed in a
vacuum or surrounded by an inert gas.
This prevents the filament from
oxidising or burning.
The electric light invention changed forever the way that society viewed
the use of electricity. Now the fluorescent globe is becoming more
popular as the energy efficient way to convert electrical energy into light
energy.
Take a look around your home and others around you. Identify some of the
devices used to provide light. List these devices in the space below.
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Light changed industry forever. Factories worked 24 hours per day.
Shift work became possible. Life in a changing society was accelerated
by the invention of electric light.
Part 6: Motors and other electrical applications
17
The toaster
The toaster converts electrical energy to heat.
In the eighteenth century English people made toast in their fireplaces
with a rack called the hanging griller. Sometimes, people simply used
long handled forks to toast their bread.
People love the smell and taste of toast. It is therefore obvious an
appliance that would make toast more quickly and easily was very
desirable. Electricity, and in particular, the electric heating element
provided the answer.
The basic principle for most toasters is cooking the bread by radiant heat.
Heat is created by passing an electric current through a wire, known as an
element. Before heating could be accomplished, a wire had to be
developed that would not burn out or oxidise in air.
In 1905 an engineer called Albert Marsh applied for a patent on an alloy
of nickel and chromium, which came to be known as nichrome.
The alloy can be described as being:
•
very low in electrical conductivity
•
very fusible
•
non–oxidising to a very high degree
•
tough and sufficiently ductile to permit drawing into wire.
The toaster element and many other devices such as the electric bar
radiator have to perform the heating task in open air. The invention of
nichrome led to the extensive use of heating devices now almost
universally preferred as the clean alternative for heating.
In 1909 General Electric made the first successful electric toaster called
‘D–12’. It was made from a wire rack and heating element attached to a
porcelain base that toasted one side of a slice of bread at a time.
Take a look around your home and others around you. Identify some of the
electrical devices which provide heating. List these devices below.
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The use of electricity for heating is not confined to the home.
Industry uses electrical resistive heating extensively. There would be a
much reduced ceramics industry without the use of the electric kiln.
Electric arc furnaces or welders would not exist.
Ask people around you about how electricity is used for heating in their
workplace. List the ways that electricity is used for heating in the space
below.
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The vacuum cleaner
The vacuum cleaner converts
electrical energy to movement.
Hubert Cecil Booth, an
Englishman, designed and patented
the first practicable vacuum cleaner
in 1901. Prior to its design
cleaning was done by sweeping or
blowing. Like so many other
electrical appliances commonly
found in the home, the vacuum
cleaner runs on electricity.
The electrical energy supplied to
the vacuum cleaner converts
electrical energy into mechanical
energy in an electric motor.
The earliest vacuum cleaners had
huge motors.
An early electric vacuum cleaner.
Often the motor had to be wheeled separately to the device or the
vacuum cleaner was of the ducted type in large buildings with a central
motor providing the power to the vacuum pump component of the
machine that did the sucking.
Part 6: Motors and other electrical applications
19
There are many devices around the home that use an electric motor to
convert electrical energy into mechanical energy to perform tasks that
require movement. Do a survey of your home to identify those devices that
convert electrical energy into mechanical energy. List those devices in the
space below.
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The use of the electrical motor is widespread in industry for everything
from public transport to cranes and pumps. The conversion of electrical
energy into mechanical energy is clean, non polluting and considered
extremely desirable. In many cities around the world where air quality is
becoming a big issue the use of electrical or hybrid electrical cars is
being encouraged and subsidised by government.
In factories where regular lifting is required in confined spaces the use of
the electrical forklift or lifting device is almost universal.
A Tangara® train from the Sydney electric train network. This and other trains
like it move hundreds of thousands of commuters around the city of Sydney
everyday as well as moving passengers between the urban centres of
Newcastle and Wollongong. (Photo: Ric Morante.)
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Identify the uses of as many electrical motors as you can in the local
industries around you. List the industrial use of those motors that convert
electrical energy into mechanical energy in the space below. Where possible
ask why the use of electrical energy is preferred to the use of the internal
combustion engine to provide the mechanical energy required. Write that
reason given down next to the device that utilises the electrical energy.
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Complete Exercises 6.5 to 6.9 now.
Part 6: Motors and other electrical applications
21
Summary
Complete the statements below to prepare your summary of this part.
•
The function of the rotor in an electric motor is to:
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•
The function of the stator in the electric motor is to:
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•
The main features of an AC electric motor are:
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•
Single phase AC motors are usually used in situations where the
required starting torque is low because:
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•
Induction electric motors are the more common electric motors
because:
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•
Electrical energy is converted into forms more useful such as light,
mechanical and heat energy. Examples of where this occurs in the
home and in industry are:
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Suggested answers
Repulsion induction electric motors
A rotor and a stator.
Induction motors
1
The spinning magnet caused the disc to spin in the same direction.
2
A rotating magnetic field induced a current in the rotor that in turn
produces a magnetic field of opposing polarity to the magnetic field
that caused it. The attraction between these opposing polarity
magnetic fields enables the rotating magnetic field to drag around
the rotor (spinning aluminium can bottom).
Part 6: Motors and other electrical applications
23
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Exercises – Part 6
Exercises 6.1 to 6.9
Name: _________________________________
Exercise 6.1
Discuss the reason why induction based electric motors are the most
common motors in use today. Make sure you list the advantages of
induction motors over commutator and brush type motors.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Exercise 6.2
Describe why a single–phase AC motor cannot start without the aid of a
device to offset the magnetic fields produced by the primary and the
secondary windings.
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Part 6: Motors and other electrical applications
25
Exercise 6.3
AC motors connected to the mains supply voltage are usually 415 V
three–phase when they are under heavy loads such as in large air
conditioning units. Explain why 415 V would be preferred over the use
of 240 V for the same motor.
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_________________________________________________________
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Exercise 6.4
Induction motors have a number of advantages over the use of other
types of motors such as universal AC/DC motors particularly in small
load applications such as their use in appliances like refrigerators and
washing machines. What are these advantages that AC induction motors
have over their other AC/DC counterparts?
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Exercise 6.5
Discuss why the majority of motors in use in the home and industry are
AC induction motors.
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Exercise 6.6
Discuss why some of the electrical appliances and tools used in the home
are connected to the mains supply via a transformer. Consider fixed
appliances as well as portable cordless appliances in your answer.
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Exercise 6.7
Televisions require large voltages on the order of 25000 V to accelerate
electrons toward the phosphor screen. How are these large voltages
created in the television set?
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Part 6: Motors and other electrical applications
27
Exercise 6.8
The iron plates in a laminated iron core transformer are always insulated
from each other. Why would this be done?
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Exercise 6.9
Identify the energy transfers and transformations that would occur in the
following electrical appliances.
a)
An AC electric drill.
b) An incandescent light globe.
c)
A fluorescent light globe.
d) A bar radiator heater.
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Motors and generators
Student evaluation of the module
Name: _______________________
Location: ______________________
We need your input! Can you please complete this short evaluation to
provide us with information about this module. This information will
help us to improve the design of these materials for future publications.
1
Did you find the information in the module clear easy to understand?
_____________________________________________________
2
What did you most like learning about? Why?
_____________________________________________________
_____________________________________________________
3
Which sort of learning activity did you enjoy the most? Why?
_____________________________________________________
_____________________________________________________
4
Did you complete the module within 30 hours? (Please indicate the
approximate length of time spent on the module.)
_____________________________________________________
_____________________________________________________
5
Do you have access to the appropriate resources? eg a computer, the
internet, scientific equipment, chemicals, people that can provide
information and help with understanding science.
_____________________________________________________
_____________________________________________________
_____________________________________________________
Please return this information to your teacher, who will pass it along to
the materials developers at OTEN–DE.
PHYHSC 43196
Learning Materials Production
Open Training and Education Network
NSW Department of Education and Training