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Three-Phase AC machines
Resource 1
Introduction to Motors and Generators
Three-Phase AC Machines
Resource 1
Introduction to Motors and Generators
Aims
• To provide an understanding of the motor and generator effect that links electricity
to magnetism
• To provide an understanding of how to apply Fleming’s left and right hand rules.
Three-Phase AC Machines
Resource 1
Introduction to Motors and Generators
Objectives
At the end of this lesson you should be able to:
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Describe the effects of placing a current carrying conductor in a magnetic field
Perform simple calculations for the force on a conductor in a magnetic field
Apply Fleming’s Left Hand Motor rule
Describe the effects of moving a conductor through a magnetic field
Perform simple calculations for the induced EMF across a conductor moving through a
magnetic field
Apply Fleming’s Right Hand Generator Rule
Describe the effects of passing a current through a coil of wire to form an
electromagnet
The Motor Effect
F =B I L
North pole
[Newtons]
B = Density of the magnetic flux in Teslas
I = Induced current in Amps
L
L = Length of conductor in field in metres
Force
B
I
South pole
F
Example 1
If a conductor of length 0.4m carrying a current
of 10.6A is placed in a magnetic field with a flux
density of 0.03T, determine the force
experienced by this conductor in newtons.
F = 0.03 x 10.6 x 0.4
= 0.1272 N
The Motor Effect
Fleming’s Left Hand Rule
North pole
Each digit of your hand must be at right
angles to both of the other two
L
Force
B
second
finger
current
thumb
motion
first finger
field
I
South pole
If the current is reversed, the direction of
motion will change
The Motor Effect
Fleming’s Left Hand Rule
Each digit of your hand must be at right
angles to both of the other two
second
finger
current
thumb
motion
North pole
F
Force
B
I
first finger
field
South pole
If the current is reversed, the direction of
motion will change
The Motor Effect
Fleming’s Left Hand Rule
Each digit of your hand must be at right
angles to both of the other two
North pole
second
finger
current
F
Force
B
I
South pole
thumb
motion
first finger
field
If the field is reversed, the motion will be
in the opposite direction
The Motor Effect
Fleming’s Left Hand Rule
Each digit of your hand must be at right
angles to both of the other two
South pole
F
Force
B
I
North pole
first finger
field
second
finger
current
thumb
motion
If the field is reversed, the motion will be in
the opposite direction
The Motor Effect
Using the following convention, we can show why Fleming’s left hand rule works
Current into page
Current out of page
field is clockwise
field is anticlockwise
The Motor Effect
Field lines in the same direction cause repulsion, field lines in opposite
directions cause attraction
North Pole
Force
North Pole
attraction
repulsion
Force
repulsion
attraction
South Pole
South Pole
The Motor Effect
The force on a conductor can be increased by forming a single turn coil
North Pole
Blue spot represents
the central pivot
point
South Pole
The Motor Effect
The force on a conductor can be increased by forming a single turn coil
Top conductor
experiences
force to left
North Pole
Force
South Pole
The Motor Effect
The force on a conductor can be increased by forming a single turn coil
Top conductor
experiences force to
left
North Pole
Force
Force
South Pole
Bottom conductor
experiences force to
right
The Motor Effect
The force on a conductor can be increased by forming a single turn coil
Top conductor
experiences force to
left
North Pole
Force
Force
South Pole
Combined action causes rotation
Bottom conductor
experiences force to
right
The Motor Effect
Forces add up to a rotational force called Torque (T) in Newtons per metre
North Pole
T
T
South Pole
The Motor Effect
For a multi-turn coil
Torque produced
T= 2nFr
North Pole
n = number of coil turns
T
T
F = force on single conductor
r = radius of coil
South Pole
The Motor Effect
For a multi-turn coil
Torque produced
T= 2nFr
North Pole
T
T
Example 2
A 100 turn coil has a radius of 0.1m and a length
of 0.15m. It is placed at right angles in a magnetic
field of flux density 0.08T and carries 12A,
calculate the force on each conductor and the
total torque produced by the coil.
F = B I L
= 0.08 x 12 x 0.15
= 0.144 N
T = 2nFr
South Pole
= 2 x 100 x 0.144 x 0.1
= 2.88 Nm
The Generator Effect
e =B L v
B = Density of the magnetic flux in Teslas
North pole
L = Length of conductor in field in metres
B
-
v = velocity in metres per second
e
Velocity
L
v
South pole
[Volts]
+
I
Example 3
Calculate the EMF induced across the ends of a
wire of length 0.3m when it is moved through a
magnetic field of flux density 0.015T at a speed
of 50m/s..
e = 0.015 x 0.3 x 50
= 0.225 Volts
The Generator Effect
Fleming’s Right Hand Rule
Each digit of your hand must be at right
angles to both of the other two
North pole
B
e
Velocity
L
+
thumb
I
motion
second
finger
current
first finger
field
v
South pole
If the motion is reversed, the polarity of
EMF will change and the current will be
reversed
The Generator Effect
Fleming’s Right Hand Rule
Each digit of your hand must be at right
angles to both of the other two
North pole
B
+
second
finger
e
L
-
Velocity
motion
v
I
South pole
current
thumb
first finger
field
If the motion is reversed, the polarity of
EMF will change and the current will be
reversed
The Generator Effect
Fleming’s Right Hand Rule
Each digit of your hand must be at right
angles to both of the other two
North pole
B
+
second
finger
e
L
-
Velocity
v
I
South pole
current
thumb
motion
first finger
field
If the field is reversed, the polarity of EMF
will change again and the current will be
reversed again
The Generator Effect
Fleming’s Right Hand Rule
Each digit of your hand must be at right
angles to both of the other two
South pole
B
e
L
+
Velocity
I
North pole
first finger
field
thumb
motion
second current
finger
If the field is reversed, the polarity of EMF
will change again and the current will be
reversed again
The Generator Effect
An EMF can be generated in a rotational motion by forming a coil
North Pole
Motion
Motion
South Pole
EMF generated in both sides of the coil add up
The Generator Effect
An EMF can be generated in a rotational motion by forming a coil
Linear velocity v of each conductor can
be worked out from the rotational speed
N and the radius r
North Pole
v
v=2πrN
60
v
South Pole
m/s
The total EMF E of a coil having n turns
moving at right angles to a magnetic field is
as follows
E=2ne
Volts
The Generator Effect
An EMF can be generated in a rotational motion by forming a coil
Example 4
A 200 turn coil has a radius of 0.12m and a
length of 0.23m.
It is placed in a magnetic field of flux
density 0.06T and rotated at 3000rpm.
When the coil is in its vertical position at
right angles to the field, calculate (a) the
EMF on each conductor (b) the total EMF
produced by the coil.
=
v =
37.7 m/s
2 π x 0.12 x 3000
60
e = 0.06 x 0.23 x 37.7
e =B L v
Volts
v =2πrN
60
E=2ne
m/s
Volts
E = 2 x 200 x 0.52
= 0.52
Volts
= 208.1
Volts
Electromagnetism
When a coil is formed of many wire turns, the magnetic fields around each wire add up to
produce a strong electromagnet.
One side of this magnet will be a North Pole while the other side will be a South Pole
If the current in the electromagnet is reversed, the magnetic poles will swap sides.
Electromagnetism
If the coil is wrapped around a soft iron core, the electromagnetic field becomes much stronger.
Electromagnets are used in motors and generators so that the strength of the field can be
varied.
In a motor, this affects the speed and torque produced. In a generator, it affects the voltage
generated.
Further Study – Types of motor
DC motors
Series Field
Shunt Field
Compound Field
AC induction
Squirrel Cage
Slip ring – wound rotor
AC synchronous
Salient Pole
Cylindrical
Further Study - DC Motor Performance
Shunt Field
Speed
Torque
Series Field
Compound Field
Further Study - AC Motor Performance
Synchronous
Cage Induction
Speed
Wound induction
Speed