Download Magnetic Fields

Electromagnetism
Topics covered in this presentation:
 Transformer Principles
 Solenoid Operation and Back-EMF
 DC Motor Principles
ClassAct SRS enabled.
Magnetic Fields
The two ends of a magnet are called the north pole and the south pole.
The diagram below shows that there is a magnetic field surrounding a
magnet. It can be seen that the field goes from the north pole to the
south pole.
The lines are closest
together at the poles,
indicating that the
magnetic field is
strongest at the poles.
Although shown here in 2
dimensions, the magnetic field
actually exists in 3 dimensions.
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Question 1
Is it true to say that the direction of the magnetic field is from
North to South?
YES
Magnetic Effect of an Electric Current
In this assignment, electromagnetic principles will
be described using conventional current flow.
When a current flows in a wire, a
magnetic field is set up around it. For
a straight conductor, the lines of force
are circles around the wire.
If two wires are placed side by side, the
magnetic fields will join together, if
current flow is in the same direction.
Winding the wire into a coil increases the
strength of the magnetic field. The
magnetic field is similar to a bar magnet.
Inserting a soft-iron core into the
middle of the coil will increase the
strength of the magnetic field.
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Transformers
A transformer is a device that transfers
electrical energy from one circuit to
another, using electromagnetic
induction (transformer action).
Electrical energy is always transferred without
a change in frequency, but may involve
changes in voltage and current magnitudes.
Transformers are commonly used in
power supplies, where the output
voltage is different to the input voltage.
They can increase voltage (step-up
transformer), or decrease voltage
(step-down transformer).
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Question 2
A transformer can directly change a high voltage to a low voltage,
as long as the voltage is alternating. Is this true or false?
TRUE
Basic Transformer
The transformer has a single
magnetic core on which two coils
are wound. They are known as the
primary and secondary windings.
The coil to which the input
voltage (V1) of a transformer
is applied is called the
Primary winding (N1).
Output voltage (V2) is taken from the Secondary winding (N2).
A transformer converts the input voltage into the output voltage using
electromagnetic principles (the effect of mutual inductance).
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Mutual Inductance
When an alternating voltage is connected to the primary winding, a changing
current flows that produces a changing magnetic field. The magnetic lines of
force expand outward, from the primary.
The secondary winding is
physically positioned (wound)
so that its winding are cut by
the expanding lines of force.
Because these lines of force are
changing, they induce a changing
(alternating) voltage in the
secondary winding.
The two coils are linked by Mutual Inductance as a change to one will have
the same effect on the other.
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Question 3
What transfers the voltage from the primary coil to the
secondary coil?
A) Mutual voltage
B) Mutual inductance
C) Electrical short circuit
D) Self inductance
Basic Transformer Operation
The primary voltage causes a magnetic field that is proportional to the
number of turns on the winding.
This magnetic field induces a
voltage on the secondary coil,
proportional to the number of
turns on the winding.
Therefore, the output
voltage is given by:
V2  V1 
N2
N1
Example: Primary winding = 100 turns,
secondary winding = 300 turns, input
voltage = 25V, what is the output voltage?
V2  V1 
N2
300
 25 
 75 V
N1
100
A typical transformer is 70% efficient, this means that only 70% of the input
power reaches the secondary windings.
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Question 4
A primary coil has 1000 turns and a secondary coil has 100
turns. What would be the output voltage, if the input voltage is
110V (assume transformer is 100% efficient)?
A) 1.1V
B) 10V
C) 11V
D) 100V
Transformer Cores
The shape of a transformer
depends on the core that is being
used. Two popular construction
types are:
Double Bobbin
This core is shaped as shown
opposite, with the primary and
secondary winding wound
together on the central pillar.
Toroidal
This core is ring shaped with the
primary and secondary windings
wound together so they are
threaded through the ring.
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Basic Solenoid
A solenoid contains a coil, a
spring and an armature.
Coil
The coil has many loops of wire. A
magnetic field is produced when a
current passes through it.
By switching the current on
and off, the magnetic field can
be switched on and off.
Magnetic
field
The coil functions as an
electromagnet. It can be
used to control
mechanical movement.
The soft-iron armature
is positioned inside the
coil, so that it can
move back and forth.
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Solenoid Device
With no current applied to
the coil, the armature is
held just out of the coil by
a spring. This creates an
air gap inside the coil.
When a current is passed through
the coil, the magnetic field that is
created pulls the armature into the
coil, closing the air gap.
The armature will be held in this position until
the current is switched off. The spring will then
return the armature to its original position.
The movement of the armature can
be used in many applications. For
example a locking mechanism.
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Question 5
What is the part of the solenoid device that moves in and out of
coil called?
A) The pole
B) The armature
C) The bar
D) The rod
Back-EMF
An undesirable current called ‘Back-EMF’ will occur in
electromagnetic devices. Just as we use ‘Mutual
Inductance’ in a Transformer to induce a current in a
conductor by controlling the current/magnetic field in
another. There will be an induced EMF in the original
winding as the magnetic field builds and collapses. This
induced EMF will always be in the opposite direction
to the original current that produced the magnetic field,
hence ‘BACK EMF’.
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Question 6
In which direction does a back-emf always act?
A) Left to right
B) + to C) Clockwise
D) In the opposite direction to the supply voltage
Relay
The relay is a mechanical switch. It uses a small input current to switch high
output currents.
Pivot point
Return
spring
Armature
It contains a soft iron core
with windings (coil) to form
an electromagnet.
It has fixed and moving contacts.
The moving contact is linked to
the armature, which moves
Windings
around a pivot point.
When the coil is energised, a
magnetic field is produced. The
armature is attracted to the coil
and the contacts close.
When the voltage is switched off, the magnetic field
collapses and the return spring pulls the contacts apart.
Fixed
contact
Moving
Moving
contact
contact
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The DC Motor
An electric motor converts electrical energy into rotational kinetic energy.
When a voltage is applied to a coil, a magnetic field is produced. If the coil is
sitting inside another magnetic field, the two magnetic fields repel to cause
rotational movement.
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Question 7
What form of energy does an electric motor convert most of its
electrical energy into?
A) Heat
B) Potential
C) Kinetic
D) Sound
Force on a Single Conductor
If a current-carrying wire is in a magnetic field, the magnetic field
surrounding the wire will react with the main field to create a directional force
on the wire.
This force will tend to push the wire out of the magnetic field.
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Motor Principle - Torque Force on a Coil
If the wire is looped several times,
it will be in the form of a coil.
The current will flow in opposite
directions on each side of the coil.
The magnetic forces on each side
of the coil will act in opposite
directions to produce a twisting
(torque) force on the coil about the
coil’s pivot.
This force will make the coil turn until it
is at right angles to the magnetic field.
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Question 8
What is another name for a twisting force?
A) Push
B) Torque
C) Pull
D) Friction
Motor Principle - Commutator
The coil can be kept turning by reversing the direction of the current
when the coil is at right angles to the magnetic field.
This is achieved using
a commutator.
The commutator ensures
that when the coil is not at
right angles to the
magnetic field, the current
is always flowing in the
same direction around the
coil (relative to the fixed
magnetic field).
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Motor Principle - Commutator
The commutator is a conductive cylinder split into two halves with each
coil terminal connected to its respective half of the commutator.
An electrical connection is
made to the commutator
using carbon brushes that
are held against the
commutator by springs.
The commutator and coil
spin freely about a central
pivot point, while the
brushes do not move.
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Question 9
What material are commutator brushes usually made from?
A) Rubber
B) Steel
C) Copper
D) Carbon
Motor Principle - Commutator Action
With a positive voltage applied to
side A of the coil, current will flow
around the coil from side A to side B.
This will create a magnetic field
around the coil that will react with
the main magnetic field, causing the
coil to turn anti-clockwise.
As the coil gets to the vertical
position the carbon brushes reach
the gap in the commutator, stopping
the current flowing in the coil.
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Motor Principle - Commutator Action
If the momentum of the coil is great
enough, the coil will continue rotating
so that the carbon brushes will
reconnect with the commutator.
The positive voltage will now be
connected to side B of the coil
causing the current to flow from
side B to side A.
The magnetic fields will again react
so that the coil continues rotating in
an anti-clockwise direction.
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Question 10
Which of the following best describes the action of a commutator:
A) To reverse the current in a coil every half turn.
B) To connect the battery to the brushes.
C) To interrupt the current in a coil every full turn.
D) To reverse the magnetic field of the magnet every half turn.
Commutator in Real Motor
For simplicity, the operation of a motor and the commutator has been
explained using a single coil and a commutator that has been split into
two halves.
A problem with this type of design is that the motor will not be able to
start, when the brushes are in contact with the insulation between the
two halves of the commutator.
In reality, a motor will contain more
than one coil, and the commutator
will be made so that at least one coil
is energised at any one time.
Motor speed is generally controlled
by adjusting the supply voltage.
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Summary
You should now be aware of:
 Transformer Principles
 Solenoid Operation and Back-EMF
 DC Motor Principles
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If you got all of the questions correct
well done.
If you got most of the questions
wrong ask your tutor to go through
the questions again and assist you.
The End