1. Topics covered in this presentation:
Electromagnetism
Topics covered in this presentation:
Transformer Principles
Solenoid Operation and Back-EMF
DC Motor Principles
ClassAct SRS enabled.

2. 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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3. Question 1
Is it true to say that the direction of the magnetic field is from North to South?
YES
Correct answer = Yes

4. 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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5. 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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6. 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
Correct answer = True

7. 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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8. 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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9. Question 3
What transfers the voltage from the primary coil to the secondary coil?
A) Mutual voltage
B) Mutual inductance
Correct answer = B
C) Electrical short circuit
D) Self inductance

10. 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:
Example: Primary winding = 100 turns, secondary winding = 300 turns, input voltage = 25V, what is the output voltage?
A typical transformer is 70% efficient, this means that only 70% of the input power reaches the secondary windings.
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11. 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
Correct answer = C
C) 11V
D) 100V

12. 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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13. Basic Solenoid A solenoid contains a coil, a spring and an armature.
Magnetic field
A solenoid contains a coil, a spring and an armature.
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.
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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14. 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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15. Question 5
What is the part of the solenoid device that moves in and out of coil called?
A) The pole
B) The armature
Correct answer = B
C) The bar
D) The rod

16. Back-EMF
A transistor can be used instead of a switch to control solenoid operation.
There is a problem with this circuit. When the transistor switches off, the collapsing magnetic field induces a voltage in the coil of the solenoid.
Back EMF
The induced voltage is very large and in the opposite direction to the supply voltage.
This voltage can cause damage to circuit components and is known as the Back-EMF.
The Back-EMF can be safely dissipated by connecting a diode in reverse bias across the coil that creates it.
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17. Question 6 In which direction does a back-emf always act?
A) Left to right
B) + to -
Correct answer = D
C) Clockwise
D) In the opposite direction to the supply voltage

18. Relay
The relay is a mechanical switch. It uses a small input current to switch high output currents.
Windings
Moving contact
Return spring
Fixed
contact
Armature
Pivot point
Windings
Moving contact
Return spring
Fixed
contact
Armature
Pivot point
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 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.
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19. 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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20. Question 7
What form of energy does an electric motor convert most of its electrical energy into?
A) Heat
B) Potential
Correct answer = C
C) Kinetic
D) Sound

21. 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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22. 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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23. Question 8 What is another name for a twisting force? A) Push
B) Torque
Correct answer = B
C) Pull
D) Friction

24. 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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25. 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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26. Question 9 What material are commutator brushes usually made from?
A) Rubber
B) Steel
Correct answer = D
C) Copper
D) Carbon

27. 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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28. 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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29. 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.
Correct answer = A
C) To interrupt the current in a coil every full turn.
D) To reverse the magnetic field of the magnet every half turn.

30. 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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31. Summary You should now be aware of: Transformer Principles
Solenoid Operation and Back-EMF
DC Motor Principles
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32. 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