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

2. The two ends of a magnet are called the north pole and the south pole. Magnetic Fields 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. Next >

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

4. 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. Inserting a soft-iron core into the middle of the coil will increase the strength of the magnetic field. Magnetic Effect of an Electric Current Next > Winding the wire into a coil increases the strength of the magnetic field. The magnetic field is similar to a bar magnet. In this assignment, electromagnetic principles will be described using conventional current flow.

5. A transformer is a device that transfers electrical energy from one circuit to another, using electromagnetic induction (transformer action). Transformers 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). Next >

6. 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? Question TRUE

7. Basic Transformer The coil to which the input voltage (V 1 ) of a transformer is applied is called the Primary winding (N 1 ). Output voltage (V 2 ) is taken from the Secondary winding (N 2 ). The transformer has a single magnetic core on which two coils are wound. They are known as the primary and secondary windings. A transformer converts the input voltage into the output voltage using electromagnetic principles (the effect of mutual inductance). Next >

8. The secondary winding is physically positioned (wound) so that its winding are cut by the expanding lines of force. 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. 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. Mutual Inductance Next >

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

10. This magnetic field induces a voltage on the secondary coil, proportional to the number of turns on the winding. Basic Transformer Operation Example: Primary winding = 100 turns, secondary winding = 300 turns, input voltage = 25V, what is the output voltage? Next > A typical transformer is 70% efficient, this means that only 70% of the input power reaches the secondary windings. The primary voltage causes a magnetic field that is proportional to the number of turns on the winding. Therefore, the output voltage is given by:

11. 4 A) 1.1V B) 10V C) 11V D) 100V 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)? Question

12. Transformer Cores The shape of a transformer depends on the core that is being used. Two popular construction types are: Toroidal This core is ring shaped with the primary and secondary windings wound together so they are threaded through the ring. Double Bobbin This core is shaped as shown opposite, with the primary and secondary winding wound together on the central pillar. Next >

13. By switching the current on and off, the magnetic field can be switched on and off. A solenoid contains a coil, a spring and an armature. Basic Solenoid 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. Coil Magnetic field Next > The coil has many loops of wire. A magnetic field is produced when a current passes through it.

14. 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. 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. When a current is passed through the coil, the magnetic field that is created pulls the armature into the coil, closing the air gap. Solenoid Device The movement of the armature can be used in many applications. For example a locking mechanism. Next >

15. 5 A) The pole B) The armature C) The bar D) The rod What is the part of the solenoid device that moves in and out of coil called? Question

16. Back-EMF Next > 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’.

17. 6 A) Left to right B) + to - C) Clockwise D) In the opposite direction to the supply voltage In which direction does a back-emf always act? Question

18. Windings Moving contact Return spring Fixed contact Armature Pivot point Windings Moving contact Return spring Fixed contact Armature Pivot point Relay Next > The relay is a mechanical switch. It uses a small input current to switch high output currents. 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.

19. 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. The DC Motor Next >

20. 7 A) Heat B) Potential C) Kinetic D) Sound What form of energy does an electric motor convert most of its electrical energy into? Question

21. 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. Force on a Single Conductor Next >

22. If the wire is looped several times, it will be in the form of a 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. The current will flow in opposite directions on each side of the coil. Motor Principle - Torque Force on a Coil Next >

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

24. 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). Motor Principle - Commutator Next >

25. An electrical connection is made to the commutator using carbon brushes that are held against the commutator by springs. The commutator is a conductive cylinder split into two halves with each coil terminal connected to its respective half of the commutator. The commutator and coil spin freely about a central pivot point, while the brushes do not move. Motor Principle - Commutator Next >

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

27. With a positive voltage applied to side A of the coil, current will flow around the coil from side A to side B. As the coil gets to the vertical position the carbon brushes reach the gap in the commutator, stopping the current flowing in the coil. This will create a magnetic field around the coil that will react with the main magnetic field, causing the coil to turn anti-clockwise. Motor Principle - Commutator Action Next >

28. 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. Motor Principle - Commutator Action Next >

29. 10 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. Which of the following best describes the action of a commutator: Question

30. 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. Commutator in Real Motor 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. 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. Motor speed is generally controlled by adjusting the supply voltage. Next >

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

32. The End 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.