1. Generators and Transformers

2. Generating Current
By moving a magnet in and out of a coil of wire an alternating current (a.c.) is being generated

3. Alternating Current

4. Generator/Dynamo
A generator works by rotating a coil in a magnetic field (or rotating a magnet in a coil). A commutator produces alternating current (a.c.)

5. AC Generator Most of our electricity comes from electric generators.
Simply put it is a coil of wire spun inside a magnet and an A.C current is produced

6. DC Generator
The easiest way to make an A.C. generator D.C. is to add a commutator.
How does a commutator work?

7. True or False
Write down the true statements and write corrected versions of the false statements where possible

8. Transformers

9. The National Grid TRANSFORMERS!!!!!!
The National Grid is the system used for distributing electrical energy around the country. The cable might have up to a hundred miles from the power station. Cables that long have very high resistance (in the example below it is about 50Ω), they heat up as the current flow through them which wastes energy.
At first, the National Grid was very inefficient.
The scientists came up with the solution ……
TRANSFORMERS!!!!!!
The power station produces electricity at a voltage of 25,000 Volts and a current of 100 Amps.
Output power = 2.5 MW
Due to the high current, the cables will heat up and most of the electrical energy will be lost as heat.
Lost power = I2R = 0.5 MW
The total energy reaching homes and factories is only a 2.0 MW

10. But since P = I x V, reducing the current means increasing the voltage
The National Grid
The solution is to reduce the current in the power lines in order to reduce the dissipated power.
But since P = I x V, reducing the current means increasing the voltage
230 Volts
132,000 Volts
25,000 Volts

11. But since P = I x V, reducing the current means increasing the voltage
The National Grid
The solution is to reduce the current in the power lines in order to reduce the dissipated power.
But since P = I x V, reducing the current means increasing the voltage
The step up transformer steps up the voltage to 132,000 Volts (by a factor of 5.28). This causes the current to drop (by a factor of 5.28) to only 19Amps .
Lost power = I2R = 0.02 MW
very little heating effect
The power station produces electricity at a voltage of 25,000 Volts and a current of 100 Amps.
input power = 2.5 MW
The step down transformer steps down the voltage to a safe 230 Volts.
The current varies according to the appliances used.

12. The Simplest Transformer
Iron core
The alternating voltage in the primary coil magnetises the iron core in alternating directions. (magnetic effect of current)
This is just like moving a magnet in and out of the secondary coil.
There will be an induced voltage in the secondary coil (electromagnetic induction).
a.c. output
Secondary coil
a.c. input
Primary coil

13. The Simplest Transformer
Iron core
The alternating voltage in the primary coil magnetises the iron core in alternating directions. (magnetic effect of current)
This is just like moving a magnet in and out of the secondary coil.
There will be an induced voltage in the secondary coil (electromagnetic induction).
a.c. output
Secondary coil
a.c. input
Primary coil

14. The Simplest Transformer
Iron core
The alternating voltage in the primary coil magnetises the iron core in alternating directions. (magnetic effect of current)
This is just like moving a magnet in and out of the secondary coil.
There will be an induced voltage in the secondary coil (electromagnetic induction).
a.c. output
Secondary coil
a.c. input
Primary coil

15. The Simplest Transformer
The output voltage produced in the secondary coil depends on the number of turns in the coil.
A higher number of turns in the secondary coil induces a higher voltage than the input voltage.
A lower number of turns in the secondary coil induces a lower voltage than the input voltage
Iron core
secondary coil
a.c. output
voltage
a.c. input
voltage
Primary coil

16. A conventional transformer consists of three main parts.

17. Symbol

18. Transformers
A changing current in the primary coil produces a changing magnetic field in the core. This changing magnetic field induces a changing current in the secondary coil.
A.C.
Current
A.C.
Current
Magnetic field

19. Transformers Represented diagramatically: Primary Coil Core
Secondary Coil
Alternating EMF
Load
2. This causes alternating magnetic field in the core
3.This causes alternating EMF at terminals of secondary coil
1.Alternating EMF causes alternating current in primary coil
4. This causes alternating current in load. .

20. Changing Voltages Vp/Vs = Np/Ns
The ratio of the number of turns on the coils equals the ratio of the voltages
# of turns = Ns
# of turns = Np
Vp/Vs = Np/Ns

21. Stepping down transformer
Primary coil
secondary coil
a.c. output
a.c. input
Iron core
Stepping up transformer
Primary coil
secondary coil
a.c. output
a.c. input
Iron core

22. Step Up or Step Down?

23. Step Up or Step Down?

24. Step Up or Step Down?

25. Step Up or Step Down?

26. Step Up or Step Down?

27. Step Up or Step Down?

28. Step Up or Step Down?

29. Step Up or Step Down?

30. Step Up or Step Down?

31. Step Up or Step Down?

32. Copy the table and fill in the blanks
Primary turns Np
Secondary turns Ns
Primary voltage Vp
Secondary voltage Vs
Step up or step down
100 20 3
400
10 000 10 50
240 12
5 000
33 000
11 000

33. Solutions
Primary turns Np
Secondary turns Ns
Primary voltage Vp
Secondary voltage Vs
Step up or step down
100 20 15 3
down
400
10 000 10 50
240 12
5 000
33 000
11 000

34. Solutions
Primary turns Np
Secondary turns Ns
Primary voltage Vp
Secondary voltage Vs
Step up or step down
100 20 15 3
down
400
10 000 10
250 up 50
240 12
5 000
33 000
11 000

35. Solutions
Primary turns Np
Secondary turns Ns
Primary voltage Vp
Secondary voltage Vs
Step up or step down
100 20 15 3
down
400
10 000 10
250 up
1 000 50
240 12
5 000
33 000
11 000

36. Solutions
Primary turns Np
Secondary turns Ns
Primary voltage Vp
Secondary voltage Vs
Step up or step down
100 20 15 3
down
400
10 000 10
250 up
1 000 50
240 12
15 000
5 000
33 000
11 000

37. Question

38. Practical Transformers
Most practical transformers have the two coils on top of one another rather than side by side
The core is laminated to reduce eddy currents and made of soft iron to allow easy magnetic reversals
Often transformers have multiple outputs allowing many voltages to be tapped off
Transformers are very efficient but larger ones require cooling (usually with oil) hence we can often assume that:
Power in = power out or
Vin Iin= Vout Iout or VPIP=VSIS