1. 6.2 Transformer and high-voltage transmission
Caution! High voltage!
Induction between two coils
Working principle of a transformer
Check-point 4
Voltage ratio
Currents in a transformer
Improving efficiency of transformers
Check-point 5
High voltage transmission of electrical energy
Power loss in transmission cable
Check-point 6 1 2 3 4 5 6 7
Book 4 Section 6.2 Transformer and high-voltage transmission

2. Book 4 Section 6.2 Transformer and high-voltage transmission
Caution! High voltage!
You may have seen a similar warning sign:
Where can you find it?
Why is there a high voltage in that place?
Book 4 Section 6.2 Transformer and high-voltage transmission

3. 1 Induction between two coils
Recall: a voltage is induced across a coil if a magnet is moved towards or away from it (Lenz’s law).
A changing current through a nearby coil can produce the same effect — even no movement is involved.
Transformer makes use of this effect.
Expt 6c
Simple transformer
Book 4 Section 6.2 Transformer and high-voltage transmission

4. Book 4 Section 6.2 Transformer and high-voltage transmission
Experiment 6c
Simple transformer
Wind 10 turns of wire round a C-core and 25 turns of wire round another C-core. Clip them together.
Connect the 10-turn coil to a switch and a battery, and 25-turn coil to centre- zero galvanometer.
Close the switch and then open it. Observe the effect on the galvanometer.
Book 4 Section 6.2 Transformer and high-voltage transmission

5. Book 4 Section 6.2 Transformer and high-voltage transmission
Experiment 6c
Simple transformer
Connect the 10-turn coil to an a.c. power supply and the 25-turn coil to a lamp. Connect another lamp to the a.c. supply. Switch on the supply and compare the brightness of lamps.
Change the 25-turn coil to coils of other no. of turns. Observe how the brightness of lamp changes.
Video
6.3 Expt 6c - Simple transformer
Book 4 Section 6.2 Transformer and high-voltage transmission

6. 1 Induction between two coils
In Expt 6c,
when the switch in the primary coil (10-turn coil) is closed, the pointer of the galvanometer in the secondary coil (25-turn coil) deflects momentarily to one side.
when the switch is open, the pointer deflects momentarily to the other side.
There is no deflection when the switch is left closed or open.
Book 4 Section 6.2 Transformer and high-voltage transmission

7. 1 Induction between two coils
Changing current in one coil
 produce an induced current in another coil
 mutual inductance
Book 4 Section 6.2 Transformer and high-voltage transmission

8. 1 Induction between two coils
When the switch is closed, a B-field builds up and ‘cuts’ through secondary coil.
By Lenz’s law, a current is induced and flows as to oppose the build-up of field lines.
 Pointer deflects momentarily to one side.
Book 4 Section 6.2 Transformer and high-voltage transmission

9. 1 Induction between two coils
Then, the current becomes steady.
 B-field stays unchanged
 no voltage induced
Pointer returns to 0.
Book 4 Section 6.2 Transformer and high-voltage transmission

10. 1 Induction between two coils
When switch is open, B-field decays in C-cores.
 Current is induced and flows as to oppose the decay of B-field
 pointer deflects momentarily to other side
Simulation
6.1 Mutual inductance
Book 4 Section 6.2 Transformer and high-voltage transmission

11. 2 Working principle of a transformer
In Expt 6c, when the primary coil is connected to an a.c. supply, the lamp in secondary coil is lit continuously.
 brighter than that in primary coil
 larger voltage across the secondary coil than across the primary coil.
Book 4 Section 6.2 Transformer and high-voltage transmission

12. 2 Working principle of a transformer
When an a.c. flows through the primary coil, magnetic field lines around the core change continuously.
 induces alternating voltage in the secondary coil. The a.c. induced in the secondary coil has the same frequency as that in the primary coil
Book 4 Section 6.2 Transformer and high-voltage transmission

13. 2 Working principle of a transformer
Primary + secondary coils + core  transformer
When an a.c. flows through the primary coil, electrical energy is transferred continuously from the primary to the secondary circuit.
Ideal transformer: 100% energy (or power) transfer
Practical transformer: energy loss (or a power loss) during transfer
efficiency =
useful output power
total input power
 100%
Book 4 Section 6.2 Transformer and high-voltage transmission

14. Book 4 Section 6.2 Transformer and high-voltage transmission
Check-point 4 – Q1
An alternating current is flowing in the primary coil. The current is taken as +ve if it flows from X to Y.
Book 4 Section 6.2 Transformer and high-voltage transmission

15. Book 4 Section 6.2 Transformer and high-voltage transmission
Check-point 4 – Q1
(a)
Primary coil’s current
Time duration
0–t1
t2–t3
Primary coil’s B-field direction (clockwise/anticlockwise)
anticlockwise
Primary coil’s B-field (increasing/decreasing)
increasing
Secondary coil’s B-field direction (clockwise/anticlockwise)
clockwise
Secondary coil’s current direction (P to Q / Q to P )
P to Q
clockwise
increasing
anticlockwise
Q to P
Book 4 Section 6.2 Transformer and high-voltage transmission

16. Book 4 Section 6.2 Transformer and high-voltage transmission
Check-point 4 – Q1
(b) Is the current induced in secondary coil a direct current or an alternating current?
Alternating current.
Book 4 Section 6.2 Transformer and high-voltage transmission

17. Book 4 Section 6.2 Transformer and high-voltage transmission
Check-point 4 – Q2
When S is closed, light bulb flashes once.
What should be done to make the bulb light continuously?
A More cells are required.
B The cell should be replaced by a.c. source.
C Solenoid P or Q should have more turns.
D Use an iron core to link up the solenoids.
Book 4 Section 6.2 Transformer and high-voltage transmission

18. Book 4 Section 6.2 Transformer and high-voltage transmission
3 Voltage ratio
As long as all B-field lines go through both coils, the ratio of voltages in them depends only on their number of turns.
This is true for transformers of any efficiency.
Book 4 Section 6.2 Transformer and high-voltage transmission

19. Book 4 Section 6.2 Transformer and high-voltage transmission
3 Voltage ratio =
Primary voltage
Secondary voltage
no. of turns in primary coil
no. of turns in secondary coil Vp Vs Np Ns
Turns ratio: primary turns to secondary turns
By choosing suitable no. of turns for coils, an a.c. voltage can be stepped up or down.
 a transformer can transform an a.c. voltage
Book 4 Section 6.2 Transformer and high-voltage transmission

20. Book 4 Section 6.2 Transformer and high-voltage transmission
3 Voltage ratio
A step-up transformer (Vs > Vp) has more turns in secondary coil than in primary coil (Ns > Np).
It is used in a power station for stepping up voltage to 400 kV for transmission by cables to users.
Book 4 Section 6.2 Transformer and high-voltage transmission

21. Book 4 Section 6.2 Transformer and high-voltage transmission
3 Voltage ratio
A step-down transformer (Vs < Vp) has fewer turns in secondary coil (Ns < Np).
It is used to, for example, supply 16 V to a notebook computer from the mains.
Simulation
6.2 Simple transformer
Expt 6d
Measuring the voltage ratio of a transformer
Book 4 Section 6.2 Transformer and high-voltage transmission

22. Book 4 Section 6.2 Transformer and high-voltage transmission
Experiment 6d
Measuring the voltage ratio of a transformer
Make up a transformer. Apply 1-V a.c. from a low voltage power supply across the primary coil.
Connect the CRO across the primary coil. Switch off the time base and measure the length of vertical trace.
Repeat step 2 with the secondary coil.
Book 4 Section 6.2 Transformer and high-voltage transmission

23. Book 4 Section 6.2 Transformer and high-voltage transmission
Experiment 6d
Measuring the voltage ratio of a transformer
Note:
For a dual-trace CRO, connect channel 1 across the primary coil and channel 2 the secondary coil to display their waveforms together.
Or use voltage sensors and data-logger to measure the primary and secondary voltages from the scope display.
Book 4 Section 6.2 Transformer and high-voltage transmission

24. Experiment 6d Measuring the voltage ratio of a transformer
Video
Video
6.4 Expt 6d - Measuring the voltage ratio of a transformer
Book 4 Section 6.2 Transformer and high-voltage transmission

25. Book 4 Section 6.2 Transformer and high-voltage transmission
3 Voltage ratio
Example 4
Turns ratio vs voltage ratio
Book 4 Section 6.2 Transformer and high-voltage transmission

26. Book 4 Section 6.2 Transformer and high-voltage transmission
Example 4
Turns ratio vs voltage ratio
The primary and secondary waveforms for Expt 6d:
Scale = 0.5 V / division
Calculate the voltage ratio of transformer and compare it with turns ratio. Are they the same?
Book 4 Section 6.2 Transformer and high-voltage transmission

27. Book 4 Section 6.2 Transformer and high-voltage transmission
Example 4
Turns ratio vs voltage ratio
Vp = 0.5  2 = 1 V
Vs = 0.5  4 = 2 V Vp Vs =
Voltage ratio
= 0.5 = Np Ns
Turns ratio = 10 25
= 0.4
Voltage ratio > turns ratio (∵ not all field lines from the primary coil cut the secondary coil).
Book 4 Section 6.2 Transformer and high-voltage transmission

28. 4 Currents in a transformer
Without power lost, all power supplied to the primary coil is transferred to the secondary.
 total input power = useful output power 
VpIp = VsIs = Vp Vs Is Ip  = Ip Is Ns Np
(if no power loss) = Vp Vs Np Ns
By ,
Book 4 Section 6.2 Transformer and high-voltage transmission

29. 4 Currents in a transformer
A step-down transformer:
turns ratio
= 20 : 1
The voltage is reduced from 220 V to 11 V.
Without power loss, Is = 10 A
With power loss, Is < 10 A
Example 5
Currents in the coils of transformer
Book 4 Section 6.2 Transformer and high-voltage transmission

30. Book 4 Section 6.2 Transformer and high-voltage transmission
Example 5
Currents in the coils of transformer
An ideal transformer (3300 turns in primary coil) operates a ‘12 V, 24 W’ lamp from the 220-V a.c. mains. Assume: no power loss in the transformer.
(a) Number of turns in secondary coil = ? = Vp Vs Np Ns
 Ns =  Np Vs Vp
=  3300 12
220
= 180
The secondary coil has 180 turns.
Book 4 Section 6.2 Transformer and high-voltage transmission

31. Book 4 Section 6.2 Transformer and high-voltage transmission
Example 5
Currents in the coils of transformer
(b) Current in secondary coil = ?
Current taken by the lamp I = P V = 24 12
= 2 A
 The current in secondary coil is 2 A.
Book 4 Section 6.2 Transformer and high-voltage transmission

32. Book 4 Section 6.2 Transformer and high-voltage transmission
Example 5
Currents in the coils of transformer
(c) Current in primary coil = ?
Assume: no power loss
VpIp = VsIs
 Ip =  Is Vs Vp
=  2 12
220
= A
The primary current is A.
Book 4 Section 6.2 Transformer and high-voltage transmission

33. 4 Currents in a transformer
Example 6
Why is a transformer preferred
Book 4 Section 6.2 Transformer and high-voltage transmission

34. Book 4 Section 6.2 Transformer and high-voltage transmission
Example 6
Why is a transformer preferred
A ‘12 V, 24 W’ lamp is connected in series with a resistor to the 220-V a.c. mains. The resistor enables the lamp to be operated at 12 V.
(a) Current drawn from the mains = ? = P V = 24 12
Current I
= 2 A
The current drawn from the mains is 2 A.
Book 4 Section 6.2 Transformer and high-voltage transmission

35. Book 4 Section 6.2 Transformer and high-voltage transmission
Example 6
Why is a transformer preferred
(b) Resistance of resistor = ?
The resistor has to take up 208 V, leaving 12 V for the lamp. R = V I =
208 2
= 104 
(c) Power supplied by the mains = ?
Power supplied by the mains
= total input power
= VI = 220  2
= 440 W
Book 4 Section 6.2 Transformer and high-voltage transmission

36. Book 4 Section 6.2 Transformer and high-voltage transmission
Example 6
Why is a transformer preferred
(d) Efficiency in supplying power to the lamp = ?
Power to lamp = useful power output
= 24 W
Efficiency =
useful output power
total input power
 100% = 24
440
 100%
= 5.45%
(e) Why is a transformer preferred to a resistor in ‘stepping down’ a voltage?
The power loss can be greatly reduced.
Book 4 Section 6.2 Transformer and high-voltage transmission

37. 5 Improving efficiency of transformers
In practical transformers, some energy supplied to the primary coil is lost as heat.
Reasons for energy loss:
1. Resistance of coils
∵ coils have resistance
∴ heated when currents flow through
Solution: Use thick wires for the coil
Book 4 Section 6.2 Transformer and high-voltage transmission

38. 5 Improving efficiency of transformers
2. Magnetization and demagnetization of core
∵ Magnetization & demagnetization need energy
∴ Heat is given out
Solution: Use soft iron to make the core (∵ easily magnetized and demagnetized)
Book 4 Section 6.2 Transformer and high-voltage transmission

39. 5 Improving efficiency of transformers
Induced currents in the core
∵ the core lies in a changing B-field
∴ eddy currents induced  heating effect
Solution: Use a laminated core (made from electrically insulated thin sheets of soft iron  high resistance)
Book 4 Section 6.2 Transformer and high-voltage transmission

40. 5 Improving efficiency of transformers
The transformer is a very efficient device. Well-designed transformers can have efficiency > 90%.
The low voltage power supply in the school laboratory contains a special transformer.
The secondary coil can be tapped at different points to give different voltages.
Book 4 Section 6.2 Transformer and high-voltage transmission

41. Book 4 Section 6.2 Transformer and high-voltage transmission
Check-point 5 – Q1
The transformer of a mobile phone:
Turns ratio and the efficiency of the transformer = ?
Turns ratio =
no. of turns in primary coil
no. of turns in secondary coil
primary voltage
secondary voltage = =
230
4.8
= 47.9
Book 4 Section 6.2 Transformer and high-voltage transmission

42. Book 4 Section 6.2 Transformer and high-voltage transmission
Check-point 5 – Q1
Efficiency =
useful output power
total input power
 100% =
4.8  0.35
230  0.05
 100%
= 14.6%
Book 4 Section 6.2 Transformer and high-voltage transmission

43. Book 4 Section 6.2 Transformer and high-voltage transmission
Check-point 5 – Q2
An ideal transformer is used to operate a lamp rated at ‘8 V, 12 W’:
What is the current in the secondary coil? Is = P V = 12 8
= 1.5 A
Book 4 Section 6.2 Transformer and high-voltage transmission

44. Book 4 Section 6.2 Transformer and high-voltage transmission
Check-point 5 – Q3
An ideal transformer is used to operate a lamp rated at ‘8 V, 12 W’:
What is the current in the primary coil? = Ip Is Ns Np
 Ip =  Is Ns Np
=  1.5
100
2500
= 0.06 A
Book 4 Section 6.2 Transformer and high-voltage transmission

45. 6 High voltage transmission of electrical energy
Electrical energy generated at a power station is transmitted over great distances.
Transmission cables are made of copper (very good conductor).
However, the resistance of a long cable is still large. How to reduce power loss (I 2R)?
Expt 6e
The model power line
Book 4 Section 6.2 Transformer and high-voltage transmission

46. Book 4 Section 6.2 Transformer and high-voltage transmission
Experiment 6e
The model power line
Set up the model power line. Switch on the 12 V d.c. power supply and compare the brightness of the two lamps.
Repeat by using a 12 V a.c. power supply.
Note any difference in brightness between the two lamps.
Book 4 Section 6.2 Transformer and high-voltage transmission

47. Experiment 6e
The model power line
3. Step up the 12 V a.c. at the ‘station’ end and step it down at the ‘consumer’ end by transformers.
Compare the brightness of the lamps.
Video
6.5 Expt 6e - The model power line
Book 4 Section 6.2 Transformer and high-voltage transmission

48. 7 Power loss in transmission cable
Expt 6e:
For the first two cases, the lamp at ‘station’ is brightly lit, but that at ‘consumer’ end is dimly lit due to the power loss in ‘cables’.
However, by using transformer, the ‘consumer’ lamp is almost as bright as the ‘station’ lamp.
Stepping up V
 Stepping down I
∴ Power loss (I 2R ) in the cables is reduced.
Book 4 Section 6.2 Transformer and high-voltage transmission

49. 7 Power loss in transmission cable
Power loss in transmission can be reduced if the electricity is transmitted at a high voltage.
A.c. is used to transmit electrical power.
∵ Voltage of a.c. can be stepped up and stepped down easily
Example 7
Power loss in cables
Book 4 Section 6.2 Transformer and high-voltage transmission

50. Book 4 Section 6.2 Transformer and high-voltage transmission
Example 7
Power loss in cables
4 kW of power is transmitted through cables of total resistance 5 .
Calculate the power loss in the cables if it is transmitted at (a) 200 V, (b) 4000 V.
What can you conclude from the results?
Book 4 Section 6.2 Transformer and high-voltage transmission

51. Book 4 Section 6.2 Transformer and high-voltage transmission
Example 7
Power loss in cables
(a) Power transmitted at 200 V: = P V
Current through cables =
4000
200
= 20 A
 power loss in cables
= I 2R
= 202  5
= 2000 W
Book 4 Section 6.2 Transformer and high-voltage transmission

52. Book 4 Section 6.2 Transformer and high-voltage transmission
Example 7
Power loss in cables
(b) Power transmitted at 4000 V: = P V =
4000
Current through cables
= 1 A
 power loss in cables
= I 2R
= 12  5
= 5 W
The power loss in the cables is much reduced by transmitting the power at a high voltage.
Book 4 Section 6.2 Transformer and high-voltage transmission

53. 7 Power loss in transmission cable
Example 8
Electric supply to MTR
Book 4 Section 6.2 Transformer and high-voltage transmission

54. Book 4 Section 6.2 Transformer and high-voltage transmission
Example 8
Electric supply to MTR
CLP Power supplies power to MTR at 33 kV a.c.
Transformer substations are built every 4 to 5 km along the railway line to step down the voltage to 1.5 kV a.c.
The 1.5 kV a.c. is then converted to d.c. and passed through the overhead cables to drive the traction motors on the MTR trains.
Book 4 Section 6.2 Transformer and high-voltage transmission

55. Book 4 Section 6.2 Transformer and high-voltage transmission
Example 8
Electric supply to MTR
(a) Why are so many transformer substations built on the railway line?
If only one substation were built to supply power to the entire railway line, there would be considerable power loss in the cables.
Book 4 Section 6.2 Transformer and high-voltage transmission

56. Book 4 Section 6.2 Transformer and high-voltage transmission
Example 8
Electric supply to MTR
(b) What turns ratio for the transformers is needed for converting 33 kV to 1.5 kV? = Np Ns Vp Vs = 33
1.5
= 22
 The turns ratio = 22 : 1
Book 4 Section 6.2 Transformer and high-voltage transmission

57. Book 4 Section 6.2 Transformer and high-voltage transmission
Example 8
Electric supply to MTR
(c) The peak value of 33 kV a.c. is 25.7 A.
Assume: no power loss in conversion of a.c. to d.c. and transformer is 100% efficient.
(i) Equivalent d.c. flowing in the primary coil of transformer = ?
Equivalent d.c. = r.m.s. value of a.c. = I0 2 =
25.7 2
= 18.2 A
Book 4 Section 6.2 Transformer and high-voltage transmission

58. Book 4 Section 6.2 Transformer and high-voltage transmission
Example 8
Electric supply to MTR
(c) (ii) Current in the overhead cable = ?
Current Np Ns
=  Ip
= 22  18.2
= 400 A
(iii) Power supplied by the secondary coil of the transformer = ?
Power supplied
= VI
= 1500  400
= 600 kW
Book 4 Section 6.2 Transformer and high-voltage transmission

59. Book 4 Section 6.2 Transformer and high-voltage transmission
Check-point 6 – Q1
What is the function of a district substation in the transmission network?
A To generate electricity
B To step down voltage
C To step down current
Book 4 Section 6.2 Transformer and high-voltage transmission

60. Book 4 Section 6.2 Transformer and high-voltage transmission
Check-point 6 – Q2
The power loss in transmission can be reduced if the electricity is transmitted at a ( low / high ) voltage.
Book 4 Section 6.2 Transformer and high-voltage transmission

61. Book 4 Section 6.2 Transformer and high-voltage transmission
Check-point 6 – Q3
3420 MW of electrical power is transmitted at 275 kV. If resistance of cable = 5 ,
power loss in cable = ?
Current flowing along the cable = P V =
3420  106
275  103
= A
Power loss in the cable
= I 2R
=  5
= 773 MW
Book 4 Section 6.2 Transformer and high-voltage transmission

62. Book 4 Section 6.2 Transformer and high-voltage transmission
The End
Book 4 Section 6.2 Transformer and high-voltage transmission