4. Transformers allow generated voltage to be either increased or decreased before it is used

Describe the purpose of transformers in electrical circuits

Transformers convert A.C. voltage to higher values (step-up) or lower values (step-down) .

From a source of fixed e.m.f, other e.m.f.’s can be obtained by varying the number of turnson the primary and secondary coils, depending on the required step-up or step down.

M.Edwards 2003

e.g. from power lines to household circuit - the voltage is lowered so that it can be usedin household appliances at 240V

M.Edwards 2003

e.g. from power station to transmission lines - the higher transmission voltage will mean lesscurrent and so less power losses

Compare step-up and step-down transformers

Transformers consist of two coils wound onto the same soft iron core.The AC current in the primary winding sets up an alternating flux in the core, and the electricfield produced by this alternating flux induces an emf in the secondary coil.

M.Edwards 2003

M.Edwards 2003

Since the same flux operates on both coils, the induced emf per turn is the same - - so the more turns, the more emf!

A transformer with more turns on thesecondary produces a higher emf (voltage)and is called a step-up transformer

A transformer with less turns on thesecondary coil produces a lower emf (voltage)and is called a step-down transformer

In both cases, the ratio of turns is equal to the ratio of voltages

s

p

s

p

n

n

v

v

4 turns 4 turns

8 turns

8 turns

Identify the relationship between the ratio of the number of turns in the primary and secondary coils and the ratio of primary to secondary voltage

Perform an investigation to model the structure of a transformer to demonstrate how secondary voltage is produced

The wires on a telegraph pole carry a potential difference of 11 kV. If the potential difference is stepped down to 240 V for household use, what would be the ratio of primary turns to secondary turns in the transformer?

1 markVp/Vs = np/ns

Vp/Vs = 11000/240Vp:Vs = 46:1

Solve problems and analyse information about transformers using:

s

p

s

p

n

n

v

v

Winona (a qualified electrician) was creating a step down transformer for her doorbell. She wanted to transform a 240 V line to a 8 V line.

Winona already had a transformer with 2000 turns in the primary coil. Using an appropriate formula she determined the number of turns she needed in the secondary coil.

She then obtained a 300 V voltmeter and although there was a potential difference of 240 V from the voltage source for the primary coil, there was a reading of 5 V across the terminals of the secondary coil.

(a) Determine the number of turns Winona would have put in the secondary coil?(b) List two reasons why Winona may have obtained a different result than what she expected.(c) Why did Winona need to install a transformer with the doorbell?

(a) 1 markns = npvs/vp

ns = 2000 x 8 / 240ns = 67 turns

(b) 2 marksThe transformer would not have worked at 100 % efficiency and therefore the potential difference across the secondary terminals would be lower than expected.The limitation of scale on the 300 V voltmeter would mean that there would be errors in reading the device.

(c) 1 markIf the supplied potential difference was too large then too much current would be drawn from the mains which would cause the wires in the doorbell to heat up and melt.

Question 4Below is a diagram of a transformer.

(a) What is the name given to this type of transformer?(b) Explain why this transformer would produce a different output voltage to that which is supplied.(c) Given that the AC power supply has a potential difference of 240 V and the coils in the diagram are proportional to the number of coils in the transformer, determine the output voltage.

4 (a) 1 markThis transformer is a step-down transformer.

(b) 2 marksThe AC power supply will produce a constant change

in the flux induced in the soft iron core. The magnetic field produced continues around the iron core and therefore there is a change in magnetic flux in the secondary coil also. Because there are less turns of wire in the secondary coil, there is less induced potential difference.

(c) 1 markvs = vpns/np

vs = 240x4/8

vs = 120 V

Explain why voltage transformations are related to conservation of energy

Gather, analyse and use available evidence to discuss how difficulties of heating caused by eddy currents in transformers may be overcome

Explain the role of transformers in electricity sub-stations

Gather and analyse secondary information to discuss the need for transformers in the transfer of electrical energy from a power station to its point of use

Discuss why some electrical appliances in the home that are connected to the mains domestic power supply use a transformer

Discuss the impact of the development of transformers on society

A.C. ELECTRIC MOTORS (2 hours)5. Motors are used in industries and the home usually to convert electrical energy into more useful forms of energy

Describe the main features of an AC electric motor

Question 5Below is a diagram of a shaded-pole induction motor.

(a) Describe what the stator is and explain why it is necessary in an AC electric motor.(b) Explain why this motor could be used in a circular saw but not in an electric car.

5 (a) 2 marksThe stator produces an apparently rotating magnetic field. An electric current is induced in the rotor. The rotating magnetic field from the stator then produces a torque due to the induced current.

(b) 2 marksAn induction motor can only provide a low amount of power. A circular saw does not require much power

therefore the induction motor which is efficient and low cost is ideal. The induction motor could not be used in an electric car because it requires too much power. The power is limited by the frequency of the alternating current..

Perform an investigation to demonstrate the principle of an AC induction motor

Gather, process and analyse information to identify some of the energy transfers and transformations involving the conversion of electrical energy into more useful forms in the home and industry

Question 3Below is a diagram of the torque provided by a 3 phase motor.

(a) Determine the maximum total torque generated.(b) If there were 2500 turns of wire per phase, and the distance from the axis of the rotor to the average wire was 5 cm, determine the magnitude of the maximum force on one turn of an average wire?(c) Describe the transformations of energy in an induction motor.

Torque vs Time

0

20

40

60

80

100

120

0 0.2 0.4 0.6

Time (s)

To

rqu

e (N

m)

Phase 1 Phase 2 Phase 3

3 (a) 1 markAdding the torques on the graph, the

maximum torque that can be determined is 200 Nm

(b) 1 markThe maximum torque on any one phase

is 100 Nm. The maximum torque on one wire is therefore 100/2500 = 0.04 Nm.

= FdF = /dF = 0.04/0.05F = 0.8 N

(c) 2 marksElectrical energy is transformed into

magnetic potential energy in the stator which is transformed into electrical energy in the armature conductors which is transformed into kinetic energy in the rotor.

Torque vs Time

0

20

40

60

80

100

120

0 0.2 0.4 0.6

Time (s)

To

rqu

e (N

m)

Phase 1 Phase 2 Phase 3