P.3.7.5.6 The operation of a transformer

A transformer has an efficiency of 80%It has 7000 turns on its primary coil and 175 turns on its secondary coil. When the primary of the transformer is connected to a 240 V ac supply, the secondary current is 8.0 A

What are the primary current and secondary voltage?

• A

• B

• C

• D

The National Grid is used to transfer electrical energy from power stations to consumers.

What conditions for the transmission voltage and the transmission current give the most efficient transfer of energy through the National Grid?

• A

• B

• C

• D

A mains transformer has a primary coil of 2500 turns and a secondary coil of 130 turns. The primary coil is connected to a mains supply where V_rms is 230 V.

The secondary coil is connected to a lamp of resistance 6.0 Ω. The transformer is 100% efficient.

What is the peak power dissipated in the lamp?

• 12 W

• 24 W

• 48 W

• 96 W

The primary coil of a step-up transformer is connected to a source of alternating pd. The secondary coil is connected to a lamp.

Which line, A to D, in the table correctly describes the ratios of flux linkages and currents through the secondary coil in relation to the primary coil?

• A

• B

• C

• D

Which one of the following statements is the main reason for operating power lines at high voltage?

• Transformers are never perfectly efficient.

• High voltages are required by many industrial users of electricity.

• Electrical generators produce alternating current.

• For a given amount of transmitted power, increasing the voltage decreases the current.

A lamp rated at 12 V 60 W is connected to the secondary coil of a step-down transformer and is at full brightness. The primary coil is connected to a supply of 230 V. The transformer is 75% efficient.

What is the current in the primary coil?

• 0.25 A

• 0.35 A

• 3.75 A

• 5.0 A

A transformer has 1150 turns on the primary coil and 500 turns on the secondary coil.

The primary coil draws a current of 0.26 A from a 230 V ac supply. The current in the secondary coil is 0.50 A. What is the efficiency of the transformer?

• 42%

• 50%

• 84%

• 100%

Which one of the following statements concerning power losses in a transformer is incorrect? Power losses can be reduced by

• laminating the core.

• using high resistance windings.

• using thick wire.

• using a core made of special iron alloys which are easily magnetised.

A transformer with 3000 turns in its primary coil is used to change an alternating pd from an rms value of 240 V to an rms value of 12 V.

When a 60 W, 12 V lamp is connected to the secondary coil, the lamp lights at normal brightness and a rms current of 0.26 A passes through the primary coil.

Which line, A to D, in the table gives correct values for the number of turns on the secondary coil and for the transformer efficiency?

• A

• B

• C

• D

Which one of the following would not reduce the energy losses in a transformer?

• using thinner wire for the windings

• using a laminated core instead of a solid core

• using a core made from iron instead of steel

• using a core that allows all the flux due to the primary coil to be linked to the secondary coil