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Lesson: Chapter 13 — Electromagnetism and electromagnetic induction

Transformers 4 of 5

Step-up and step-down transformers

Transformers are named by what they do to the voltage.

Step-up transformer

Gives an output voltage higher than the supply voltage.

The secondary has more turns than the primary.

Step-down transformer

Gives an output voltage lower than the supply voltage.

The secondary has fewer turns than the primary.

Figure to be added

Circuit diagrams of a step-up transformer (secondary coil drawn with more turns) and a step-down transformer (secondary drawn with fewer turns).

Where transformers are used

  • The AC electricity generated at power stations is raised by step-up transformers to high voltages such as 132 000 V (132 kV) or 220 000 V (220 kV) and fed into the national grid.
  • Step-down transformers reduce the electricity from the main grid substations to 230 V for distribution to homes.
  • Step-down transformers are used in power supplies and in electrical equipment such as computers and radios.
  • Step-up transformers give the high voltages needed for microwave ovens, X-ray tubes and similar equipment.

Figure to be added

From power station to home: power station 11 kV → step-up transformer → 220 kV transmission → step-down transformer → 11 kV distribution → step-down transformer → 230 V to the home.

Why send power at such a high voltage

Power lines run for hundreds of kilometres, and their resistance turns some of the energy they carry into heat. The heat depends on the current, not the voltage: more current, much more heat. By the power page that follows, the same power can be sent as a high voltage with a small current. So the generator's 11 kV is stepped up to 220 kV, cutting the current to a twentieth and the losses far more, and stepped down again near the town, and once more to the safe household 230 V. Only alternating current can be stepped up and down like this, which is a large part of why mains electricity is AC.

Up for the grid, down for the home

Step-up: more secondary turns, higher voltage. Step-down: fewer secondary turns, lower voltage. Power is sent at 132–220 kV and used at 230 V.