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A.C. generators

An a.c. (alternating current) generator transforms mechanical energy into electrical energy using electromagnetic induction.

This set-up is very similar to that of the d.c. motor!

A coil is rotated inside a magnetic field which induces an electromotive force in the coil.

Both ends of the coil are attached to a slip ring which maintains contact with the coil as it rotates.

The rings are then connected to a complete circuit so that the potential difference (e.m.f.) induced can cause a current to flow.

The induced potential difference changes direction every half turn so the output is alternating current.

An a.c. generator produces an output voltage that varies sinusoidally (looks like a sine function) with time.

In the image below, the coil lies between a north pole on the left, and a south pole on the right.

The voltage is at a maximum or a minimum when the coil is horizontal (parallel to the magnetic field lines). The voltage is zero when the coil is vertical (perpendicular to the field lines).

The output voltage of an a.c. generator over one complete rotation of the coil. The coil lies between a north pole on the left, and a south pole on the right.
The output voltage of an a.c. generator over one complete rotation of the coil. The coil lies between a north pole on the left, and a south pole on the right.

The voltage swaps from positive to negative when the current changes direction.

Remembering Faraday's experiment, the output voltage can be increased by:

  • rotating the coil faster;
  • using stronger magnets; or
  • increasing the number of turns in the coil.