Generators

A generator turns motion into electricity by electromagnetic induction. It is a motor running backwards: instead of supplying current to produce rotation, rotation is supplied to produce current.

The a.c. generator (alternator):

1. A coil is rotated in a magnetic field between the poles of a magnet.
2. As it turns, the coil cuts field lines and an e.m.f. is induced in it.
3. The coil is connected to slip rings — two complete rings, one for each end of the coil — which press against carbon brushes.

Slip rings, not a commutator. Because each end of the coil stays connected to its own ring, the connections never swap. As the coil turns past the vertical the direction of cutting reverses, so the induced current reverses — and that is exactly what alternating current is. Using a split-ring commutator instead would swap the connections every half turn and produce d.c.

The output through one rotation:

• The e.m.f. is greatest when the coil is horizontal (in the plane of the field), because there it is cutting field lines fastest.
• The e.m.f. is zero when the coil is vertical (at right angles to the field), because at that instant it is moving along the field lines rather than cutting them.
• One complete rotation gives one complete cycle of the output.

This is often stated the wrong way round, and it is worth reasoning it out rather than memorising: the output is largest where the motion is most nearly perpendicular to the field lines.

On an oscilloscope the output is a smooth wave, positive for half a turn and negative for the other half.

Making the output larger:

1. Rotate the coil faster — this also increases the frequency.
2. Use a stronger magnet.
3. Use more turns on the coil.
4. Wind the coil on a soft iron core.

Note that turning faster changes two things at once: a larger peak e.m.f. and a higher frequency, so the waves become both taller and more closely spaced.

Example: a bicycle dynamo turns a magnet near a coil as the wheel rotates. Ride faster and the lamp gets brighter, because the field through the coil changes more rapidly and a larger e.m.f. is induced.