AC Generators

An AC generator (alternator) converts kinetic energy to electrical energy by rotating a coil in a magnetic field.

How it works

1. A coil of wire rotates between the poles of a magnet.
2. As the coil rotates, the magnetic flux through it changes.
3. This changing flux induces an electromotive force (e.m.f.) in the coil (Faraday’s law).
4. The e.m.f. reverses direction every half turn, producing alternating current (AC).

Slip rings and brushes

• Slip rings are smooth metal rings connected to each end of the coil. They rotate with the coil.
• Carbon brushes press against the slip rings and connect the rotating coil to the external circuit.
• Unlike a commutator (which reverses contacts every half turn), slip rings maintain the same connection, so the output is AC.

Output voltage graph

The voltage varies sinusoidally:

• Maximum when the coil is horizontal (cutting field lines at the greatest rate).
• Zero when the coil is vertical (momentarily moving parallel to field lines, not cutting them).

Increasing the output

Change	Effect on peak voltage	Effect on frequency
Faster rotation	Higher peak voltage	Higher frequency
Stronger magnet	Higher peak voltage	No change
More turns on coil	Higher peak voltage	No change
Larger coil area	Higher peak voltage	No change

Common errors and how to correct them

• Confusing slip rings (AC output) with a commutator (DC output). A commutator reverses connections every half cycle.
• Saying the voltage is constant. The output of an AC generator varies sinusoidally.

How to apply this in an exam

Describe the structure (coil, magnets, slip rings, brushes). Explain that the changing flux induces an e.m.f. State how the output changes with rotation speed, field strength, number of turns and coil area.