DC generator principle; function of carbon brush

A DC generator converts mechanical energy into electrical energy using Faraday's law of electromagnetic induction: when a conductor cuts magnetic flux, an emf is induced in it, $e = Blv$ volts (direction by Fleming's right-hand rule).

Working:

1. The field poles produce a flux $\Phi$ across the air gap.
2. The prime mover rotates the armature, so the armature conductors cut this flux and an emf is induced in each conductor.
3. In a single coil the induced emf is alternating, because each coil side passes under a north pole and then a south pole.
4. The commutator and brushes act as a mechanical rectifier. Each time the coil emf reverses, the segments reverse their connection to the brushes, so the brush terminals give a unidirectional (DC) voltage.
5. With a load connected, current flows out through the positive brush.

Functions of carbon brushes

1. They provide a sliding contact with the rotating commutator and collect the current from the armature to the external circuit (or supply it, in a motor).
2. They connect the armature coils to the load at the right moment, so that coils undergoing commutation are short-circuited by the brush and the current in them reverses.
3. Carbon is used because it is self-lubricating, has a high contact resistance (which helps commutation and reduces sparking), and is softer than copper, so the brush wears away instead of the commutator.

The brushes are set on the magnetic neutral axis, where the emf in the short-circuited coil is nearly zero, so sparking is minimum.