A little about DC generators

Simple direct current (DC) generators contain an armature (or rotor), a commutator, brushes, and a field winding.

A basic DC generator has four basic parts: (1) a magnetic field; (2) a
single conductor, or loop;(3) a commutator; and (4) brushes. The
magnetic field may be supplied by either a permanent magnet or
an electromagnet. For now, we will use a permanent magnet to
describe a basic DC generator.

In a direct current generator, the commutator's job is to change the alternating current (AC), which flows into its armature, into direct current. To put it another way, commutators keep the current flowing in one direction instead of back and forth. They accomplish this task by keeping the polarity of the brushes stationed on the outside of the generator positive. The commutator is made up of copper segments, with a pair (of segments) for every armature coil being insulated from all the others.

The stationary brushes, which are graphite connectors on the generator, form contact with opposite parts of the commutator. As the armature coil turns, it cuts across the magnetic field, and current is induced. At the first half turn of the armature coil (clockwise direction), the contacts between communicator and brushes are reversed. The first brush now contacts the opposite segment that it was touching during the first half turn, while the second brush contacts the segment opposite the one it touched during the first half turn. By doing this, the brushes keep current going on one direction, and deliver it to and from its destination.

(source: http://micro.magnet.fsu.edu/electromag/java/generator/dc.html)

Introduction To Generators And Motors