A basic starter motor consists of:
The armature is the revolving component of the direct current motor. The armature shaft is supported at each end by bushes pressed into end frames which locate the armature centrally in the outer casing or yoke of the motor.
The commutator serves as a sliding electrical connection between the motor windings and the brushes and is mounted on one end of the armature shaft. The commutator has many segments that are insulated from each other. As the windings rotate away from the pole shoe (piece), the commutator segments change the electrical connection between the brushes and the windings. This action reverses the magnetic field around the windings. The constant changing electrical connection at the windings keeps the motor spinning.
The commutator end frame carries the copper-impregnated carbon brushes which conduct current through the armature when it is being rotated in operation. The brushes are mounted in brush holders and are kept in contact with the commutator by tensioned spiral springs.
Half of the brushes are connected directly to the end-frame and via the ground return of the vehicle frame to the battery negative terminal. The other half are insulated from the end-frame and connected to the positive battery terminal via the main starter solenoid input terminal.
This can be a direct connection in the case of a permanent magnet type starter or indirectly via the electro-magnetic field poles of a series wound motor.
Source: CDX Global & Wikipedia - en.wikipedia.org
