A conventional brushed DC machine uses a stationary magnetic field and a rotating winding system arranged so that the external electrical terminals carry direct current while the rotor continues producing torque in one direction.

Stator and rotor

The stator provides the stationary magnetic field, using field windings or permanent magnets.

The rotor, traditionally called the armature, carries conductors in which current interacts with the magnetic field and in which rotational EMF is induced.

Brushes and commutator

The armature winding connects to a segmented commutator on the shaft. Stationary brushes make sliding electrical contact with the rotating segments.

As the rotor turns, the commutator changes which armature conductors are connected to each external terminal.

Why commutation is needed

Without changing the winding connections, the torque contribution of a rotor coil would reverse after it passed through half a turn.

The commutator reverses the electrical connection at the appropriate rotor position so that the resulting electromagnetic torque continues in the desired direction during motor operation.

In generator operation, the same mechanical switching converts the alternating EMF induced in rotating armature coils into a unidirectional voltage at the brushes.

Commutation is therefore a mechanical means of coordinating the rotating winding with fixed DC terminals.