Commutator

It is the process of conversion of generated ac voltage into the armature of a dc generator to dc voltage at the terminal of the dc generator by use of pair of brushes and commutator.
OR It is the process of conversion of given dc voltage at the terminal of the dc motor to ac voltage in the armature windings in a dc motor by use of pair of brushes and commutator.

Since the armature of a motor is the same as that of a generator, the current from the supply line must divide and pass through the paths of the armature windings.
In order to produce unidirectional force (or torque) on the armature conductors of a motor, the conductors under any pole must carry the current in the same direction at all times. This is illustrated in Fig. (4.10). In this case, the current flows away from the observer in the conductors under the N-pole and towards the observer in the conductors under the S-pole. Therefore, when a conductor moves from the influence of N-pole to that of S-pole, the direction of current in the conductor must be reversed. This is termed as commutation. The function of the commutator and the brush gear in a d.c. motor is to cause the reversal of current in a conductor as it moves from one side of a brush to the other. For good commutation, the following points may be noted:
(i) If a motor does not have commutating poles (compoles), the brushes
must be given a negative lead i.e., they must be shifted from G.N.A.
against the direction of rotation of, the motor.
(ii) By using interpoles, a d.c. motor can be operated with fixed brush
positions for all conditions of load. For a d.c. motor, the commutating
poles must have the same polarity as the main poles directly back of
them. This is the opposite of the corresponding relation in a d.c.
generator.
Note. A d.c. machine may be used as a motor or a generator without changing the commutating poles connections. When the operation of a d.c. machine changes from generator to motor, the direction of the armature current reverses. Since commutating poles winding carries armature current, the polarity of commutating pole reverses automatically to the correct polarity.