What is Commutation?
Commutation in DC machines is the process by which the reversal of current takes place. In DC generator this process is used to convert the induced AC in the conductors to a DC output. In DC motors commutation is used to reverse the directions of DC current before being applied to the coils of the motor.
How Does the Commutation Process Take Place?

One can be aware of the fact that emf induced into the rotating conductor or winding is always sinusoidal or alternating in nature. If the machine is of DC nature that is DC Generator then it is bounding for someone to convert it (sinusoidal emf) into a unidirectional emf. This process of converting AC voltage into a DC is commonly called as commutation. Hence the commutator is a device which normally acts as a rectifier and performs the process of rectification. Coil(s) current is constant and unidirectional so long as the coil is under the influence of given pole pair(s), while it reverses when the coil passes onto the next pole pair as the armature rotates. Commutation takes place when the coil is passing through the interpolar region and during this period the coil is shorted via the commutator segments by the brushes located (electrically) in the interpolar region. Commutation takes place simultaneously for P coils in a lap wound machines and two coil sets of P/2 coils each in a wave wound machine.
Various symbols used in the figure are:


