Commutator is simply a mechanical rectifier.The voltage generated within the armature, placed during a rotating magnetic, of a DC Generator is alternating in nature. The commutation in DC machine or more specifically commutation in DC generator is that the method during which generated AC within the armature winding of a dc machine is converted into DC after browsing the commutator and thus the stationary brushes.Again in DC Motor, the input DC is to be converted in alternating form in armature which is additionally done through commutation. This transformation of current from the rotating armature of a DC machine to the stationary brushes must maintain continuously moving contact between the commutator segments and thus the brushes. When the armature starts to rotate, then the coils situated under one pole (let it's N pole) rotates between a positive brush and its consecutive negative brush and thus the present flows through this coil is during a direction inward to the commutator segments.
Then the coil is brief circuited with the help of a brush for a very short fraction of time(1⁄500 sec). it's called commutation period. After this short-circuit time the armature coils rotates under S pole and rotates between a negative brush and its succeeding positive brush. Then the direction is reversed which is within the away from the commutator segments. This phenomena of the reversal of current is termed as commutation process. We get DC from the comb terminal. The commutation is known as ideal if the commutation process or the reversal of current is completed by the highest of the short time or the commutation period. If the reversal of current is completed during the short time then there's sparking occurs at the comb contacts and thus the refore the commutator surface is broken because of overheating and therefore the machine is known as poorly commutated.