What is commutation process in D.C. Generator ?

Commutation :

In general, the commutation process it is the process of reversal of current in an armature coil by means of brush of commutator sigments.

Commutation process

The currents induced in the armature conductors of a d.c. generator are alternating in nature. The communication process involves the change from a generated alternating current to an externally applied direct current.

These induced currents flow in one direction when the armature conductors are under north pole. They are in opposite direction when they are under south pole.

As conductors pass out of the influence of north pole and enter the south pole, the current in them is reversed. The reversal of current takes place along the MNA or brush axis. Whenever a brush spans two commutator segments , the winding element connected to those segments is short circuited.

By commutation we means the change that take place in a winding element during the period of short circuit by a brush.

Consider a simple ring winding the current I flowing towards the brush from the left-hand side passes round the coil in a clockwise direction.

Current collection at the commutator

In position this coil carries the same current in the same direction, but the coil is to short circuited by the brush.

Current collection at the commutator

In position the brush makes contact with bars a and b , thereby short circuiting coil 1. The current is still I from the left-hand side and I from the right-hand side. It is seen that these two current can reach the brush without passing through coil 1.

Current collection at the commutator

That bar b has just left the brush and the short circuit of coil 1 has ended. It is now necessary for the current I reaching the brush from the right-hand side in the anticlockwise direction.

Current collection at the commutator

From the above discussion it is seen that during the period of short circuit of an armature coil by a brush the current in the coil must be reversed and also brought up to its full value in the reversed direction. The time of short circuit is called the Period of commutation.

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