Electrical Engineering ⇒ Topic : Commutation and its types

Commutation means the process of current collection by the brush or the changes which take place in a coil during the period of short-circuit by a brush. The period during which this happens is called the commutation period Fig. (x).

                                                                       figure (x)              

Two methods are available for improving commutation, i.e., of making the current in the short-circuited coil attain its full value in the reverse direction by the end of the short-circuit period. These are :

1. Resistance commutation. In this method the low-resistance copper brushes are replaced by comparatively high resistance carbon brushes.

2 .e.m.f. Commutation. In this method an arrangement is made to neutralise the reactance voltage by producing a reversing e.m.f. in the short-circuited coil under commutation. The reversing e.m.f. may be produced by the following two methods :

• By giving the brush a forward lead sufficient enough to bring the short-circuited coil under the influence of next pole of opposite polarity.
• By using interpoles or compoles