Protection of Motor from Switching Surge by Vacuum Switch

A way of surge suppression by a reactor is presented for the protection of turn-to-turn insulation of motors from steep front surges produced by vacuum switch. In the first part of this paper, the construction and the functions of the reactor are explained. Discussions are given on the possibilities of turn-to-turn faults by the switching surges under the effect of the reactor, taking voltage distribution in motor winding into account. The result is expressed as the probability of turn-to-turn fault per switching operation for a given circuit condition. Examples of numerical evaluation of probability are obtained for a circuit breaker with average chopping current of 4A and for a contactor with average chopping current of 1 A.