Protection circuit for three-phase motor - uses multivibrator operating relay to disconnect motor if one phase or each connection fails

摘要

The protection circuit disconnects a 3-phase motor from its mains supply when one phase or earth connection fails. The phase conductors are connected to the primary windings of subtransformers, whose secondaries are coupled via rectifiers to a circuit that controls the protection switch. The first outputs of the rectifiers are each connected to the base of a transistor. These 3 transistors are all in collector/emitter parallel with their collectors connected to a fourth transistor controlled by the shared second outputs of the rectifiers. The fourth transistor's output controls a multivibrator that operates a protection relay.