Power efficient motor starter and circuit breaker for an induction motor

Abstract

A motor starter and protection circuit is disclosed wherein a bimetallic contact support supports moveable contacts which are electrically interconnected with stationary contacts during normal non-overload conditions. Current from the power support passes through the stationary moveable contact sets as well as the bimetallic contact support and through the main coil of the motor. A positive temperature coefficient resistance is mounted on the contact support in electrical interconnection therewith. Current passes through this resistance, a TRIAC, and the motor starting winding. Conduction of the TRIAC is controlled by sensing the current which passes through the main motor coil. Only when the current passing through the main coil is substantially greater than normal load current is the TRIAC gated into conduction. A phase control capacitance is interposed between the coil sensing current through the main coil of the motor and the TRIAC gate. This capacitance ensures substantially complete conduction of the TRIAC when gated ON. The mounting of the positive temperature coefficient resistance directly on the contact support ensures good thermal transfer of the heat generated by this resistance and also maintains this resistance heated during normal operation after starting due to heat developed within the bimetallic contact support.