A solid state contactor for use in 60 Hz, 3 ϕ service

A solid-state step-start contactor using back-to-back thyristors has been developed at NAL. The unit described here employs water-cooled thyristors designed to switch 3 phase 500A rms at 480V, 60 Hz. The thyristors are turned on by a rectified 50 kHz carrier signal transmitted to the thyristor gate circuit via a specially developed high-isolation transformer that prevents the impulse noise on the 480V line from affecting the logic-level control circuits. A 3-phase current sensor and a solid-state interlock and status-monitor display unit form an integral part of the contactor. DTL compatible logic-level signals are used to control the contactor from local or remote positions and to display and monitor the status of the interlocked variables. These principles appear to be quite applicable to contactors designed for lower or higher power levels.