Three=phase motor starter circuit - uses thyristors as starter impedances varying their firing time in dependence on line phase current drawn by stator windings

摘要

A starter circuit for a three phase a synchronous motor (10) with Squirrel cage rotor comprises a respective pair of thyristors arranged head to tail in series with the respective motor phase windings and controlled at a function of the phase currents by respective firing multivibrators in turn controlled by bistable switches. A synchronisation circuit applies a respective synchronising pulse at the zero crossing of each phase half cycle directly to the respective bistable switch and also controls the application of a sampled integrated signal to the switch. The signal is derived through a rectifier from a current transformer in the phase line. The sampler includes a capacitor charged by the integrator under control of the synchronising pulses and a comparator which applies a setting signal to the bistable switch in dependence on the capacitor charge, the direct synchronising pulses resetting the switch.