Zero-cross Switch for the Starter of Single-phase Induction Motors

Abstract

A simple electronic starter is proposed for single-phase induction motors. Peak voltage _??_0.7V of a tiny series-resistor R, of main coil, triggers TRIAC on, in its reverse-trigger mode. At the peak 90° of sinusoidal voltage of R, current of auxiliary coil is already at 180° due to starting capacitor. So, the TRIAC is triggered in a narrow interval including the point 180° of auxiliary current, that is zero-cross switching. Thus, the auxiliary current is perfectly sinusoidal in each cycle, and so the starting characteristics are just the same as those of metal contacts. As rotation increases, main current decreases and stops triggering.