Starting-current limiting device for single-phase induction motors used

Abstract

Disclosed is a starting current limiting device for single-phase induction motors, driven by an AC power supply, used in household electrical equipment. The device is adapted to provide, as the power switch of the induction motor is just turned on, a low and gradually increasing starting current flowing into the induction motor. The device is capable of cutting some portions of each cycle in the waveform of the AC power off such that the average voltage input to the single-phase induction motor is reduced, and/or converting some portions of each cycle to oscillate in the reverse directions such that the waveform of the AC power input to the single-phase induction motor appears to have a lower frequency. The device is provided with a switching circuit including a plurality of thyristors, which can be silicon controlled rectifiers or triacs. A means is provided for generating the triggering signals to fire the thyristors, the triggering signals being predetermined and their timing is controlled by a sequence of timing signals. The sequence of timing signals is generated by a timing signal generator capable of generating a single timing signal every time the AC voltage has a zero-crossing. When the current flowing into the induction motor has reached a predetermined percentage of the rating current, the device would switch off the switching circuit and connect the induction motor directly to the AC power supply.