Single phase to three phase converter capable of variable speed motor

Abstract

A converter, for providing three phase power to a three phase induction motor from a single phase supply, includes an inverter connected to the single phase input power lines which has two switching devices, and a bi-directional switch connected between one of the single phase input power and one output line to the motor. The other two outputs provided to the motor are a direct connection from the other AC input line and the output of the inverter. In a first or start-up mode of operation, the bi-directional switch and the inverter are controlled to provide currents to the motor which are at a fundamental frequency lower than the single phase AC input frequency to provide controlled start-up of the motor with limited in-rush current. After the motor has started, the converter may be switched to a second mode wherein the bi-directional switch is turned off and the inverter provides single phase power at increasing frequency to the motor to drive the motor up in speed. In a third mode, where the motor is at or near its rated speed, the hi-directional switch and the inverter may be operated to provide power at the rated frequency to the motor, but at variable and controllable voltage levels to control the speed of the motor. In a fourth mode, at or near the rated speed of the motor, the bi-directional switch is controlled to conduct constantly, and the inverter is operated to provide balanced three phase voltages to the motor. The converter may be implemented with only two switching devices in the inverter and one additional bi-directional switch.