A novel variable-frequency three-phase induction motor drive system using only three controlled switches

This paper presents a novel variable-frequency motor drive system for a three-phase induction machine. In this drive system, the machine is excited by unidirectional stator currents shaped and sequenced such that a rotating air-gap flux is induced so that the machine can operate. Due to the unidirectional nature of the currents, an "inverter" that incorporates only three active, insulated-gate-bipolar-transistor-type switches, and three freewheeling diodes can synthesize them. The drive system requires an asymmetric induction machine that can easily be obtained by externally rewiring a motor with a dual-wound stator. The inverter is supplied by a single-phase input via a voltage-doubler diode rectifier; hence, it can be operated with residential service. In this paper, theoretical control methods are presented and both simulation and experimental results are provided proving the validity of the proposed drive system.