Strategy With Smooth Transitions and Improved Torque–Speed Region and Stator Copper Loss for Two-Level Asymmetrical Six-Phase Induction Motor Drives Under Switch Faults

Abstract

Asymmetrical six-phase drives are very attractive to obtain fault tolerance. Most faults arise in the power electronics. Two conventional methods to handle switch/diode faults in two-level converters are keeping the corresponding phases open (in two-level multiphase drives) or connecting them to the dc-link midpoint (in two-level three-phase drives) through a bidirectional switch (e.g., triac/relay). However, the former yields larger stator copper loss (SCL) for given torque and smaller maximum torque, because of current constraints; on the other hand, the latter reduces the maximum speed, due to voltage constraints, and has not been considered for two-level multiphase drives. This article presents several proposals to improve fault-tolerant behavior in two-level asymmetrical six-phase induction motor drives. First, to adequately combine and alternate said two approaches in these particular drives, depending on the speed, neutral-point configuration, and faulty legs. In this manner, the SCL-per-torque decreases and the total torque–speed region of the drive is extended, i.e., the maximum torque and maximum speed rise compared with each of the aforementioned conventional strategies. Second, to also turn on / off a switch between stator neutral points for further improvement in this regard. Third, a pulsewidth modulation procedure to ensure smooth transitions between drive configurations. Experimental results confirm the theory.