Multiphase Electric Machines for Variable-Speed Applications

Although the concept of variable-speed drives, based on utilization of multiphase machines, dates back to the late 1960s, it was not until the mid- to late 1990s that multiphase drives became serious contenders for various applications. These include electric ship propulsion, locomotive traction, electric and hybrid electric vehicles, ldquomore-electricrdquo aircraft, and high-power industrial applications. As a consequence, there has been a substantial increase in the interest for such drive systems worldwide, resulting in a huge volume of work published during the last ten years. An attempt is made in this paper to provide a brief review of the current state of the art in the area. After addressing the reasons for potential use of multiphase rather than three-phase drives and the available approaches to multiphase machine designs, various control schemes are surveyed. This is followed by a discussion of the multiphase voltage source inverter control. Various possibilities for the use of additional degrees of freedom that exist in multiphase machines are further elaborated. Finally, multiphase machine applications in electric energy generation are addressed.

I. Introduction

Variable-Speed ac drives are nowadays invariably supplied from power electronic converters. Since the converter can be viewed as an interface that decouples three-phase mains from the machine, the number of machine's phases is not limited to three any more. Nevertheless, three-phase machines are customarily adopted for variable speed applications due to the wide off-the-shelf availability of both machines and converters. Such a situation is expected to persist in the future and multiphase variable speed drive utilization is always likely to remain restricted to specialized niche applications where for one reason or the other, a three-phase drive does not satisfy the specification or is not available off-the-shelf either.