Torque capabilities of a five-phase PM BLDC drive vs. a three-phase one and various supply modes

Electric drives with a three-phase structure are commonly utilized in road vehicles for propulsion and in aeronautics as control-surface actuators. In recent times, some concerns have emerged on the use of three-phase drives in such applications in regard to power capability, efficiency and fault-tolerance. A way to effectively cope with these concerns is the adoption of multiphase drives. In this sense, a five-phase drive is a promising solution as it does not imply an excessively complicated structure. In this paper a five-phase PM BLDC drive is considered and its torque capabilities are investigated for square-wave current supply. Specifically, the torque developed by a five-phase drive is first compared to the three-phase counterpart; torque comparison is carried out by keeping motor size and loading unchanged and by considering two hypotheses: equal phase back-emf and equal phase rms current. Then the torque available from a five-phase drive is determined under various supply modes, characterized by the simultaneous conduction of a reduced number of phases; the torque available is determined by imposing an equal rms phase current in the supply modes.