DC-bus utilisation in series-connected multi-phase machines supplied from a VSI with a composite phase number

Multiphase series-connected motor drives can be supplied from a two-level voltage source inverter (VSI), which has either a prime or a composite number of legs (phases). It has been shown recently that, in the case of a prime number of inverter/machine phases, limits of the linear PWM region get extended over those valid for a particular phase number due to the phase transposition applied in the series connection. A study of the dc bus voltage utilisation in the linear modulation region is reported in this paper for series-connected multiphase drives with a composite phase number. The limits are independent of the actual PWM technique used and are equally applicable to both carrier-based and space vector PWM techniques. A detailed simulation and experimental investigation is conducted using a symmetrical six-phase VSI, which supplies series connection of a symmetrical six-phase and a three-phase machine. The results thus obtained fully verify theoretical considerations.