A PWM method for seven- and nine-phase open-end winding motor drives

Although three-phase open-end winding drives have been under investigation for two decades, there is hardly any work available related to multiphase (more than three phases) open-end winding drive structures. This paper extends one possible PWM technique, recently introduced in conjunction with five-phase drives, to seven- and nine-phase open-end winding structures. Both multiphase inverters are two-level and require two isolated dc sources of equal voltage values. The technique, termed unequal reference sharing (URS), is explained and subsequently applied in simulations to examine the performance of the drive systems. Benchmarking is performed with respect to the equivalent single-sided supply with a two-level inverter and with respect to another PWM method recently developed for the dual-inverter supply structure. This enables a comparison of the achievable performance in single-sided and dual-inverter supply modes. Simulation results are given for both phase numbers. The THDs, obtained in this manner, are compared throughout the modulation index range for both phase numbers as well. The simulation results are verified experimentally on a seven-phase laboratory set-up in the whole linear modulation index range.