A nine-arm modular multilevel converter (9A-MMC) for six-phase medium voltage motor drives

Dual three-phase (asymmetrical six-phase) induction motors have received significant attention in high power medium voltage drive applications. Typically, two three-phase voltage source converters are needed to feed such a drive system. In medium voltage applications, multilevel inverters are preferred as they provide a stepped output voltage waveform that reduces dv/dt stresses and provides a better output current waveform, which reflects positively on the machine developed torque and overall drive performance. Among different multilevel inverters, the Modular Multilevel Converter (MMC) has recently shown promise in medium and high voltage applications. For a dual three-phase machine, two MMCs with 6 arms each, are needed. This paper proposes a nine-arm MMC (9A-MMC) to drive such a motor. The proposed approach reduces the number of involved arms by 25%, i.e. reduces the number of required dc capacitors, semiconductor devices and their gate drive circuits, which reduces system complexity and cost. A complete analysis of the proposed system is presented and a simulation model is used to verify the proposed concept.