Zero-/Low-Speed Operation of Multiphase Drive Systems with Modular Multilevel Converters

Modular Multilevel Converter (MMC)-based AC motor drives are emerging trend-research. They ease medium-/high-voltage handling in high-power applications with reduced footprint and life-cycle costs. A known challenge concerning their operation is the zero-/low-speed operating condition. The recent techniques, proposed to improve operation during that interval, implement complex hardware/software approaches. To cope with this issue, this work proposes multiphase machines for MMC-based AC-drives. Among several advantages regarding the power splitting, multiphase machines provide additional degrees of freedom compared to their three-phase counterparts. A novel exploitation to these additional degrees of freedom is proposed in this paper by injecting a secondary current component in the load current with specific magnitude and frequency during zero-/low-speed intervals enabling the motor to function duly. Since the control of these secondary components is already inherited in the current controller structure of any multiphase machine, no additional algorithms or sensors will be required. In this paper, a three-level five-phase MMC-based distributed winding Induction Machine (IM) drive system for medium-/high-voltage applications is investigated during low-speed operation as well as starting from standstill to rated speed. Two different control options are proposed using capacitor voltage measurements or under sensorless operation in order to reduce the cost and complexity in case of a high number of MMC-levels. Experimental results have been obtained with a downscaled drive system to verify the proposed solution.