A Medium-Voltage Motor Drive With a Modular Multilevel PWM Inverter

This paper describes the control and operating performance of a modular multilevel PWM inverter for a transformer-less medium-voltage motor drive. The inverter is prominent in the modular arm structure consisting of a cascaded stack of multiple bidirectional chopper-cells. The dominant ac-voltage fluctuation with the same frequency as the motor (inverter) frequency occurs across the dc capacitor of each chopper-cell. The magnitude of the voltage fluctuation is inversely proportional to the motor frequency. This paper achieves theoretical analysis on the voltage fluctuation, leading to system design. A downscaled model rated at 400 V and 15 kW is designed and built up to confirm the validity and effectiveness of the nine-level (17-level in line-to-line) PWM inverter for a medium-voltage motor drive.