A modular multilevel PWM inverter for medium-voltage motor drives

This paper presents a modular multilevel PWM inverter for medium-voltage motor drives, focusing on its control method and operating performance. It is especially suitable for fans, blowers, pumps, and compressors, the load torque of which is proportional to a square of rotating speed. Particular attention is paid to dc-capacitor voltage fluctuation of each chopper-cell because it may affect the voltage rating of power switching devices used. This paper derives theoretical equations related to an amount of the voltage fluctuation. A laboratory system 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-toline) PWM inverter intended for medium-voltage motor drives.