A Speed-Sensorless Startup of an Induction Motor Driven by a Modular Multilevel Cascade Inverter (MMCI-DSCC) -Applications to Quadratic-Torque Loads Like Fans, Blowers, and Compressors

A modular multilevel cascade inverter based on double-star chopper-cells (MMCI-DSCC) has been expected as one of the next-generation multilevel PWM inverters for medium-voltage high-power motor drives. This paper has theoretical and experimental discussions on a speed-sensorless startup of an induction motor driven by the MMCI-DSCC, especially when this motor drive is applied to fans, blowers, and compressors, the torque of which is proportional to a square of a motor mechanical speed. A theoretical analysis focusing on ac-voltage fluctuation of each dc capacitor indicates that the ac-voltage fluctuation can be kept within an acceptable level over a wide range of motor mechanical speeds by controlling the three-phase stator currents. Experimental steady-state waveforms obtained from a downscaled motor drive verify operating performance with a quadratic-torque load.