A speed-sensorless startup method of an induction motor driven by a modular multilevel cascade inverter (MMCI-DSCC)

The 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 mediumvoltage motor drives. This paper has theoretical and experimental discussions on a practical speed-sensorless startup method for an induction motor driven by the MMCI-DSCC from the standstill to a middle speed. This motor drive is suitable, especially for large-capacity fan-/blower-like loads, the torque of which is proportional to a square of the motor mechanical speed. Unlike the so-called “voltz-per-heltz” or “slip-frequency” controls, three-phase stator currents are based on “feedback” control, whereas their amplitude and frequency are based on “feedforward” control. Although the motor drive has no speed sensor attached to the motor shaft, this method makes a slow startup stable with the help of a stator-current feedback loop. Experimental results obtained from a 400-V, 15-kW downscaled system verify stable operating performance from the standstill to a middle speed of 588 min^-1 loaded with 60%.