A Speed-Sensorless Start-Up Method for an Induction Motor Driven by a Modular Multilevel Cascade Inverter Based on Double-Star Chopper Cells (MMCI-DSCC)

A modular multilevel cascade inverter based on double-star chopper cells (MMCI-DSCC) is expected as one of the next-generation multilevel PWM inverters for medium-voltage motor drives. This paper presents theoretical and experimental discussions on a practical speed-sensorless start-up method for an induction motor driven by the MMCI-DSCC from a standstill to a middle speed. This motor drive is suitable, especially for a large-capacity fan- or blower-like load, the torque of which is proportional to a square of the motor mechanical speed. Unlike the so-called “volts-per-hertz” control, 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 start-up stable with the help of a stator current-feedback loop. Experimental results obtained from a 400-V, 15-kW downscaled system verify the stable operating performance from a standstill to a middle speed of 590min-1 with a 40% load torque.