Startup and low-speed operation of an adjustable-speed motor driven by a modular multilevel cascade inverter (MMCI)

A modular multilevel cascade inverter based on double-star chopper-cells (MMCI-DSCC) is expected to be used as one of the next-generation medium-voltage PWM inverters suitable to motor drives for energy savings. This three-phase inverter is formed by six modular arms, each of which consists of a cascaded stack of multiple bidirectional chopper-cells. It suffers from ac-voltage fluctuation in the dc-capacitor voltage of each chopper-cell at low speed. The frequency of the fluctuation is equal to the stator-current frequency. This paper attempts to suppress the fluctuation by injecting a common-mode voltage of 45 Hz and circulating currents among the three legs. Experimental results obtained from a 400-V 15-kW downscaled system verify that stable operation is achieved at an ultra-low speed of 17 min^-1 with a load torque of τ_L= 40%, as well as “three-phase” dc-current feeding operation. Moreover, the motor can start up from a standstill without producing any overvoltage or overcurrent.