Subharmonic reduction for six step operation of three-phase inverter

The six step operation of an AC electric machine drive system under high speeds has a few advantages compared with the pulse width modulation (PWM) operation including the reduced switching loss, better utilization of DC bus voltage, and enhanced speed capability. The six step operation is generally applied when the fundamental electrical frequency is high. However, the low carrier-fundamental frequency ratio will make power electronics not able to switch on and off exactly at the zero crossing points of the phase voltage. This can result in a DC offset in the phase voltage and cause the subharmonic oscillation in the phase current. The subharmonic oscillation will create issues such as higher power loss, lower efficiency, and excessive heat that may permanently demagnetize permanent magnets on the rotor for a permanent magnet synchronous machine. This paper investigates a control method to significantly reduce the subharmonic current for the six step operation of a three-phase inverter. Simulation and experimental results demonstrate the algorithm can reduce the low frequency oscillation in the phase current by more than 90%.