Low-Frequency Current Oscillation Reduction for Six-Step Operation of Three-Phase Inverters

This paper investigates a control method to significantly reduce the low-frequency current oscillation for six-step operation of a three-phase inverter. The six-step operation of an ac electric machine drive system under high speeds has a few advantages compared with pulse width modulation (PWM) operation including reduced switching losses, a better utilization of the dc bus voltage, and an enhanced speed capability. In digital implementation of the six-step control, power electronics may not be able to switchonandoffexactly at the zero crossing points of reference voltages, which will result in a dc offset in phase voltages and cause a low-frequency oscillation in phase currents. The low-frequency oscillation will create issues such as higher power losses, lower efficiency, and excessive heat that may demagnetize magnets on the rotor for a permanent magnet synchronous machine. This research presents a control algorithm using asymmetric PWM to ensure that power electronics switchonandoffexactly at the reference voltage zero crossing points. Both simulation and experimental results demonstrate the algorithm can reduce the low-frequency oscillation in phase currents by more than 90%.