A new PWM strategy for common-mode voltage reduction in neutral-point-clamped inverter-fed AC motor drives

In this paper, the relationship between common-mode voltage and switching states in a neutral-point-clamped (NPC) inverter is clarified, and a new pulsewidth-modulation (PWM) strategy for reducing the common-mode voltage in an NPC inverter-fed AC motor drives is proposed. Among a total of 27 switching states in the NPC inverter, the proposed PWM uses only 19 switching states that generate common-mode voltage of which the magnitude is the same or less than one-sixth of DC-bus voltage. Moreover, the proposed PWM strategy satisfied the constraint that the output voltage vector should be changed by only one switching action. Not requiring extra hardware, the NPC inverter with the proposed PWM results in a remarkable reduction in peak-to-peak of common-mode voltage, compared to a two-level inverter and an NPC inverter controlled by conventional PWM. The proposed PWM strategy can be easily implemented in software without degradation of control performance in AC motor drives.