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 neutral-point clamped (NPC) inverters is clarified, and a new pulse-width modulation (PWM) strategy for reducing the common mode voltage in NPC inverter-fed AC motor drives is proposed. Among total twenty seven switching states in NPC inverter, the proposed PWM uses only nineteen switching states that generates common mode voltage of which magnitude is same or less than one-sixth of DC bus voltage. Moreover, the proposed PWM strategy satisfies the constraint such 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 the remarkable reduction in peak-to-peak of common mode voltage, compared to two-level inverter and 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