Third Harmonic Injection SPWM Method Based on Alternating Carrier Polarity to Suppress the Common Mode Voltage

In this paper, a third harmonic injection sinusoidal pulsewidth modulation (THISPWM) method based on alternating carrier polarity (ACP) is proposed for three-phase PWM inverter. Compared with the space vector pulsewidth modulation (SVPWM), the THISPWM has the advantage of lower low-frequency common mode voltage (CMV). However, the phase of the third harmonic injected must be strictly synchronous with the phase of the reference voltage. The synchronization will cause the difficulty of the injected third harmonic generation. First, the relationship between the injected third harmonic and the reference voltage of the PWM inverter output is analyzed to obtain a real-time conversion method, thereby solving the synchronization problem. Then, a modulation method based on ACP is proposed to eliminate zero switch states of the PWM inverter. The middle amplitude-modulated voltage is used to select the negative polarity carrier. In addition, the low-frequency and high-frequency CMV, which are separately defined according to the modulated fundamental voltage frequency and the carrier frequency, are taken into account. Therefore, the analysis results manifest that the proposed method could suppress both the low-frequency and high-frequency CMVs and has the same voltage utilization as the SVPWM. Finally, a three-phase PWM inverter circuit based on an FPGA was built, and the experimental results verified the proposed method.