A Modified Carrier-Based PWM Strategy for Common Mode Voltage Elimination and Neutral Point Voltage Balance in a Unidirectional Three-Level Converter for AC Motor Drives

High-frequency and high-magnitude common mode voltage (CMV) generated by the operation of transformerless multilevel converters is one of the main causes for decreasing the lifespan of ac motors. In this article, the principle of CMV generated by rectifier and inverter stages of a recently proposed transformerless unidirectional three-level converter for ac motor drives is studied, a modified carrier-based pulsewidth modulation strategy for both CMV elimination and neutral point (NP) voltage balance is proposed. The strategy can effectively eliminate the CMV of the system by modifying the pulse voltage generation and replacing the phase of the carrier signal of the two active switches in the three-level power circuit according to the polarity of each phase current. Moreover, by superposing the NP voltage balance control signal onto the reference voltage of those selected active switches (whose switching action has no impact on the CMV) in the rectifier stage, the NP voltage balance control is integrated into the CMV elimination. The merit of this method is that it does not require the complex calculation of phase shift position and has little impact on the multilevel voltages of the rectifier and inverter stages. The experiments verified the effectiveness of the method.