Reduction of common mode voltage of 2-level voltage source inverter-fed machine

Pulse width modulation (PWM) converter based variable-frequency drives to control AC machines have been widely used in motor drive system. However, it also leads to early bearing degradation in inverter driven machine due to the transient shaft voltage and high frequency bearing currents resulted from the PWM switching. To reduce the machine bearing current and extend the bearing lifetime, one possible solution is to reduce the common-mode (CM) voltage generated by the three-phase inverter. This paper aims to explore different PWM switching strategies to reduce the CM voltage of inverter. Various PWM techniques in reduction of common mode voltage are theoretically investigated. The conventional space vector PWM (SVPWM) is set as the baseline and other PWM methods including discontinuous PWM (DPWM), near state PWM (NSPWM) and active zero space PWM (AZSPWM) are compared with the baseline. In the study, in addition to the CMV characteristics, various practical performance characteristics such as voltage linearity range, inverter output current ripple, total harmonic distortion (THD), and output line-to-line voltage pulse pattern are also analyzed. NSPWM is concluded as the superior RCMV-PWM method since its smaller THD level.