Analysis And Suppression Of Common-mode Voltage In PWM Varied Frequency Supply Power System

With the development of power electronic technology, the comprehensive performance of PWM inverter has been greatly improved, and variable frequency drive systems have been widely applied in industry. However, the switch elements and PWM modulating technique induce many negative effects on driving systems. Coupling with motor’s stray capacitances, the common mode voltage of the output of PWM inverter could induce shaft voltage and severe bearing current problems. Therefore, the analysis on common mode voltage mechanism and the effective suppression method on common mode voltage are extremely important to the reliability of variable frequency drive systems.Firstly, the common mode voltages of SPWM and SVPWM inverters are analyzed. The characteristics and influence factors of common mode voltage are investigated through three different methods, analytical solution, MATLAB simulation and experiment. It can be concluded that harmonic components of common mode voltage are mainly concentrated at the integer times of the carrier frequency, and which reach a maximum at carrier frequency; with the increasing utilization of DC voltage, common mode voltage effective value decreases, and also the effective value of the common mode voltage component at the carrier frequency decreases; with the increase of the carrier frequency, common mode voltage effective value changes a little, and the effective value at the carrier frequency doesn’t change significantly; with the increasing of dead time, the effective value of common mode voltage also increases and the effective value of the harmonic component on the carrier frequency increases. Under the same utilization of DC voltage, common mode voltage RMS and component at the carrier frequency of SPWM and SVPWM were similar, but the biggest utilization of DC voltage of SVPWM is 15% higher than SPWM. Considering the efficiency of electricity utilization, SVPWM should be chosen in the first place. Then the suppression method of common mode voltage is investigated. Through AZSPWM1 modulation strategy the common mode voltage is suppressed significantly. The effectiveness of this method is testified by simulation and experiment. The problem in AZSPWM1 modulation method is improved, and the simulation result shows that the scheme can eliminate common mode voltage spike caused by the dead time. Finally, the inverter-induction motor experimental platforms are established. The hardware and software designs of the inverter are introduced and the measurement scheme of common mode voltage is illustrated.