Influence of power shielded cable and ground on distribution of common mode currents flowing in variable-speed AC motor drive systems

Parasitic common mode (CM) currents unavoidably generated by power converters, in particular PWM inverters, which are widely used in variable-speed AC motor drives in many industrial applications, causes many Electromagnetic Compatibility (EMC) problems. In order to reasonably solve these problems, the understanding of behavior and the knowledge of propagation path of CM currents flowing through the system are indispensable. This paper presents an investigation on influence of power shielded cable length and its position in comparison to ground and on ground effect. These two elements are very important to make CM currents flowing to other parts of the system, especially through the motor. Measurement of the proportion of CM currents returning to CM voltage source via ground and cable shield connected at motor terminal in various configurations, in frequency domain, is carried out to illustrate which propagation path is preferred in the considered frequency range. This allows us to apply an appropriate method to restrict or eliminate CM currents in the desired frequency range. An analytical model corresponding to this experiment will be also established; its results will be compared with the measured ones. The validated model could be later used to predict proportion of CM currents distributed in the system for other configurations.