DC bus grounding capacitance optimizatio for common-mode EMI minimization

This paper analyzes the effects of dc bus grounding capacitance on the input common-mode (CM) electromagnetic interference (EMI) currents in three-phase motor drive systems using uncontrolled or PWM rectifier as the front end. The dc bus grounding capacitance considered includes the parasitic capacitance of the bus bar and semiconductor devices, as well as capacitors that are purposely added between both positive and negative bus bars to the ground. Simple CM EMI models are presented for both cases to provide a basis for analysis. The dc bus grounding capacitance is shown to decrease the input CM current with diode rectifier as the front end. With a PWM rectifier as the front end, however, the dc bus grounding capacitance is shown to increase the CM current generated by the rectifier while decreasing the CM currents generated by the inverter. The bus grounding capacitance can be optimized to minimize the overall CM current emission from the motor drive systems. In both cases, large grounding capacitance may lead to CM resonance that adds to CM current emission and many manifest itself into a low-frequency power quality problem. Experimental measurements are presented to support the analyses.