Modeling of common mode currents induced by motor cable in converter fed AC motor drives

Investigation of conducted EMI generation in AC motor fed by pulse width modulated frequency converters requires to consider parasitic capacitances in converters, motor windings and feeding cables to be taken into account. Motor voltage transients and related common mode currents are significantly correlated with resonance effects occurring in load circuits. An analysis of frequency converter load impedance-frequency characteristics allows for identification and determination of frequency ranges in which the foremost contributions to EMI noise generation have the voltage ringing phenomena associated with the load parasitic capacitances. This paper presents a method to model an AC motor with a feeding cable in conducted EMI frequency range up to 30 MHz. Distributed parasitic capacitances of AC motor windings are modeled by a ladder circuit. The proposed circuit model allows for an analysis of the influence of motor feeding cable parameters on common mode currents generated in AC motor drive system, particularly in AC motor itself. The simulation results obtained based on the proposed ladder circuit model are verified by the experimental tests which have been carried out for an exemplary adjustable speed AC motor drive application.