Motor cable effect on the converter fed AC motor common mode current

Analysis of conducted EMI in AC motor drives fed by pulse width modulated voltage 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. The levels of intensity of these phenomena depend noticeably on frequency dependant impedance - frequency characteristics of converter load, motor windings and feeding cable. An analysis of frequency converter load impedance characteristics allows for identification and determination of frequency ranges in which the foremost contributions to EMI noise generation have 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 modelled as a ladder circuit. The proposed circuit model allows for an analysis of the influence of the 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 were carried out for an exemplary adjustable speed AC motor drive application.