Broadband modeling of motor cable impact on common mode currents in VFD

Examination of conducted EMI generation and propagation in AC motor drives fed by frequency converter requires to consider parasitic capacitances in converters, motor windings and feeding cables to be taken into account. AC motor winding voltage transients and related common mode currents are significantly correlated with resonance effects occurring in converter´s load circuits. Intensity levels of these phenomena depend significantly on impedance - frequency characteristics of converter´s load, motor windings and feeding cable. An analysis of converter´s load impedance in frequency domain allows for identification and determination of frequency ranges in which the foremost contributions to EMI noise generation have voltage ringing phenomena associated with 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 using a ladder circuit. The proposed circuit model allows for an analysis of the influence of the motor feeding cable parameters on generated common mode current spectra in AC motor drive, particularly in AC motor itself. Obtained simulation results based on the proposed ladder circuit model are verified by the experimental investigations which were carried out for an exemplary AC motor drive.