Mode Transformation of EMI Noise Due to Unbalanced Filter Capacitors in Three-Phase Motor Drive Systems

This paper deals with the transformation from differential to common mode noise due to slightly unbalanced filter capacitors in a three-phase motor drive system. Indeed, filter components, i.e., inductors and capacitors, can be slightly different due to the tolerance of commercial products usually varying from 5% to 20% Such components discrepancies result in noise transformation from differential to common mode. This phenomenon is clearly evident in experimental setups where differential mode resonances can be observed in the common mode current. In this paper, noise transformation is described in terms of an approximate equivalent circuit based on the symmetrical components transformation. The equivalent circuit derived in the paper allows straightforward analytical evaluation of the contribution to the common-mode current ascribed to the discrepancies of the filter capacitances with respect to the nominal symmetrical value.