Minimization and cancellation of common-mode currents, shaft voltages and bearing currents for induction motor drives

This paper presents the theoretical analyses and experimental comparisons of different solutions aimed at minimizing and canceling common-mode (CM) currents, shaft voltages and bearing currents. In particular, it discusses the use of CM chokes for minimizing CM currents; illustrates the use of a shaft-grounding method combined with a CM choke; analyzes the effectiveness of the "passive" method of the CM transformer (CMX); and evaluates the "active" method of the CMX that effectively cancels the CM voltages (and thus simultaneously the CM currents, shaft voltages and bearing currents). The latter is the best solution to both CM (EMI) and bearing current problems at the expense of being the most costly. Experimental results are presented to substantiate the theoretical analyses.