In depth study of two solutions for common mode current reduction in six-phase machine drive inverters


Abstract: Common mode current (CMC) destroy machine bearings in the long run and increase electromagnetic interference. According to standards, the RMS value of CMC must be lower than 0.3A. Theories show that CMC is originated by applying zero states to the inverter. In this paper, the performance of two solutions in reducing CMC for six-phase machines is investigated. In the first solution, the traditional six-phase inverter is modified by adding two serial power switches on its input terminal. This method is an extended version of a method that has been presented for three-phase inverters, reduces CMC by optimizing the circuit structure. In the second solution, a new space vector modulation (SVM) strategy is proposed, and the inverter topology remains unchanged. In the first method, the modified inverter disconnects from its DC-bus whenever zero vectors are applied, while in the second method, zero vector times are modified. Experimental results evaluate the performance of these two approaches to diminishing the CMC problem. For this purpose, a 380V/5.5kW prototype has been prepared. According to the results, the first solution decreases CMC by 75% below the standard value, and the second method decreases CMC by 45%. Efficiency analysis indicates that the topology modification plan is useful for low-power applications, while modulation improvement can be used for high-power applications due to lower power losses.

Keywords: Six-phase, inverter, space vector modulation, common mode current, common mode voltage

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