New optimal common-mode modulation for three-phase inverters with DC-link referenced output filter

A two-level inverter followed by a DC-link referenced output filter is a promising solution for high-speed low-voltage drive systems. A limitation of this power electronics topology, however lies in the high current ripple and hence additional losses occurring within the filter inductors. This shortcoming can be addressed by means of an appropriate modulation technique. This paper details two new modulation strategies, tailored to the specific characteristics of three-phase inverters with DC-link referenced output filter, utilizing the instantaneous common-mode (CM) voltage as a degree of freedom, in order to decrease the output inductor's current ripple. It is deduced that a constant (DC) CM injection modulation (DCCMM) or an AC CM injection modulation (ACCMM) with arbitrarily large amplitude yields the best performance depending on the operating point. The CM pattern that minimizes the inductor current ripple envelope results from the combination of the two schemes and constitutes the optimal CM injection modulation (OCMM). Furthermore, the proposed modulation schemes are tested on a 300 W hardware demonstrator driving a 300 krpm custom designed motor, where a reduction of the total inverter losses of up to 11% is observed.