A novel modulation method in a four-level inverter to eliminate bearing currents

Among the voltage vectors of even level inverters there is no voltage vector with zero common mode voltage. Therefore bearing currents, which may cause premature motor bearing failures, will not be eliminated due to generated common mode voltages. To eliminate these currents in even level inverters, instead of elimination of common mode voltage, the main idea is based on an extreme increment of the parasitic coupling impedance through which currents are flying. This increment of the impedance is resulted by reducing frequency of common mode voltage to zero. To achieving this aim, only voltage vectors with an equal common mode voltage are used. All calculations for switching between these vectors have been done with two independent methods. Comparison between these methods has shown that none of them has the absolute prominence.