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A four-level inverter drive could be obtained by feeding an open-end winding induction motor by two two-level inverters from either side with unequal dc-link voltages, which are in the ratio of 2:1. This inverter modulation scheme is capable of producing 64 space-vector combinations. Some of the space vector combinations could result in the overcharging of the dc-link capacitor corresponding to the inverter operating with the lower voltage. In this paper, a new power circuit topology is proposed, in which, a rectifier-inverter combination is nested within a conventional two-level inverter configuration. The output of the conventional two-level inverter feeds one end of the open-end winding induction motor, while the output of the nested rectifier-inverter combination feeds the other end. The total dc-link voltage needed in the proposed topology is only 77% of the one needed in the conventional multilevel inverters. Also, the problem of zero-sequence current, commonly encountered in open-end winding induction motor drives, is avoided by resorting to a decoupled space vector modulation PWM scheme, which eliminates the zero-sequence voltage of the dual-inverter system by forcing the sampled average of the zero sequence voltage of individual inverters to zero.
Date of Publication: May 2013