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Operating a dual three-level inverter system from a single dc link circuit is an economic solution for generating five-level output voltage waveforms and to double the output power compared to a single three-level inverter. Such an inverter system is used for feeding a drive motor with open stator windings. A mechanism is then required to reduce the common-mode voltage components of the motor voltages. This paper describes an offline optimization method that minimizes both the harmonic distortion of the motor currents and the common-mode voltage components. The optimization permits reduction of the switching frequency to a very low value of 200 Hz without compromising the harmonic distortion. High performance operation of the drive system at low switching frequency is experimentally demonstrated.Index Terms-Common-mode voltage, medium-voltage ac drives, multilevel inverters, optimal pulsewidth modulation (PWM), total harmonic distortion (THD).

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