Optimal pulsewidth modulation of a dual three-level inverter system operated from a single dc link

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 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 off-line optimization method that minimizes both the harmonic distortion of the motor currents and the common mode voltage components. The optimization permits reducing the switching frequency to a very low value of 200 Hz without compromising on harmonic distortion. High performance operation of the drive system at low switching frequency is experimentally demonstrated.