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A new approach to improve the current harmonic content on open-end winding AC motors supplied by multi-level inverters

In open-end winding AC motor drives, terminals of stator phases are normally connected between two inverters, normally, of the same type and size. In the present paper, an alternative configuration is studied where the machine is supplied exploiting two different converters: a main multi-level inverter and an auxiliary two-level inverter. The last operates as active power filter in order to eliminate undesired low-frequency voltage harmonics produced by the multi-level modulation process. Furthermore, the two level inverter is operated at a rather low voltage, thus it can be equipped with power MOSFETs. Consequently, a voltage PWM at a remarkably higher frequency is allowed, if compared with standard multilevel inverters. According to the proposed approach, the phase current harmonic content is remarkably reduced, the torque ripple is mitigated while the drive efficiency is increased. Numerical simulations and experimental results confirm the consistency of such new methodology. A particular emphasis is given to stator current harmonic content and power losses analysis as well as to specific implementation issues.