Modulation Effects in an Inverter Fed Nine-Phase Induction Motor Drive

Using electricmotors with a number of phases greater than three present some advantages over conventional three-phase machines: higher reliability, reduced power per phase, increased torque density, and reduced torque pulsations. As the number of phases increases, the complexity of the modulation also increases due to a larger number of switching vectors and harmonic planes. A nine-phase inverter implies 512 switching vectors in each one of its four harmonic planes. In this article, a modulation technique for a nine-phase inverter based on a subset of the voltage vectors that yields a reduced number of ON/OFF transitions in a switching period is shown. The resulting harmonics and their effects in multiphase machines are analyzed. Third harmonic injection is considered from the perspective of both torque improvement and dc bus voltage utilization. Simulation, finite element, and experimental results are presented to support the authors' conclusions.