Novel PWM strategy for direct self-control of inverter-fed induction motors

Direct self-control (DSC) of an inverter-fed induction machine is a signal processing technique based on two-level hysteresis control of the machine flux and torque. The advantages of DSC are simplicity, requiring no field coordinate transformations, and high dynamic performance even with high power, low switching frequency converters. One disadvantage, however, is that the output inverter current is not sinusoidal, containing harmonic components typical of those in six-pulse square-wave inverters. In this paper, a novel PWM strategy is proposed for inverter-fed induction machines with DSC which reduces distortion in the inverter input and output currents. This is achieved by introducing 12 sectors into the stator flux alpha - beta plane in which stator voltage vectors are chosen which are two steps in advance (two-forward) or behind (two-backward) relative to the DSC-derived vectors, instead of zero-vector switching. The method requires no additional vector switchings to form a stator flux trajectory closer to circular compared with the classic DSC flux hexagon. Moreover, the technique does not require elaborate pre-processing or online calculation, and is based on logic circuits and an EPROM look-up table.