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High speed control of induction machines

Square wave inverters are the most feasible choice for high speed and high power applications. However, for a square wave inverter, the harmonics problem becomes more severe specifically, at the lower frequencies below the base speed, where the harmonics current is increased which significantly decreases the machine efficiency. Another problem with the square wave inverter at low frequency is that if volt per hertz speed control is used, it becomes difficult to control the voltage level for maintaining a constant flux operation below the base speed. Therefore, this paper designs a speed controller with a hybrid approach in which a Sine Triangle Pulse Width Modulated (SPWM) inverter is used below the base speed and a square wave inverter is employed above the base speed. The transition between the SPWM and square wave is realized in a unique way, which is by simply setting the frequency of the carrier signal equal to zero. Thus, the proposed method in a way uses only SPWM and the transition from the SPWM to square wave is accomplished in a seem-less manner. Therefore, the torque discontinuity problem which normally appears in the traditional method where there is a switching between SPWM and square wave does not occur in this proposed strategy. Computer simulation results for a V/f drive system are presented to validate the performance of the proposed algorithm.