Efficiency-optimized control of medium-size induction motor drives

The efficiency of a variable-speed induction motor drive can be optimized by adaptation of the motor flux level to the load torque. In small drives (< 10 kW), this can be done without considering the relatively small converter losses, but for medium-size drives (10-1000 kW) the losses cannot be disregarded without further analysis. The importance of the converter losses on efficiency optimization in medium-size drives is analyzed in this paper. Based on the experiments with a 90-kW drive, it is found that it is not critical if the converter losses are neglected in the control, except that the robustness toward load disturbances may unnecessarily be reduced. Both displacement power factor and model-based efficiency-optimizing control methods perform well in medium-size drives. The last strategy is also tested on a 22-kW drive with good results.