Abstract: An efficiency control system for an induction motor system includes an induction motor, a power transducer connected to the induction motor, and a detector adapted for separate real-time detection of a voltage signal, a current signal, and a frequency signal of electric power supplied from the power transducer to the induction motor so that an output of the induction motor during operation may be controlled by these signals in response to a load torque. The efficiency control system includes an actual slip signal processor for calculating an actual slip of the induction motor during operation on the basis of a motor constant and the voltage signal, current signal, and frequency signal of the electric power supplied from the power transducer to the induction motor. The efficiency control system further includes an optimum slip signal processor for calculating an optimum slip desired to obtain the maximum efficiency of the induction motor.