Abstract: The theory of the electric motor and torque-speed characteristic curve of the induction motors was used to analyze linear load and thruster loads. The results showed that for an induction motor with frequency speed control, slip did not change with change in frequency when it was drived by linear load. The changes of slip had a relation with frequency and rating slip when it was drived by thruster load. In approximate calculation, the ratio of the slip for the changed frequency to rated slip is equivalent to the square of the ratio of the changed frequency to rated frequency.