Abstract: In recent years the induction motor speed has come to be controlled by changing the frequency and voltage of the commercial power supply by means of an invertor. The frequency of the commercial power supply, however, differs from country to country and even in the same country differs from region to region. If the motor controller designed for a supply frequency of f.sub.L is operated connected to the supply having a frequency f.sub.H higher than f.sub.L, an overvoltage will result. Conversely, if the motor controller designed for a supply frequency of f.sub.H is connected to the supply with a frequency f.sub.L lower than f.sub.H, then a sufficient torque cannot be obtained. To eliminate the above drawback, the present invention detects the input frequency of the motor controller and selects or calculates control constants in accordance with the detected value so as to enable stable operation even when the input frequency changes.