PURPOSE: To control the speed and torque of an induction motor accurately even in a very low speed region.

CONSTITUTION: A speed adaptive secondary flux observer comprising a flux observer 4 of same dimension and a speed adaptive mechanism 7 estimates the actual speed of an induction motor 1 and a current control section is controlled by means of a comparison error signal between an estimated motor speed ωr# and motor speed command ωr#. A PWM control converter 2 at the current control section compares primary voltage control command voltages vu, vv, vw with a carrier signal (triangular wave signal) and performs PWM control of an inverter depending on the comparison result. A PWM control inverter 2 sets the frequency of carrier signal at a conventional high level when an estimated motor speed ωr# is higher than a set value and decreases the frequency of carrier signal when the estimated motor speed ωr# drops below the set value.