PURPOSE: To execute stable control over a wide range by calculating voltage model magnetic flux and current model magnetic flux independently and installing a function generator specially.

CONSTITUTION: A three-phase induction motor 6 driven from a dc voltage source 1 through a PWM inverter 3 is equipped with a voltage detector 2, current detectors 5, a position sensor 6a, and a controller 7. The controller 7 includes a current model operation block 706, a low-order priority block, 708, a magnetic flux vector length 1 calculation block 710, and control flags fθ-fτ operation blocks 715 and 716. Thereby current model magnetic flux can be used together in a low-frequency band and setting of the switching characteristic thereof is not restricted.