PURPOSE: To permit the control of a rotational speed without attaching any rotational speed sensor, by a method wherein the primary flux of an AC motor is detected to operated a primary flux speed from a primary flux detection signal and supply it as a ratational speed control signal.

CONSTITUTION: An output voltage from a 3-phase AC power source 1 is supplied to an AC motor 3 through a rectifying circuit 11, a capacitor 12 and the switching unit 21 of an inverter. The voltage and the current of respective phases of the AC motor 3 are converted by 3-phase/2-phase converting circuits 4, 5 while respective components of a primary flux vector are obtained from both converted output signals through adders 52a, 52c, subtractors 53a, 53c and integrating circuits 6a, 6c. Respective components of the primary flux vector, obtained in such a manner, are supplied to a primary flux speed operation unit 7 and a primary flux speed ω is operated to supply it to an inverter control unit 22. According to this method, a rotational speed detection device, such as an encoder or the like, may be made unnecessary.