PURPOSE: To provide a vector controller for a thermally stable induction motor, which does not use a secondary resistance value as a parameter.

CONSTITUTION: Currents Iu, Iw and voltages Vu, Vw detected by a current detector 2 and a voltage detector 3 are converted into (d) axis, (q) axis primary currents I1d, I1q and (d) axis, (q) axis primary voltages V1d, V1q by a rotary biaxial coordinate system by a three-phase two-phase converter 4. First and second subtracters 5, 6 compare the primary currents I1d, I1q of (d) axis and (q) axis with each command value, and output the primary-current deviation signals of the (d) axis and the (q) axis. First and second current controllers 7, 8 receive the deviation signals, and output the primary-voltage command values V1d, V1q of the (d) axis and the (q) axis. A two-phase three-phase converter 9 receives the command values, and forms the primary-voltage command values Vu, Vv, Vw of three phase. A PWM inverter 10 controls applied voltage by the command values. A power-supply angular-frequency computing element 11 arithmetically operates power-supply angular frequency ω, integrates the power-supply angular frequency ω to an integrator, and forms the included angle θ of a U-phase winding axis and the (d) axis.