PURPOSE: To enhance performance of an air-conditioning unit without causing degradation of motor efficiency or increase of the capacity of inverter element by performing constant control of the instantaneous power factor of an induction motor based on a preset reference value so that maximum efficiency can be achieved without being subjected to pulsating instantaneous load torque.

CONSTITUTION: In order to perform constant control of the instantaneous power factor at a high efficiency point without being subjected to pulsation of load, phase difference are taken out between phase voltages Vu, Vv, Vw outputted from an inverter 10d and motor currents Iu, Iv, Iw thus producing phase difference signals. Assuming the phase difference signals and an induction motor 1a have four poles, signals are produced twelve times per single revolution. The phase difference signals are fed to an instantaneous power factor detecting circuit 19 thus producing an instantaneous power factor signal cos. The instantaneous power factor signal is then compared, in a comparator 21, with a reference power factor command signal cos* fed from a reference frequency command-reference power factor command converting circuit 20, and an instantaneous power factor control circuit 22 performs learning control thus controlling the power factor constantly so that the induction motor 1a exhibits a maximum efficiency.