It is an object of the invention to realize minimization of the total loss from magnification relationships among a fundamental wave loss generated by an AC motor, a loss of an inverter that drives the motor, and a loss generated by a harmonic of the inverter. Since the fundamental wave loss is increased along with the increase in load of the AC motor, a switching frequency of the inverter is lowered to reduce the fundamental wave loss, and the loss caused by fundamental wave current is lowered, so that the efficiency of a system is improved. In contrast, since the harmonic loss of the AC motor is dominant in a light load area, the efficiency is maximized by increasing the switching frequency of the inverter or the number of pulses and reducing a harmonic current.
