PURPOSE: To suppress electromagnetic vibration without increasing the size or the weight of unit and to realize silent operation by providing a first voltage control means for decreasing V/F ratio as the motor speed decreases and weakening excitation of motor as the motor speed decreases thereby suppressing electromagnetic vibration.

CONSTITUTION: A controller 10 comprises a V/F setting circuit 14 outputting a frequency command f* and a voltage signal VS in response to a speed command S*, a voltage decreasing circuit 11 outputting a voltage reference V* in response to the voltage signal VS, a frequency control circuit 13 controlling a power converter 1 according to the V*, and a voltage control circuit 12. The voltage signal VS is fed to the voltage decreasing circuit (first voltage control means) 11 where it is converted into a voltage command V* for decreasing the V/F ratio at low speed as shown by a solid line on V*-Vs characteristics. In other words, the voltage decreasing circuit 11 is provided with a function of function generator. (Conventional characteristics are shown by broken line.) The frequency control circuit 13 and the voltage control circuit 12 control output frequency and output voltage of the power converter 1 according to respective command values.