To achieve smooth switching of control without fluctuations in speed and torque, an excitation current command is allowed to transit linearly or in accordance with the function of speed between a value under sensorless vector control and a value under low-speed region control in accordance with a speed command or estimated speed in a speed region where the control is switched or in an adjacent speed region where sensorless vector control is performed. Therefore, abrupt variations in excitation current are reduced before and after the switching of the control.