PURPOSE: To prevent the direct intrusion of line noises to a control circuit, and to obviate leakage currents through the control circuit by transmitting a position sensing signal by using a photo-coupler, etc.

CONSTITUTION: A higher harmonic component is removed by filters 1a∼1c in a signal waveform acquired by synthesizing a pulse-width modulated voltage waveform given to a winding and winding induced voltage, and an approximately sine wave is outputted. Outputs from the filters 1a∼1c and potential at a vertual neutral point are compared by comparators 2a∼2c, and outputs from the comparators are transmitted over photo-couplers 10a∼10c. Outputs from the photo-couplers 10a∼10c are inputted to a control circuit 3 as position sensing signals, and a proper conduction mode is selected.