more current flows to the motor windings and the rotor rotates more rapidly also the above described pwm is performed in a period taken from a rising edge of the start sensor to a falling edge thereof then the rotating speed and the phase of the motor is detected using the start sensor and the detected rotating speed is compared with the reference speed determined by the system step 806 if the detected rotating speed of the motor is faster than the reference speed as a result of comparison at step 806 the base of commutation is changed from the start sensor to the operation sensor and a dwell time control is performed while if the detected rotating speed is slower than the reference speed the process of adjusting the pwm duty ratio is continuously performed the dwell time control is performed in a manner that the current is supplied or cut off at a time for a time determined by the microcomputer instead of turning on or off the switching elements according to the pwm duty ratio in comparison to the pwm the dwell time control greatly reduces the number of switching operations of the switching elements of the.