Patent ID: 7427841

Claim:
A method of driving a brushless DC motor comprising: rectifying an AC voltage by a rectifier circuit configured with a diode bridge circuit, a capacitor being coupled between output terminals of the rectifier circuit and the AC voltage of an AC power source being input to the rectifier circuit; driving the brushless DC motor by an inverter coupled to the rectifier circuit; detecting a rotor position of the brushless DC motor by a position detector based on one of a back electromotive force of the brushless DC motor and a motor current; estimating the rotor position by a position estimator when a voltage between terminals of the capacitor is lower than a predetermined voltage, wherein the rotor position is not detectable by the position detector; and controlling the inverter by a controller based on one of the rotor position detected by the position detector and the rotor position estimated by the position estimator, wherein the capacitor has a capacitance that a ripple content in an output voltage of the rectifier circuit becomes not less than 90% in an output range for practical use in driving the brushless DC motor.