Sensorless start-up method for driving a brushless DC motor

At First, a plurality of detection modes are applied in sequence to a brushless DC motor, such that with respect to each of the detection modes a composite magnetic field generated by a motor current flowing through motor coils points in a different direction. Then, each of peak values of the motor currents generated by the detection modes is compared with respect to one another so as to select, from the detection modes, one detection mode which generates a largest one of the peak values of the motor currents. An initial position of a rotor is determined on a basis of a direction of a composite magnetic field generated by the detection mode which generates the largest one of the peak values of the motor currents. Based on the initial position of the rotor, the brushless DC motor is driven to start rotating.