Patent ID: 7102306

Claim:
A brushless DC motor driving method comprising: a brushless DC motor having a stator and a rotor including a permanent magnet, an inverter for supplying electric power to the brushless DC motor, a drive unit for driving the inverter, a position detecting unit for outputting rotor position signal based on a voltage induced on the stator of the brushless DC motor, a rotating speed detecting unit for detecting a rotating speed of the brushless DC motor from the rotor position signal, a first waveform generating unit for outputting drive signal based on the rotor position signal, while performing PWM duty control, a second waveform generating unit for outputting drive signal driving the brushless DC motor as a synchronous motor, while keeping PWM duty constant, and a commutation judging unit for driving the inverter through the drive unit by selecting one of the drive signal from the first waveform generating unit and the drive signal from the second waveform generating unit, wherein the method comprises the steps of: when a rotating speed commanded of the brushless DC motor is smaller than the maximum rotating speed capable of being driven by the drive signal of the first waveform generating unit, driving the inverter by the drive signal outputted from the first waveform generating unit through the drive unit, and when a rotating speed commanded of the brushless DC motor is larger than the maximum rotating speed capable of being driven by the drive signal of the first waveform generating unit, driving the inverter first by the drive signal outputted from the first waveform generating unit through the drive unit until reaching the maximum rotating speed capable of being driven by the drive signal of the first waveform generating unit, and then the commutation judging unit changing over from driving by the drive signal of the first waveform generating unit to driving by the drive signal of the second waveform generating unit, and the second waveform generating unit changing the frequency while keeping the PWM duty constant, and raising the rotating speed of the brushless DC motor to the rotating speed commanded.