The brushless motor is a synchronous motor, which means that the rotation of the shaft is synchronized with the frequency of the supply current; the rotation period is exactly equal to an integral number of AC cycles.
It is not possible to start the motor using a sine-wave drive, because the stopped rotor does not follow the frequency when starting.
Therefore, when starting the motor, the acceleration operation is required by feeding back the rotor position.
- (1) Detect the rotor position.
- (2) Apply a sinusoidal voltage slowly to the coil according to the rotor position.
- (3) Rotate while gradually increasing the voltage.
- (4) Control the period of the sine-wave according to the timing of the position detection sensor.
- (5) If the rotor rotates with a certain speed, steady operation starts to reach the target speed.
In steady operation, the speed can be maintained against load fluctuations by feeding back the target speed deviation.