The speed of a brushless motor is adjusted by changing the voltage.
The rotation speed is increased by raising the voltage and increasing the frequency of the drive command waveform.
If only the frequency of the drive command waveform is increased, the rotation of the rotor will not be able to follow and the rotor will stop.
Also, if only the voltage is increased, the rotor will rotate faster, but the frequency of the drive command waveform will be too slow to stop the rotor.
Therefore, it is necessary to adjust both the voltage and drive command waveform.
(The frequency of the drive command waveform of the brushless motor is determined by the position detection of the rotor.)