There are two ways of DC motor speed regulation: voltage regulation and frequency regulation.

The speed of the common fan with the brush motor can be changed by the way of voltage regulation, while the fan with the variable frequency motor can only be adjusted by the way of frequency modulation.

For a brush motor, changing the supply voltage is to change the current in the rotor winding so as to change the magnetic field strength and torque. The speed of the motor increases with the increase of torque and decreases with the decrease of torque. When the load resistance increases, the speed of the motor will decrease. In order to keep the speed constant when the load changes, the closed-loop control must be adopted, which is realized by the speed negative feedback, so the control circuit is relatively complex.