There are several ways to control the speed of a DC motor:

1. Adjust the voltage: the speed of the DC motor is proportional to the voltage, and the speed is controlled by increasing or decreasing the voltage.

2. Using PWM control: By controlling the duty cycle of the pulse width modulation (PWM) signal, the average voltage and speed of the motor can be changed. When the duty cycle of the PWM signal increases, the average voltage of the motor increases, and the speed also increases.

3. Use of electronic speed controller: Electronic speed controller is an electronic device that can change the voltage and frequency of AC or DC motors, so as to achieve precise control of the speed of the motor.

4. Use of frequency converter: The frequency converter is a power supply device that can change the fixed frequency power supply voltage into variable frequency and variable voltage, so it can control the speed of the motor.

5. Adjust the magnetic field of the motor: By changing the magnetic field of the motor, the speed of the motor can be changed. This can be achieved by adjusting the field current of the motor.