[Introduction] How does a DC brushless motor achieve speed regulation? This is a question our electromechanical company encounters every few days. It's a common question from potential customers who need brushless motors, so it's essential to explain this topic.
DC brushless motors do not have brush wear, making maintenance relatively simple and more reliable than brushed motors, but they require the addition of a drive (commutation) circuit.
The first method for speed control of a brushless DC motor is primarily voltage-controlled, while torque is mainly controlled by current. This usually comes with a matching motor driver; changing the driver's output voltage controls the motor speed. If you don't have a driver and want to control the motor directly, you need to consider the motor's power and operating current. For low-power motors, resistance speed control can be used (not recommended; the method is simple—just connect a potentiometer in series—but this reduces efficiency). High-power motors cannot use resistance speed control because it requires a small-value, high-power resistor (the motor's operating resistance is very small), which is difficult to find, and this solution is too inefficient. It's best to use a matching driver.
The second speed control method for brushless DC motors is PWM speed control. The principle of PWM speed control for DC motors differs from that of AC motors. It doesn't adjust the motor speed through frequency modulation, but rather by adjusting the pulse width of the drive voltage. This, in conjunction with corresponding energy storage components in the circuit, changes the amplitude of the voltage delivered to the armature, thereby altering the DC motor's speed. Its modulation method is amplitude modulation (AM). PWM control has two modes:
1. Using a PWM signal to control the conduction time of the transistor, the longer the conduction time, the longer the work is done, and the higher the motor speed.
2. Use PWM control signals to control the conduction time of the transistor, and change the control voltage to achieve the desired effect.
The advantages of brushed DC motors are high starting torque, simple speed control system structure, and low price. However, they also have many disadvantages, such as high noise, easy damage, and the need to replace carbon brushes. Therefore, they have been gradually replaced by brushless DC motors.
