Many practitioners and even engineers in the new energy sector have long been unclear about the differences between brushless motors and brushed motors? What is the difference between synchronous motors and asynchronous motors? Are all servo motors AC motors? Are all servo motors synchronous motors? Do stepper motors belong to DC motors or AC motors? Do servos belong to servo motors? ... Below, we will explain systematic knowledge in simple language and use a lot of animations and images to vividly express obscure knowledge.

DC Motor - Brushed Motor

Anyone who has studied high school physics knows that in order to study the force on a current-carrying conductor in a magnetic field, we have trained our left hand to be a palm-up gesture, which is also the principle of the DC motor.

All motors consist of a stator and a rotor. In a DC motor, to make the rotor spin, the direction of the current needs to be constantly changed; otherwise, the rotor can only turn half a circle, similar to a bicycle pedal. Therefore, a commutator is needed for a DC motor. Broadly speaking, DC motors include both brushed and brushless motors.

Brushed motors, also known as DC motors or carbon brush motors, refer to brushed DC motors. They use mechanical commutation, where the external magnetic poles are stationary and the internal coil (armature) moves. The commutator and rotor coil rotate together, while the brushes and magnets remain stationary, causing friction between the commutator and brushes to switch the direction of the current.

Disadvantages of brushed motors: 1. Sparks generated by mechanical commutation cause friction between the commutator and brushes, leading to electromagnetic interference, high noise, and short lifespan.

2. Poor reliability and frequent failures, requiring regular maintenance.

3. The presence of the commutator limits the rotor's inertia, restricts the maximum speed, and affects dynamic performance.

Given these many disadvantages, why are they still widely used? Because they have high torque, simple structure, easy maintenance (i.e., replacing carbon brushes), and are inexpensive.

DC Motor - Speed Control Principle

Speed control of DC motors: Speed control refers to obtaining the required torque by adjusting the motor speed.

DC (brushed) motors can adjust speed by regulating voltage, connecting resistors in series, or changing excitation, but in practice, adjusting voltage is the most convenient and commonly used method. Currently, PWM speed control is mainly used, which actually achieves voltage regulation of DC through high-speed switching. In one cycle, if the on-time is long, the average voltage is high; if the off-time is long, the average voltage is low. It is very convenient to adjust, as long as the switching speed is fast enough, the harmonics in the power grid are reduced, and the current is more continuous. However, brushes and commutators wear out over time, and during commutation, there are significant changes in current, which can easily produce sparks. The commutator and brushes limit the capacity and speed of DC motors, creating a bottleneck in speed control.

For brushless DC motors, during speed control, the input voltage is superficially controlled, but the motor's self-controlled variable frequency speed control system (the brushless DC motor itself is equipped with rotor position detectors and other devices to obtain rotor position signals, using these signals to control the commutation timing of the voltage and frequency speed control device) automatically controls the frequency based on voltage changes, making it almost as convenient to use as a DC (brushed) motor. Since the rotor uses permanent magnets, there is no need for a dedicated excitation winding. Under the same capacity, the motor is smaller, lighter, more efficient, more compact, more reliable in operation, and has better dynamic performance, making it widely used in applications such as electric vehicle drives.