The driving principle of brushless DC motor

    1. Characteristics of Brushless DC Motors

    There are two main types of DC motors: brushless DC motors and brushless DC motors.

    (1) Brushed DC motor

    DC motors are known for their excellent starting performance, speed regulation performance, and other advantages. Among them, brushless DC motors belonging to the category of DC motors use mechanical commutators, making the driving method simple. The model diagram is shown in Figure 1.2.

    The motor is mainly composed of a stator made of permanent magnet materials, a rotor (armature) wound with coil windings, a commutator, and brushes. As long as a certain amount of DC current is applied to both ends A and B of the brush, the commutator of the motor will automatically change the magnetic field direction of the motor rotor, so that the DC motor rotor will continue to operate.

    It can be seen that commutators and brushes play important roles in DC motors. Although they can simplify the structure of motor controllers, they themselves have certain drawbacks:

    a. The structure is relatively complex, increasing manufacturing costs;

    b. Easy to be affected by the environment (such as dust), reducing the reliability of work;

    c. Sparks may be generated during commutation, limiting the scope of use;

    d. Easy to damage, increases maintenance costs, etc.

    (2) Brushless DC motor

    The birth of Brushless Direct Current Motor (BLDCM) overcame the inherent defects of brushless DC motors and replaced mechanical commutators with electronic commutators. Therefore, BLDCM not only has the advantages of good speed control performance of DC motors, but also has the advantages of simple AC motor structure, no commutation sparks, reliable operation, and easy maintenance.

    The brushless DC motor model shown in Figure 1.3 is a model transformed from Figure 1.2. It mainly consists of a rotor made of permanent magnet materials, a stator with coil windings, and a position sensor (optional). It can be seen that it has many similarities with DC motors. The structure of the stator and rotor is similar (the original stator becomes the rotor, and the rotor becomes the stator), and the wiring of the windings is also basically the same. However, there is a clear difference in their structure: brushless DC motors do not have commutators and brushes in DC motors, but are replaced by position sensors. In this way, the motor structure is relatively simple, reducing the manufacturing and maintenance costs of the motor. However, the brushless DC motor cannot automatically reverse (phase), and the sacrificial cost is the increase in the cost of the motor controller (for example, the drive bridge of a brushless DC motor requires 4 power transistors, while the drive bridge of a brushless DC motor requires 6 power transistors).

    Figure 1.3 shows one of the low-power three-phase, star connected, single pair magnetic opposite pole brushless DC motors, with an inner stator and an outer rotor. The structure is similar to that of the DC motor shown in Figure 1.2. The structure of another brushless DC motor is just the opposite, with the stator on the outside and the rotor on the inside. The stator is a frame composed of coil windings, and the rotor is made of permanent magnet material.