Detailed explanation of brushless DC motor composition and working principle

Brief introduction of brushless DC motor

The brushless DC motor (BLDC) replaces the mechanical commutator with an electronic commutator, so the brushless DC motor not only has the characteristics of good speed regulation performance of the DC motor, but also has the simple structure of the AC motor, no commutation spark, and reliable operation And easy maintenance and other advantages.

Brushless DC motor model

Figure 1. Brushless DC motor model

　　 Brushless DC motor is mainly composed 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 a lot in common with a DC motor. The structure of the stator and the rotor are similar (the original stator becomes the rotor, and the rotor becomes the stator), and the winding connections are basically the same. However, they have an obvious difference in structure: the brushless DC motor does not have the commutator and brushes in the DC motor. Instead, it is a position sensor. In this way, the motor structure is relatively simple, which reduces the cost of manufacturing and maintenance of the motor, but the brushless DC motor cannot automatically commutate (phase), and the sacrifice is the increase in the cost of the motor controller (such as the same three-phase DC motor, there are The drive bridge of the brushed DC motor needs 4 power tubes, and the drive bridge of the brushless DC motor needs 6 power tubes).

　　 Figure 1 shows one of the low-power three-phase, star-connected, single-pair brushless DC motors with a stator inside and a rotor outside. The structure of another brushless DC motor is just the opposite. Its stator is outside and the rotor is inside. That is, the stator is a base composed of coil windings, and the rotor is made of permanent magnet materials.

　　Brushless DC motor has the following characteristics:

　　● The brushless DC motor has good external characteristics and can output large torque at low speed, so that it can provide large starting torque;

　　● The brushless DC motor has a wide speed range and can run at full power at any speed;

　　● The brushless DC motor has high efficiency and strong overload capacity, which makes it have excellent performance in the driving system;

　　● The brushless DC motor has a good regenerative braking effect. Because its rotor is made of permanent magnet material, the motor can enter the generator state when braking;

　　● The brushless DC motor is small in size and high in power density;

　　● The brushless DC motor has no mechanical commutator and adopts a fully enclosed structure, which can prevent dust from entering the motor and has high reliability;

　　● The drive control of the brushless DC motor is simpler than that of the asynchronous motor.

Two brushless DC motor working principle

　　 The stator of the brushless DC motor is a coil winding armature, and the rotor is a permanent magnet. If only a fixed DC current is applied to the motor, the motor can only generate a constant magnetic field, and the motor cannot rotate. Only the position of the motor rotor is detected in real time, and then the different phases of the motor are connected with corresponding currents according to the position of the rotor. The stator generates a rotating magnetic field with uniform changes in direction, and the motor can rotate with the magnetic field.

Figure 2 shows the schematic diagram of the rotation principle of the brushless DC motor. For the convenience of description, the center tap of the motor stator coil is connected to the motor power supply POWER, the end points of each phase are connected to the power tube, and the G pole of the power tube is connected when the position sensor is turned on. 12V, the power tube is turned on, and the corresponding phase coil is energized. As the three position sensors turn on as the rotor rotates, the corresponding phase coils are also energized in turn, so that the direction of the magnetic field generated by the stator changes continuously, and the motor rotor also rotates. This is the brushless DC motor. The basic principle of rotation-detecting the position of the rotor, energizing each phase in turn, so that the direction of the magnetic field generated by the stator changes continuously and uniformly.

Schematic diagram of brushless DC motor rotation principle

Figure 2: Schematic diagram of brushless DC motor rotation principle

Three driving methods of brushless DC motor

　　The drive mode of brushless straight machine motor can be divided into multiple drive modes according to different categories, and they have their own characteristics.

　　 Press drive waveform:

　　● Square wave drive, this drive mode is convenient to implement and easy to realize motor position sensorless control;

　　● Sinusoidal drive, this drive mode can improve the motor running effect and make the output torque uniform, but the realization process is relatively complicated. At the same time, this method has two methods, SPWM and SVPWM (Space Vector PWM), and the effect of SVPWM is better than SPWM.