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The driving principle of brushless DC motor

Time:2023-11-14 Views:595
1、 Working 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.
2. Characteristics of brushless DC motors
     (1) The brushless DC motor has good external characteristics and can output large torque at low speeds, allowing it to provide large starting torque;
    (2) The brushless DC motor has a wide speed range and can operate at full power at any speed;
    (3) The high efficiency and strong overload capacity of brushless DC motors enable them to perform excellently in driving systems;
    (4) The regenerative braking effect of brushless DC motor is good, as its rotor is made of permanent magnet material, the motor can enter the generator state during braking;
    (5) The brushless DC motor has a small size and high power density;
    (6) The brushless DC motor has no mechanical commutator and adopts a fully enclosed structure, which can prevent dust from entering the interior of the motor and has high reliability;
    (7) The drive control of brushless DC motors is simpler than that of asynchronous motors.
3. Working principle of brushless DC motor
     The stator of a 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 cannot rotate. Only by detecting the position of the motor rotor in real time, and then providing corresponding currents to different phases of the motor based on the rotor position, can the stator generate a uniformly changing rotating magnetic field, and the motor can rotate with the magnetic field.
    The schematic diagram of the rotating principle of a brushless DC motor is shown in Figure 1.4. For the convenience of description, the center tap of the motor stator coil is connected to the motor power supply POWER, and the endpoints of each phase are connected to power tubes. When the position sensor is connected, the G pole of the power tube is connected to 12V, and the power tube is connected. The corresponding phase coil is energized. Due to the fact that the three position sensors are sequentially conductive as the rotor rotates, the corresponding phase coils are also sequentially energized, causing the direction of the magnetic field generated by the stator to constantly change, and the motor rotor to rotate accordingly. This is the basic rotation principle of brushless DC motors - detecting the position of the rotor, sequentially energizing each phase, and continuously and uniformly changing the direction of the magnetic field generated by the stator.
    In the following section, which introduces the position sensor drive of brushless DC motors, the rotating principle of brushless DC motors will be further introduced.
    For the convenience of understanding, the following content of this document uniformly uses these two symbols as shown in Figure 1.5 as the model introduction. Figure A shows the motor rotor and stator on the same center, while Figure B shows the internal magnetic field of the motor on a different center. Please refer to the following text for details.
4. The Driving Method of Brushless DC Motor
    The driving methods of brushless direct machine motors can be divided into multiple types according to different categories, each with its own characteristics.
    According to the driving waveform:
    (1) This driving method is convenient to implement and easy to achieve sensorless control of the motor;
    (2) This driving method can improve the operation effect of the motor and make the output torque uniform, but the implementation process is relatively complex. At the same time, this method also has two methods: SPWM and SVPWM (Space Vector PWM), and the effect of SVPWM is better than that of SPWM.
 












   
      
      
   
   


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