Brushless DC motor (BLDC) is a kind of permanent magnet synchronous motor, but not a real DC motor. Different from brushed DC motors, brushless DC motors do not use mechanical brush devices. They use square wave self-controlled permanent magnet synchronous motors, replace carbon brush commutators with Hall sensors, and use NdFeB as the permanent magnet material of the rotor. Compared with ordinary traditional DC motors, it has great advantages in performance and is the most ideal speed-regulating motor today.

 

 

Brief Introduction of 1. Brushed DC Motor

 

Before introducing brushless DC motors, let's take a look at brushed motors:

 

 

DC motors are known for their good starting performance and speed regulation performance. Among them, brushed DC motors, which belong to the class of DC motors, use mechanical commutators to make the driving method simple. The model diagram is shown in the figure below.

 

Schematic diagram of DC motor model

 

Operation diagram of DC motor (brush motor)

 

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

DC motors have the following disadvantages

The structure is relatively complex, which increases the manufacturing cost;

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

Sparks will be generated when reversing, which limits the scope of use;

Easy to damage, increased maintenance costs, etc.

 

Brief Introduction of 2. Brushless DC Motor

 

The "BL" in BLDC motor means "brushless", that is, the "brush" in DC motor (brush motor) is gone.

 

The brushless DC motor (BLDC) replaces the mechanical commutator with the 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 advantages of simple structure, no commutation spark, reliable operation and easy maintenance.

 

Figure 1. Brushless DC motor model

 

Operation diagram of BLDC motor

 

The brushless DC motor is mainly composed of a rotor made of permanent magnet material, a stator with coil windings and a position sensor (optional). It can be seen that it has a lot in common with the DC motor. The structure of the stator and the rotor is similar (the original stator becomes the rotor, and the rotor becomes the stator), and the winding connection is basically the same. However, they have an obvious difference in structure: brushless DC motors do not have commutators and brushes in DC motors, and are replaced by position sensors. In this way, the motor structure is relatively simple, reducing the manufacturing and maintenance costs of the motor, but the brushless DC motor cannot automatically commutate (phase), and the cost of sacrifice is the increase in the cost of the motor controller (for example, it is also a three-phase DC motor. The drive bridge of a brushed DC motor requires 4 power tubes, while the drive bridge of a brushless DC motor requires 6 power tubes).

 

Figure 1 shows one of the low-power three-phase, star-connected, single-pair magnetic pole brushless DC motors. Its stator is inside and its rotor is outside. The structure of another brushless DC motor is just the opposite of this one. 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;

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

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

Brushless DC motor drive control is simpler than asynchronous motor.

 

Working principle of 3. 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 the motor cannot rotate. Only when the position of the motor rotor is detected in real time, and then the different connections 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 a schematic diagram of the rotation principle of a brushless DC motor. For convenience of description, the center tap of the coil of the motor stator is connected to the motor power POWER, and the end of each phase is connected to the power tube. When the position sensor is turned on, the G pole of the power tube is connected to 12V, the power tube is turned on, and the corresponding phase coil is energized. As the three position sensors turn on in turn with the rotation of the rotor, the corresponding phase coils are also energized in turn, so that the direction of the magnetic field generated by the stator is constantly changing, and the motor rotor also rotates. This is the brushless DC motor. The basic principle of rotation is to detect the position of the rotor and energize each phase in turn, so that the direction of the magnetic field generated by the stator changes continuously and evenly.

 

Figure 2: Brushless DC motor rotation principle diagram

 

In order to facilitate understanding, the following contents of this article use these two symbols as shown in the following figure as a model introduction. Figure A shows the rotor and stator of the motor in the same center of the circle, and Figure B shows the different center of the circle, which is to facilitate the explanation of the internal magnetic field of the motor.

 

Brushless DC motor structure diagram

 

Brushless DC motor working principle video

The stator windings of the motor are mostly made of three-phase symmetrical star connection, which is very similar to the three-phase asynchronous motor. The rotor of the motor is glued with a magnetized permanent magnet. In order to detect the polarity of the motor rotor, a position sensor is installed in the motor. The driver is composed of power electronic devices and integrated circuits. Its functions are: receiving the start, stop, and brake signals of the motor to control the start, stop and brake of the motor; receiving position sensor signals and forward and reverse signals to control The on and off of each power tube of the inverter bridge generates continuous torque; accepts speed commands and speed feedback signals to control and adjust the speed; provides protection and display, etc.

 

The DC motor has fast response, large starting torque, and can provide rated torque from zero speed to rated speed. However, the advantage of the DC motor is also its disadvantage, because the DC motor needs to produce constant load. For the performance of torque, the armature magnetic field and the rotor magnetic field must be maintained at 90 °, which requires carbon brushes and commutators. Carbon brushes and commutators will produce sparks and carbon powder when the motor rotates, so in addition to damage to the components, the use of the occasion is also limited. AC motor without carbon brush and commutator, maintenance-free, strong, wide application, but the characteristics to achieve the equivalent of DC motor performance must be achieved with complex control technology. Today's rapid development of semiconductor power components to speed up the switching frequency of many, improve the performance of the drive motor. The speed of the microprocessor is also getting faster and faster, which can realize the control of the AC motor in a rotating two-axis rectangular coordinate system, and properly control the current components of the AC motor in the two axes to achieve similar DC motor control and have comparable performance with the DC motor.

 

In addition, many microprocessors have done the necessary functions to control the motor in the chip, and the volume is getting smaller and smaller; like analog/digital converter (analog-to-digital converter,adc), pulse width modulation (pulse wide modulator,pwm)... Wait. The brushless DC motor is an application that electronically controls the commutation of the AC motor to obtain similar DC motor characteristics without the lack of a DC motor mechanism.

 

Driving Method of 4. Brushless DC Motor

 

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

 

1. Press the drive waveform:Square wave drive, this drive is convenient to achieve, easy to achieve motor position sensorless control;

 

2. Sine drive:This driving mode can improve the operation effect of the motor and make the output torque uniform, but the implementation process is relatively complicated. At the same time, this method has SPWM and SVPWM (space vector PWM) two ways, SVPWM effect is better than SPWM.

 

Application field of 5. brushless DC motor

 

Application field one, office computer peripheral equipment, electronic digital consumer goods field.

 

This is the most popular and largest number of brushless DC motors. For example, printers, fax machines, copiers, hard disk drives, floppy disk drives, movie cameras, tape recorders, etc., which are common in life, have brushless DC motors in the drive control of their spindles and auxiliary movements.

 

Application field two, industrial control field.

 

In recent years, due to the large-scale research and development of brushless DC motor and the gradual maturity of technology, the distribution range of its drive system in industrial production has also expanded, and it has gradually become the mainstream of industrial motor development. Research and attempts to reduce production costs and improve operating efficiency have achieved significant benefits, and major manufacturers also provide different types of motors to meet the needs of different drive systems. At this stage in the textile, metallurgy, printing, automated production lines, CNC machine tools and other industrial production, brushless DC motors are involved.

 

Application area three, medical equipment field.

 

In foreign countries, the use of brushless DC motor has been more common, can be used to drive the artificial heart in the small blood pump; in China, high-speed surgical equipment for high-speed centrifuge, thermal imaging camera and thermometer infrared laser modulator are used brushless DC motor.

 

Application field four, automotive field.

 

According to analysis, in the market, the average family car needs 20-30 permanent magnet motors, while each luxury car needs as many as 59. In addition to the core engine, it is used in wipers, electric doors, car air conditioners, and electric windows. There are motors in other parts. With the development of the automobile industry towards energy saving and environmental protection, the motor used must also meet the standards of high efficiency and low energy consumption. The advantages of brushless DC motors such as low noise, long life, no spark interference, and convenient centralized control are fully in line. As its speed control technology becomes more mature, the cost performance will become higher and higher. It is used in all aspects of automobile motor drive. The application will be more extensive.

Application field five, the field of household appliances.

"Frequency conversion" technology has been very common. As a symbol of Chinese home appliances, it has gradually occupied most of the consumer market. "DC frequency conversion" is favored by manufacturers, and it has gradually replaced the trend of "AC frequency conversion. This transformation is essentially the transition of the motor used in home appliances from an induction motor to a brushless DC motor and its controller to meet the requirements of energy saving, environmental protection, low noise, intelligence, and high comfort. The development direction of brushless DC motor is the same as that of power electronics, sensors, control theory and other technologies. It is the product of a combination of multiple technologies, and its development depends on the innovation and progress of each related technology.