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Brushless DC motor (BLDC) composition, application fields and working principle detailed explanation

Date:2022-05-31   Author:XINDA MOTOR

A brushless DC motor (BLDC) is a type of permanent magnet synchronous motor, not a real DC motor, abbreviated as BLDC in English. Different from the brushed DC motor, the brushless DC motor does not use a mechanical brush device, but adopts a square wave self-controlled permanent magnet synchronous motor, replaces the carbon brush commutator with a Hall sensor, and uses NdFeB as the permanent magnet material of the rotor. , Compared with the general traditional DC motor, it has great advantages in performance and is the most ideal speed regulating motor today.

1. Introduction of 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 belonging to DC motors use mechanical commutators, which makes the driving method simple. The schematic diagram of the model is shown in the figure below.


Schematic diagram of the DC motor model


Schematic diagram of operation of DC motor (brush motor)

The motor is mainly composed of a stator made of permanent magnet material, a rotor (armature) with coil windings, a commutator and a brush. As long as a certain DC current is passed through the A and B ends of the brush, the commutator of the motor will automatically change the magnetic field direction 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;

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

Sparks will be generated during commutation, limiting the scope of use;

Easily damaged, increased maintenance costs, etc.

2. Introduction of brushless DC motor

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

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 sparks, and reliable operation. and easy maintenance.


Figure 1. Model of a brushless DC motor


Schematic diagram of the operation of the 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 structures of the stator and the rotor are similar (the original stator becomes the rotor, and the rotor becomes the stator), and the connection of the windings is basically the same. Structurally, however, they have one distinct difference: BLDC motors do not have the commutator and brushes found in DC motors, but instead use position sensors. In this way, the motor structure is relatively simple, which reduces the manufacturing and maintenance costs of the motor, but the brushless DC motor cannot automatically commutate (phase), and the sacrificed price 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, while 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-pole-pair BLDC motors, with the stator inside and the 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 frame composed of coil windings, and the rotor is made of permanent magnet materials.

Brushless DC motors have 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 drag system;

The regenerative braking effect of the brushless DC motor is good, because its rotor is a 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;

Brushless DC motors are simpler to drive and control than 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 supplied to the motor, the motor can only generate a constant magnetic field, and the motor cannot rotate. Only the position of the rotor of the motor can be detected in real time, and then according to the position of the rotor, the corresponding current is supplied to the different phases of the motor, so that the The stator generates a rotating magnetic field with a uniform change in direction, so that 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 coil of the motor stator 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 to the G pole when the position sensor is turned on. 12V, the power tube is turned on, and the corresponding phase coil is energized. Because the three position sensors will be turned on in turn with the rotation of the rotor, so that the corresponding phase coils are also energized in turn, so that the direction of the magnetic field generated by the stator also changes continuously, and the motor rotor also rotates, which is the brushless DC motor. Basic rotation principle - Detect the position of the rotor, energize each phase in turn, so that the direction of the magnetic field generated by the stator changes continuously and evenly.


Figure 2: Schematic diagram of the rotation principle of the brushless DC motor

In order to facilitate understanding, the following two symbols as shown in the following figure are used as the model introduction. Figure A shows the rotor and stator of the motor on the same center, and Figure B shows the different centers. It is for the convenience of explaining the internal magnetic field of the motor.


Schematic diagram of brushless DC motor structure

Video of the working principle of brushless DC motor

The stator winding of the motor is mostly made into a three-phase symmetrical star connection, which is very similar to the three-phase asynchronous motor. A magnetized permanent magnet is attached to the rotor of the motor. In order to detect the polarity of the rotor of the motor, a position sensor is installed in the motor. The driver is composed of power electronic devices and integrated circuits. Its functions are: accept the start, stop and braking signals of the motor to control the start, stop and braking of the motor; accept the position sensor signal and the forward and reverse signals to control the reverse. Change the on-off of each power tube of the bridge to generate continuous torque; accept speed commands and speed feedback signals to control and adjust the speed; provide protection and display, etc.The DC motor has fast response, large starting torque, and can provide rated torque from zero speed to rated speed. The performance of the 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 generate sparks and carbon powder when the motor rotates, so in addition to causing damage to the components, the application is also limited. The AC motor has no carbon brushes and commutators. It is maintenance-free, sturdy, and widely used. However, in order to achieve the performance equivalent to the DC motor, complex control technology can be used to achieve it. With the rapid development of semiconductors today, the switching frequency of power components is much faster, which improves 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 Cartesian coordinate system, and properly control the current component of the AC motor in the two axes, which is similar to the control of the DC motor and has the equivalent of the DC motor. performance.

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

Fourth, the driving method of brushless DC motor

The driving mode of the brushless direct motor can be divided into various driving modes according to different categories, and they each have their own characteristics.

1. According to the driving waveform: square wave driving, this driving method is convenient to realize, and it is easy to realize the control of the motor without position sensor;

2. Sinusoidal drive: This drive method 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, the effect of SVPWM is better than SPWM.

Five, brushless DC motor application field

Application field 1, the field of office computer peripheral equipment, electronic digital consumer goods.

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

Application field two, industrial control field.

In recent years, due to the large-scale research and development of brushless DC motors 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. The 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, brushless DC motors are involved in industrial production such as textiles, metallurgy, printing, automated production lines, and CNC machine tools.

Application field three, the field of medical equipment.

In foreign countries, the use of brushless DC motors is relatively common, which can be used to drive small blood pumps in artificial hearts; in China, high-speed centrifuges for high-speed surgical instruments, infrared laser modulators for thermal imagers and thermometers Both use brushless DC motors.

The fourth application area is the automotive field.

According to the analysis on 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, electric windows, etc. There are motors everywhere. With the development of the automobile industry in the direction of energy saving and environmental protection, the motors 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. With the increasingly mature speed regulation technology, the cost performance will become higher and higher. application will be more extensive.

Application field five, the field of household appliances.

"Inverter" technology has become very common. As a symbol of Chinese home appliances, it has gradually occupied most of the consumer market. "DC inverter" is favored by manufacturers and has gradually replaced "AC inverter". This transformation is essentially the transition from induction motors to brushless DC motors and their controllers for electric motors used in home appliances, in order to meet the requirements of energy conservation, 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 the combination of various technologies, and its development depends on the innovation and progress of each technology related to it.