Difference between brush motor and brushless motor

1. Working principle of brush motor

Brush motor is the first kind of motor we came into contact with. It was also used as a model to show the motor introduced in physics class in middle school. The main structure of brush motor is stator + rotor + brush. The rotating torque is obtained through rotating magnetic field, so as to output kinetic energy. The brush and commutator are in constant contact and friction, which plays the role of conduction and commutation in rotation.

The brush motor adopts mechanical commutation, the magnetic pole does not move and the coil rotates. When the motor works, the coil and commutator rotate, the magnetic steel and carbon brush do not rotate, and the alternating change of coil current direction is completed with the commutator and brush rotating with the motor.

In the brush motor, this process is to arrange the two power input terminals of each group of coils into a ring in turn, which are separated from each other by insulating materials to form something like a cylinder, which is connected with the motor shaft. The power supply passes through a small column (carbon brush) made of two carbon elements, which is from two specific fixed positions under the action of spring pressure, Press the two points on the upper coil power input ring cylinder to energize a group of coils.

As the motor rotates, different coils or different poles of the same coil are energized at different times, so that there is a suitable angle difference between the N-S pole of the magnetic field generated by the coil and the N-S pole of the nearest permanent magnet stator. The magnetic field attracts and repels each other, generating force and driving the motor to rotate. The carbon electrode slides on the coil connector like a brush on the object surface, so it is called carbon "brush".

Sliding with each other will rub the carbon brush and cause loss, so the carbon brush needs to be replaced regularly; Alternating on-off between the carbon brush and the coil connector will produce electric spark, produce electromagnetic break and interfere with electronic equipment.

2. Working principle of brushless motor

In the brushless motor, the commutation is completed by the control circuit in the controller (generally Hall sensor + controller, and the more advanced technology is magnetic encoder).

The brushless motor adopts electronic commutation, the coil does not move and the magnetic pole rotates. The brushless motor uses a set of electronic equipment to sense the position of the magnetic pole of the permanent magnet through the hall element. According to this perception, the electronic circuit is used to timely switch the direction of the current in the coil to ensure that the magnetic force in the correct direction is generated to drive the motor. The disadvantage of brush motor is eliminated.

These circuits are motor controllers. The controller of brushless motor can also realize some functions that cannot be realized by brush motor, such as adjusting the power switching angle, braking the motor, reversing the motor, locking the motor, and stopping the power supply to the motor by using the braking signal. Now the electronic alarm lock of battery car makes full use of these functions.

Brushless DC motor is a typical mechatronics product, which is composed of motor body and driver. Since the brushless DC motor operates in a self-control mode, it will not add a starting winding on the rotor like the synchronous motor started under heavy load under variable frequency speed regulation, nor will it produce oscillation and out of step when the load changes suddenly.

3. Difference of speed regulation mode

In fact, the control of the two motors is voltage regulation. Only because the brushless DC adopts electronic commutation, it can be realized with digital control. The brushless DC is commutation through carbon brush, which can be controlled by traditional analog circuits such as thyristor, which is relatively simple.

1. The speed regulation process of brush motor is to adjust the power supply voltage of the motor. The adjusted voltage and current are converted through the commutator and brush to change the strength of the magnetic field generated by the electrode, so as to change the speed. This process is called variable voltage speed regulation.

2. The speed regulation process of brushless motor is that the voltage of the motor power supply remains unchanged, the control signal of electric regulation is changed, and the switching rate of high-power MOS transistor is changed by microprocessor to change the speed. This process is called variable frequency speed regulation.

4. Performance differences

1. The brush motor has simple structure, long development time and mature technology

As early as the 19th century, when the motor was born, the practical motor was brushless, that is, AC squirrel cage asynchronous motor, which was widely used after the generation of AC. However, asynchronous motor has many insurmountable defects, resulting in the slow development of motor technology. In particular, brushless DC motor has not been put into commercial operation. With the rapid change of electronic technology, it has not been put into commercial operation until recent years. In essence, it still belongs to the category of AC motor.

Soon after the birth of brushless motor, people invented DC brushless motor. Because the DC brush motor has simple mechanism, easy production and processing, convenient maintenance and easy control; DC motor also has the characteristics of fast response, large starting torque, from zero speed to rated speed, and can provide rated torque, so it has been widely used as soon as it came out.

2. DC brushless motor has fast response speed and large starting torque

The DC brush motor has the advantages of fast starting response, large starting torque, stable speed change, almost no vibration from zero to maximum speed, and can drive greater load during starting. Brushless motor has large starting resistance (inductive reactance), so the power factor is small, the starting torque is relatively small, there is a buzzing sound during starting, accompanied by strong vibration, and the driving load is small during starting.

3. The DC brush motor runs stably and has good starting and braking effect

The brush motor is regulated by voltage regulation, so the starting and braking are stable, and the constant speed operation is also stable. Brushless motor is usually controlled by digital frequency conversion, which first changes AC into DC, then DC into AC, and controls the speed through frequency change. Therefore, brushless motor runs unsteadily and vibrates greatly during starting and braking, and it will be stable only when the speed is constant.

4. High control precision of DC brushless motor

DC brushless motor is usually used together with reducer and decoder to make the output power of the motor larger and the control accuracy higher. The control accuracy can reach 0.01 mm, which can make the moving parts stop at almost any desired place. All precision machine tools are controlled by DC motor. Since the brushless motor is not stable during starting and braking, the moving parts will stop at different positions every time. It can only stop at the desired position through the locating pin or stopper.

5. The DC brush motor has the advantages of low cost and convenient maintenance

Due to the simple structure, low production cost, many manufacturers and mature technology, the DC brush motor is also widely used, such as factories, processing machine tools, precision instruments, etc. if the motor fails, just replace the carbon brush. Each carbon brush costs only a few yuan, which is very cheap. The technology of brushless motor is immature, the price is high and the scope of application is limited. It should be mainly used in constant speed equipment, such as variable frequency air conditioner, refrigerator, etc. if the brushless motor is damaged, it can only be replaced.

6. Brushless, low interference

The brushless motor removes the brush, and the most direct change is that there is no electric spark generated when the brushless motor is running, which greatly reduces the interference of electric spark to remote control radio equipment.

7. Low noise and smooth operation

Brushless motor has no brush, the friction is greatly reduced, the operation is smooth, and the noise will be much lower. This advantage is a great support for the operation stability of the model.

8. Long service life and low maintenance cost

Without the brush, the wear of the brushless motor is mainly on the bearing. From a mechanical point of view, the brushless motor is almost a maintenance free motor. When necessary, it only needs to do some dust removal maintenance