BG Motor - What is a DC Brushless motor

What is a brushless DC motor:

The electric motor converts the electrical energy provided into mechanical energy. Various types of motors are universal. Among them, the brushless DC motor (BLDC) has the characteristics of high efficiency, good controllability, etc., and is widely used. BLDC motors have energy-saving advantages over other motor types.

A motor is a device for energy conversion

When engineers face the challenge of designing electrical equipment to perform mechanical tasks, they will consider how to convert electrical signals into energy. Therefore, actuators and motors are one of the devices that convert electrical signals into motion. The motor exchanges electrical energy into mechanical energy or other forms of energy.

Brushless DC motor:

The brushless DC motor is composed of a motor body and a driver, and is a typical mechatronic product. 

Because the brushless DC motor runs in a self-control mode, it will not add a start winding to the rotor like a synchronous motor that is started under heavy load under variable frequency speed regulation, nor will it cause oscillation and loss of step when the load changes suddenly. 

The permanent magnets of medium and small capacity brushless DC motors are now mostly made of rare earth neodymium iron boron (Nd-Fe-B) materials with high magnetic energy levels. Therefore, the volume of the rare-earth permanent magnet brushless motor is one size smaller than the three-phase asynchronous motor of the same capacity.

working principle

Brushless DC motors use semiconductor switching devices to achieve electronic commutation, that is, electronic switching devices are used to replace traditional contact commutators and brushes. It has the advantages of high reliability, no reversing sparks, low mechanical noise, etc., and is widely used in high-end audio jacks, video recorders, electronic instruments and automated office equipment.

The brushless DC motor is composed of a permanent magnet rotor, a multi-pole winding stator, and a position sensor. The position sensor commutation of the stator winding current in a certain sequence according to the change of the rotor position (that is, it detects the position of the rotor pole relative to the stator winding, and generates a position sensing signal at a certain position, which is processed by the signal conversion circuit To control the power switch circuit, switch the winding current according to a certain logical relationship). The working voltage of the stator winding is provided by an electronic switch circuit controlled by the output of the position sensor.

There are three types of position sensors: magnetic sensitive, photoelectric and electromagnetic.

Brushless DC motors using magnetic-sensitive position sensors, whose magnetic-sensitive sensor components (such as Hall elements, magnetic diodes, magnetic sensitive pole tubes, magnetic resistors or application-specific integrated circuits, etc.) are mounted on the stator assembly. To detect the change of the magnetic field generated when the permanent magnet and the rotor rotate.

The brushless DC motor with photoelectric position sensor is equipped with a photoelectric sensor at a certain position on the stator assembly, a light shield is installed on the rotor, and the light source is a light-emitting diode or a small light bulb. When the rotor rotates, the photosensitive components on the stator will generate pulse signals intermittently at a certain frequency due to the effect of the light shield.

Brushless DC motors using electromagnetic position sensors are equipped with electromagnetic sensor components (such as coupling transformers, proximity switches, LC resonance circuits, etc.) on the stator assembly. When the position of the permanent magnet rotor changes, the electromagnetic effect will cause the electromagnetic sensor Produces a high-frequency modulation signal (the amplitude of which varies with the position of the rotor).

Structure analysis

In terms of structure, brushless motors and brushed motors have similarities. There are also rotors and stators, but the structure is opposite to that of brushed motors. The rotor of brushed motors is a coil winding connected to the power output shaft, and the stator is a permanent magnet. Steel; the rotor of the brushless motor is permanent magnet steel, which is connected to the output shaft together with the casing. The stator is a winding coil. The commutation brush used by the brush motor to alternately transform the electromagnetic field is removed, so it is called a brushless motor ( Brushless motor), then there is a problem now. Without the transformation of the electromagnetic field, how to make the brushless motor rotate?

To put it simply, by changing the alternating frequency and waveform of the current wave input to the stator coil of the brushless motor, a magnetic field rotating around the geometric axis of the motor is formed around the winding coil. This magnetic field drives the permanent magnet on the rotor to rotate. The motor starts to rotate. The performance of the motor is related to the number of magnets, the strength of the magnetic flux, and the input voltage of the motor. It also has a lot to do with the control performance of the brushless motor, because the input is direct current, and the current needs electronics. The governor turns it into a 3-phase alternating current, and it also needs to receive a control signal from the remote control receiver to control the speed of the motor to meet the needs of the model. In general, the structure of the brushless motor is relatively simple, and the real decision of its performance is the brushless electronic governor. A good electronic governor requires the process of single-chip control program design, circuit design, and complex processing technology. The overall control, so the price is much higher than the brush motor.

Related comparison

The comparison between brushless motors and brushed motors is as follows:

Brushed motors advantages

(1) The friction is large, the loss is large

I have encountered this problem when playing with brushed motors in the past, that is, after using the motor for a period of time, the motor needs to be turned on to clean the carbon brushes of the motor.

(2) High heat generation and short life span

Due to the structure of the brush motor, the contact resistance between the brush and the commutator is very large, which causes the overall resistance of the motor to be large and easy to generate heat. The permanent magnet is a thermal element. If the temperature is too high, the magnet will demagnetize , The performance of the motor is reduced, and the life of the brushed motor is affected.

(3) Low efficiency and low output power

The heat problem of the brushed motor mentioned above is largely due to the fact that the current is done on the internal resistance of the motor, so the electrical energy is converted to heat to a large extent, so the output power of the brushed motor is not high and the efficiency is not high. 

Brushless motor advantages

(1) No brush, low interference

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

(2) Low noise and smooth operation

The brushless motor has no brush, the friction is greatly reduced during operation, the operation is smooth, and the noise is much lower. This advantage is a huge support for the stability of the model.

(3) Long 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, only need to do some dust removal maintenance. Comparing the previous and the next, you know where the advantages of the brushless motor over the brushed motor are, but everything is not absolute. The performance characteristics of the brushless motor such as excellent low-speed torque performance and large torque are irreplaceable. However, In terms of the ease of use of brushless motors, with the decreasing cost of brushless controllers and the development and market competition of brushless technologies at home and abroad, brushless power systems are in the stage of rapid development and popularization, which also greatly promotes The development of the model movement.

Other differences

(1) Scope of application:

Brushless motor: usually used in equipment with relatively high control requirements and high speed, such as aircraft models, precision instruments, etc., which have strict control of motor speed and high speed.

Carbon brush motor: usually power equipment uses brush motors, such as hair dryers, factory motors, household range hoods, etc. In addition, the speed of the series motor can reach very high, but due to the wear of the carbon brushes, use The service life is not as good as the brushless motor.

(2) Service life:

Brushless motors: Usually the service life is in the order of tens of thousands of hours, but the service life of brushless motors is also very different due to different bearings.

Carbon brush motor: Generally, the continuous working life of a brush motor is several hundred to more than 1,000 hours. The carbon brush needs to be replaced when the use limit is reached, otherwise it is easy to cause the wear of the bearing.

(3) Use effect:

Brushless motor: usually digital frequency conversion control, strong controllability, from a few revolutions per minute to tens of thousands of revolutions per minute can be easily achieved.

Carbon brush motor: The old carbon brush motor generally works at a constant speed after starting, and the speed adjustment is not very easy. The series motor can also reach 20,000 rpm, but the service life will be relatively short.

(4) Energy saving:

Relatively speaking, the brushless motor controlled by inverter technology will save much energy than the series motor. The most typical ones are inverter air conditioners and refrigerators.

(5) In terms of future maintenance:

The carbon brush motor needs to be replaced. If it is not replaced in time, the motor will be damaged. The brushless motor has a long service life, usually more than 10 times that of the brush motor. However, if it is broken, the motor needs to be replaced, but the daily maintenance is basic No need.