Brushless DC motor is a typical mechatronic product composed of motor body and driver. Brushless motor refers to the motor without brush and commutator ( or collector ring ), also called commutator - less motor. As early as the 19th century when the motor was born, the practical motor was a brushless form, that is, an AC squirrel cage asynchronous motor, which was widely used. However, asynchronous motors have many insuperable defects, resulting in the slow development of motor technology. Transistor was born in the middle of last century, so the DC brushless motor with transistor commutation circuit instead of brush and commutator came into being. This new type of brushless motor is called an electronically commutated DC motor, which overcomes the defects of the first generation of brushless motors.

DC brushless motor

Brushless DC motor is composed of motor body and DC brushless driver, which is a typical mechatronic product. The stator winding of the motor is usually made into a three-phase symmetrical star connection, which is very similar to a three-phase asynchronous motor. The rotor of the motor is stuck with magnetized permanent magnets. In order to detect the polarity of the motor rotor, a position sensor is installed in the motor. The driver is composed of power electronics and integrated circuits. Its function is to receive the start, stop and brake signals of the motor to control the start, stop and brake of the motor. receiving the position sensor signal and the positive and negative rotation signal for controlling the on-off of each power tube of the inverter bridge to generate continuous torque; Accepting speed commands and speed feedback signals for controlling and adjusting the rotation speed; Provide protection and display, etc.

The DC motor has a fast response, a large starting torque and the capability of providing rated torque from zero speed to rated speed, but the advantages of the DC motor are also its disadvantages, because if the DC motor is to produce a constant torque under rated load, the armature magnetic field and the rotor magnetic field must be maintained at 90 degrees, which requires the use of carbon brushes and commutator. The carbon brush and commutator will produce sparks and carbon powder when the motor rotates, so besides causing damage to the components, the use situation is also limited. The AC motor has no carbon brush and commutator, which is maintenance - free, strong and widely used, but the performance equivalent to that of the DC motor can only be achieved by complex control techniques. At present, the semiconductor industry is developing rapidly, and the switching frequency of power components is much faster, thus improving the performance of the drive motor. Microprocessor speed is also getting faster and faster, which can realize AC motor control in a rotating two-axis rectangular coordinate system and properly control AC motor current components in two axes to achieve similar DC motor control and have the same performance as DC motor.

In addition, many microprocessors have made the necessary functions to control the motor in the chip, and the volume is getting smaller and smaller. Such as analog - to - digital converter ( ADC ), pulse width modulation ( PWM ), etc. Brushless DC motor is an application that electronically controls the commutation of AC motor to obtain characteristics similar to DC motor without missing DC motor mechanism.

Characteristics of Brushless DC Motor

1. It can replace DC motor speed regulation, frequency converter + variable frequency motor speed regulation, asynchronous motor + speed reducer speed regulation;

2. It has the advantages of traditional DC motor and eliminates the structure of carbon brush and slip ring.

3, can run at low speed and high power, and can save the reducer from directly driving a large load;

4, small size, light weight and large output;

5. Excellent torque characteristics, good middle and low speed torque performance, large starting torque and small starting current;

6. stepless speed regulation, wide speed regulation range and strong overload capacity;

7, soft start and soft stop, good braking characteristics, can save the original mechanical braking or electromagnetic braking device;

8, high efficiency, no excitation loss and carbon brush loss of the motor itself, eliminating multi-stage deceleration loss, and the comprehensive power saving rate can reach 20 % - 60 %.

9. High reliability, good stability, strong adaptability and simple maintenance.

10. Resistance to turbulence and vibration, low noise, small vibration, smooth operation and long service life;

11, do not produce sparks, especially suitable for explosive places, explosion-proof type;

12. Select trapezoid wave magnetic field motor and sine wave magnetic field motor as required.

Selection of Brushless DC Motor

It is very important to select the right motor for practical application. According to the load characteristics of the motor, it is necessary to determine the appropriate motor parameters. The main parameters are as follows:

( 1 ) The maximum torque requirement of the application;

( 2 ) square root ( RMS ) torque requirements;

( 3 ) Speed requirements.

1. Maximum torque

The maximum torque can be obtained by adding the load torque, moment of inertia and friction. In addition, there are some additional factors that affect the maximum required torque, such as resistance of air gap air, which requires a torque margin of at least 20 %. In conclusion, the following equation exists:

TP = (TL TJ TF) * 1.2

TJ is the rotational torque that needs to be overcome in the process of starting or accelerating the motor, which mainly includes the rotational torque of the motor rotor and the rotational torque of the load, which is expressed as:

TJ = JL M * α

In the above formula, α is the acceleration, JL m is the rotational moment of the stator and the load. The mechanical axis of the motor determines the load moment and friction force of the motor.

2. Square root torque

The square root torque can be approximately considered as the continuous output torque required in practical application. It is determined by many factors: maximum torque, load torque, moment of inertia, acceleration, deceleration and running time. The following equation represents the calculation of square root torque, where TA is the acceleration time, TD is the deceleration time, and TR is the running time.

TRMS = √ [{TP2 TA (TL TF)2TR (TJ – TL – TF)2 TD}/(TA TR TD)]

3. Speed

This is the speed required for application. For example, the speed requirement of the blower is that the maximum speed is not different from the average speed. Obviously, in some point-to-point positioning systems, such as conveyor belts and mechanical arm systems, a motor with a large speed range is required. The speed requirement of the motor can be determined according to the trapezoidal curve ( ) of the motor speed. Generally, due to other factors, a 10 % margin is required when calculating the motor speed requirement.

Characteristics, Selection and Application of Brushless DC Motor

Application of Brushless DC Motor

BLDC has a wide range of applications, such as automobiles, tools, industrial control, automation, aerospace and so on. In general, BLDC can be divided into the following three main uses:

( 1 ) Continuous load application

( 2 ) Variable load applications

( 3 ) Positioning application

1. Continuous load application

This kind of application is mainly used in those fields that require a certain speed but do not require high speed accuracy, such as fans, water pumps, blowing air and so on. In general, such applications are relatively low-cost and mostly open-loop control.

2. Variable load applications

This kind of application mainly refers to the application where the motor speed needs to change within a certain range. In this kind of application, the high speed characteristic and dynamic response characteristic of the motor are mainly required. Washing machines, dryers and compressors in household appliances are good examples. In the automotive industry, oil pump control, electric controller, engine control and electronic tools are also good examples. There are also many applications in the aviation field, such as centrifuges, pumps, mechanical arms, gyroscopes, etc. In this field, motor feedback devices are often used to form a semi-open loop and a closed loop for control. This requires complex control algorithms, increases the complexity of the controller and increases the system cost.

3. Positioning application

Most industrial control and automatic control applications fall into this category. In these applications, energy transmission, such as gears or conveyor belts, is often completed. Therefore, the system has special requirements for the dynamic response and torque of the rotating speed of the motor. At the same time, these applications may need to change the steering of the motor at any time. The motor may work in uniform speed, acceleration and reduction stages, and the load may also be changing in these stages. Therefore, higher requirements are put forward for the controller. Usually, this control uses closed loop control, and even there may be three control loops: torque loop, speed loop and position loop. Photoelectric encoders and some synchronization equipment may be used for speed measurement. Sometimes these sensors are used to measure relative positions and sometimes absolute positions. Many of the process control, mechanical control and transportation control belong to such applications.