Knowledge of the principle of brushless DC motor

The number of clicks: Updated: 2018-04-14 09:12:04 [ close ] share:

    Brushless DC motor:

    The brushless DC motor is composed of a motor body and a driver, and is a typical mechatronic product. Brushless motor refers to the motor without brush and commutator (or slip ring), also known as the commutatorless motor. When the motor was born in the nineteenth century, the practical motor produced was a brushless form, that is, an AC squirrel-cage asynchronous motor, which has been widely used. However, asynchronous motors have many insurmountable shortcomings, resulting in the slow development of motor technology. Transistors were born in the middle of the last century, so DC brushless motors using transistor commutation circuits instead of brushes and commutators came into being. This new type of brushless motor is called an electronic commutated DC motor, which overcomes the defects of the first generation of brushless motor.

    DC Countless Motor Principle:

    The brushless DC motor is composed of a motor body and a driver, and is a typical mechatronic product. The stator winding of the motor is mostly made of 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 motor rotor, a position sensor is installed in the motor. The driver is composed of power electronics and integrated circuits. Its function is to accept the start, stop, and brake signals of the motor to control the start, stop, and brake of the motor. It also receives position sensor signals and forward and reverse signals to control the inverse. The on-off of each power tube of the bridge produces continuous torque; it accepts speed commands and speed feedback signals to control and adjust the speed; provides protection and display and so on.
    DC motors have fast response, large starting torque, and the ability to provide rated torque from zero speed to rated speed. However, the advantages of DC motors are also its disadvantages, because DC motors must produce constant rotation at rated load. The torque performance requires the armature magnetic field and rotor magnetic field to be maintained at a constant 90 °. This requires carbon brushes and commutators. Carbon brushes and commutators generate sparks and toner when the motor rotates. Therefore, in addition to causing component damage, the use of the brush is limited. AC motors do not have carbon brushes and commutators. They are maintenance-free, sturdy, and widely used. However, in order to achieve the performance equivalent to DC motors, complex control technology can be used. Nowadays, the rapid development of semiconductors has accelerated the switching frequency of power components, which has improved the performance of drive motors. 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 appropriately control the current component of the AC motor in the two axes to achieve similar control to the DC motor and have the equivalent performance.
    In addition, many microprocessors have built the functions necessary to control the motor in the chip, and the size is getting smaller and smaller; like analog-to-digital converter (adc), pulse wide modulator, pwm) ... etc. The DC brushless motor is an application that controls the AC motor commutation electronically, and obtains similar DC motor characteristics without missing the DC motor mechanism.

    Brushless DC motor features:

    1. Can replace DC motor speed control, inverter + frequency conversion motor speed control, asynchronous motor + reducer speed control;
    Brushless DC motors (3 photos)
    2. It has the advantages of traditional DC motors, while eliminating the structure of carbon brushes and slip rings;
    3. It can run at low speed and high power, which can save the reducer to directly drive large loads;
    4, small size, light weight, high output;
    5. Excellent torque characteristics, good medium and low speed torque performance, large starting torque and small starting current;
    6. Stepless speed regulation, wide speed range, 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, the motor itself has no excitation loss and carbon brush loss, eliminating multi-stage deceleration consumption, the comprehensive power saving rate can reach 20% ~ 60%.
    9, high reliability, good stability, strong adaptability, simple repair and maintenance;
    10. Resistance to bumps and vibrations, low noise, small vibration, smooth operation and long life;
    11. Does not generate sparks, especially suitable for explosive places, with explosion-proof type;
    12. Optional trapezoidal wave magnetic field motor and sine wave magnetic field motor are available.

    Brushless DC motor applications:

    The applications of brushless DC motors are very wide, such as automobiles, tools, industrial industrial control, automation, and aerospace. In general, brushless DC motors can be divided into the following three main uses:
    Continuous load applications: mainly applications that require a certain speed but not high speed accuracy, such as applications such as fans, pumps, hair dryers, etc. These applications have lower cost and are mostly open-loop control.
    Variable load applications: mainly applications in which the speed needs to be changed within a certain range, and there is a higher demand for motor speed characteristics and dynamic response time characteristics. For example, in household appliances, dryers and compressors are good examples. In the automotive industry, oil pump control, electric controllers, engine control, etc., the system cost of such applications is relatively higher.
    Positioning applications: Most industrial control and automatic control applications fall into this category. In such applications, energy transmission is often completed, so there are special requirements for the dynamic response and torque of the speed, and the requirements for the controller are also high. . Photoelectricity and some synchronization equipment may be used for speed measurement. Many process control, mechanical control, and transportation control fall into this category of applications.
    The practical new type of brushless motor is closely related to the development of electronic technology, microelectronic technology, digital technology, automatic control technology and materials science. It is not limited to the field of AC and DC, but also involves the fields of energy conversion and signal sensing of electric and power generation. In the field of motors, there are many new types of brushless motors, but brushless motors with excellent performance are not widely used due to price restrictions. The following are the main new brushless motors for exploration and research.

    Brushless DC motor classification:

    DC motor
    Generally, DC motors have the same working principle and application characteristics, but their composition is different. In addition to the motor itself, the former also has a commutation circuit, and the motor itself and the commutation circuit are tightly integrated. The motor of many low-power motors is integrated with the commutation circuit. From the appearance, the DC brushless motor is exactly the same as the DC motor.
    The motor of the DC brushless motor itself is an electromechanical energy conversion part. In addition to the two parts of the motor armature and permanent magnet excitation, it also has a sensor. The motor itself is the core of the brushless DC motor. It is not only related to performance indicators, noise and vibration, reliability and service life, etc., but also involves manufacturing costs and product costs. Due to the use of a permanent magnetic field, the DC brushless motor is freed from the traditional design and structure of general DC motors, meets the requirements of various application markets, and develops in the direction of saving copper and saving materials and manufacturing. The development of permanent magnetic field is closely related to the application of permanent magnetic materials. The application of the third generation of permanent magnetic materials has promoted the DC brushless motors towards high efficiency, miniaturization and energy saving.
    In order to achieve electronic commutation, a position signal is required to control the circuit. Earlier, the position signal was obtained by using an electromechanical position sensor, and the position signal has been gradually obtained by an electronic position sensor or other methods. The simplest method is to use the potential signal of the armature winding as the position signal.
    A speed signal is required to control the motor speed. The speed signal is obtained by using the similar method of obtaining the position signal. The simplest speed sensor is a combination of frequency-measuring speed-measuring generator and electronic circuit.
    The commutation circuit of a DC brushless motor is composed of two parts: the drive and the control. The two parts are not easy to separate. Especially for low-power circuits, the two are integrated into a single application-specific integrated circuit.
    In a high-power motor, the driving circuit and the control circuit can be integrated into one. The driving circuit outputs electric power, drives the armature winding of the motor, and is controlled by the control circuit. The driving circuit has been switched from a linear amplification state to a pulse width modulation switching state, and the corresponding circuit composition has also been transformed from a transistor discrete circuit to a modular integrated circuit. Modular integrated circuits are composed of power bipolar transistors, power field effect transistors, and isolated gate field effect bipolar transistors. Although the isolated gate field effect bipolar transistor is more expensive, it is still more suitable from the perspective of reliability, safety and performance.
    The control circuit is used to control the speed, steering, current (or torque) of the motor and protect the motor from overcurrent, overvoltage, overheating, etc. The above parameters are easily converted into analog signals, and it is relatively simple to use them for control, but from the development point of view, the parameters of the motor should be converted into digital quantities, and the motor should be controlled by a digital control circuit. At present, the control circuit has three kinds of components: an application specific integrated circuit, a microprocessor, and a digital signal processor. It is a simple and practical way to form a control circuit with an application-specific integrated circuit where the requirements for motor control are not high. The use of digital signal processors to form control circuits is the future development direction. The related digital signal processors will be introduced in the following AC synchronous servo motors.
    In the micropower category, DC brushless motors are new types of motors that are developing rapidly. Because each application field requires its own unique DC brushless motor, there are many types of DC brushless motors. Generally, there are external memory for computers and flat coreless motor structures for VCD, DVD, and CD spindle drives, outer rotor motor structures for small fans, multi-pole magnetic field structures and built-in structures for home appliances, and multi-pole and outer rotors for electric bicycles. Structure and so on. The motor and the circuit of the above-mentioned DC brushless motor are integrated into one body, which is very convenient to use and its output is also very large. In order to meet the needs of high-volume, low-cost markets, the production of DC brushless motors must form economies of scale. Therefore, the DC brushless motor is a high-input, high-output industry. At the same time, we should consider that the market is also constantly developing. For example, the motors for home air conditioners are changing from 3A to 3D, and a large number of small and medium power DC brushless DC motors are required. Research and development of small and medium power DC brushless motors are also urgent.

    Brushless Motor
    (BLDCM) was developed on the basis of brushed DC motors, but its drive current is uncompromising AC; brushless DC motors can be divided into brushless rate motors and brushless torque motors. Generally, there are two types of drive currents for brushless motors, one is a trapezoidal wave (generally a "square wave") and the other is a sine wave. Sometimes the former is called a DC brushless motor, and the latter is called an AC servo motor, which is exactly a type of AC servo motor.
    In order to reduce the moment of inertia, the brushless DC motor usually adopts a "slim" structure. Brushless DC motors are much smaller in weight and volume than brushed DC motors, and the corresponding moment of inertia can be reduced by about 40-50%. Due to the processing problems of permanent magnet materials, the general capacity of brushless DC motors is below 100kW.
    This kind of motor has good linearity of mechanical characteristics and adjustment characteristics, wide speed range, long life, convenient maintenance, low noise, and there is no series of problems caused by brushes, so this kind of motor has a lot in the control system. Application potential.
    The stator winding of the motor is mostly made of 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 motor rotor, a position sensor is installed in the motor. The driver is composed of power electronics and integrated circuits. Its function is to accept the start, stop, and brake signals of the motor to control the start, stop, and brake of the motor. It also receives position sensor signals and forward and reverse signals to control the inverse. The on-off of each power tube of the bridge produces continuous torque; it accepts speed commands and speed feedback signals to control and adjust the speed; provides protection and display and so on.
    Because the brushless DC motor runs in a self-controlled manner, it will not add a starting winding to the rotor like a synchronous motor with heavy-duty starting under variable frequency speed regulation, and it will not cause oscillation and step loss when the load changes suddenly.
    The permanent magnets of small and medium capacity brushless DC motors are mostly made of high magnetic energy product rare earth neodymium iron boron (Nd-Fe-B) material. Therefore, the volume of the rare-earth permanent magnet brushless motor is smaller than that of the three-phase asynchronous motor of the same capacity by one frame size.
    In the past thirty years, the research on the variable frequency speed regulation of asynchronous motors is in the final analysis looking for a method to control the torque of asynchronous motors. Small errors and other characteristics show advantages in the field of speed regulation.
    动的需求。 Because brushless DC motors have the characteristics of DC brushed motors and are also devices that change frequency, they are also known as DC frequency converters. The international common term is BLDC. The operation efficiency, low speed torque, and speed accuracy of brushless DC motors are better than The control technology of the inverter is better, so it deserves the attention of the industry. This product has been produced more than 55kW and can be designed to 400kW, which can meet the needs of the industry to save energy and high-performance drives .

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