Application prospect of the hottest permanent magn

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The application prospect of permanent magnet synchronous motor

I. overview

as we all know, DC motor has excellent control performance, its mechanical characteristics and speed regulation characteristics. The experimental results of aluminum alloy electronic universal experimental machine are accurate parallel straight lines, which is a characteristic that all kinds of AC motors do not have. In addition, DC motor has the characteristics of large starting torque, high efficiency, convenient speed regulation, good dynamic characteristics and so on. The excellent control characteristics made DC motor almost the only choice in the occasions with speed regulation and control requirements for a long time before the 1970s. However, the structure of DC motor is complex, its stator has exciting windings to produce the main magnetic field, and DC motors with high power are often equipped with commutation poles to improve the commutation performance of the motor. The armature winding and commutator are placed on the rotor of the DC motor. The DC power supply sends the DC power to the armature winding through the brush and commutator and converts it into the alternating current in the armature winding, that is, mechanical current commutation. The complex structure limits the further reduction of the volume and weight of the DC motor, especially the sliding contact between the brush and the commutator causes mechanical wear and sparks, which makes the DC motor have many faults, low reliability, short service life and heavy maintenance workload. Commutation sparks not only cause electric corrosion of the commutator, but also a source of radio interference, which will have a harmful impact on the surrounding electrical equipment. The larger the capacity and the higher the speed of the motor, the more serious the problem will be. Therefore, the brush and commutator of ordinary DC motor limit the development of DC motor to high speed and large capacity

in AC, people also widely use AC asynchronous motors to drive working machinery. AC asynchronous motor has the advantages of simple structure, reliable operation, long service life, low cost and simple maintenance. However, compared with DC motor, it has poor speed regulation performance, small starting torque, low overload capacity and efficiency. The generation of its rotating magnetic field requires the absorption of reactive power from electricity, so the power factor is low, especially under light load, which greatly increases the loss of lines and electricity. For a long time, in occasions that do not require speed regulation, such as the drive of fans, water pumps and ordinary machine tools, asynchronous motors have played a leading role. Of course, in this kind of drive, a large amount of electric energy has been virtually lost

in the past, there were few synchronous motors in electric drive. The main reason was that synchronous motors could not start automatically under electric voltage, and the average torque of stationary rotor magnetic poles was zero under the action of rotating magnetic field. People also know that variable-frequency power supply can solve the starting and speed regulation problems of synchronous motors, but before the 1970s, variable-frequency power supply was a conceivable but unavailable equipment. Therefore, in the past, synchronous motors were rarely used as prime movers in electric drive. In the high power range, there are occasional examples of synchronous motor operation, but it is often used to improve the electric power factor of large enterprises

since the 1970s, the development of science and technology has greatly promoted the development and application of synchronous motors. The main reasons are:

1. The development of high-performance permanent magnet materials

permanent magnet materials have been developed rapidly in recent years. There are three categories of aluminum nickel cobalt, ferrite and rare earth permanent magnets. Rare earth permanent magnets also include the first generation samarium cobalt 1:5, the second generation samarium cobalt 2:17 and the third neodymium iron boron. Al Ni Co is a permanent magnet material successfully developed in the 1930s. Although it has the advantages of high remanence induction strength and good thermal stability, it has low coercivity, poor anti demagnetization ability, and the use of precious metal cobalt has high cost. These shortcomings greatly limit its application in motors. Ferrite magnets are permanent magnet materials developed in the early 1950s. Their biggest characteristics are low price and high coercivity. Their shortcomings are low remanence induction intensity and magnetic energy product. Samarium cobalt rare earth permanent magnet material came out in the mid-1960s. It has the same high remanence induction strength as aluminum nickel cobalt, and the coercivity is higher than ferrite, but the price of samarium rare earth material is higher. Neodymium iron boron rare earth permanent magnet material appeared in the early 1980s. It has high remanence induction strength, high coercivity and high magnetic energy product, which are especially suitable for use in motors. Their shortcomings are high temperature coefficient, low Curie point, easy to oxidize and rust and need coating treatment. After several years of continuous improvement, most of these shortcomings have been overcome. Now the maximum working temperature of Nd-Fe-B permanent magnet material has reached 180 ℃, generally 150 ℃, which is enough to meet the requirements of most motors. Table 1 shows the performance comparison of various permanent magnet materials. Table 1 performance comparison of various permanent magnet materials

the development of permanent magnet materials has greatly promoted the development and application of permanent magnet synchronous motors. The advantages of using permanent magnet to replace the traditional electrically excited magnetic pole in synchronous motor are:

using permanent magnet to replace the electrically excited magnetic pole simplifies the structure, eliminates the slip ring and brush of the rotor, realizes the brushless structure, and reduces the volume of the rotor; The exciting DC power supply is omitted, and the exciting loss and heating are eliminated. Nowadays, the vast majority of small and medium power synchronous motors have adopted permanent magnet structure

2. The development of power electronic technology has greatly promoted the development and application of permanent magnet synchronous motor

power electronics technology is an important interface between information industry and traditional industry, and a bridge between weak current and strong current. Since the invention of the first power semiconductor switch thyristor in the world in 58, power electronic components have experienced the first generation of semi controlled thyristors, The third-generation composite field control devices (insulated gate power transistor IGBT, electrostatic induction transistor sit, MOS controlled transistor MCT, etc.) of the second-generation semiconductor devices with self turning off ability (high-power transistor GTR, turn off thyristor GTO, power MOSFET) until the fourth generation power integrated circuit IPM appeared in the 1990s. With the continuous improvement of the performance of semiconductor switching devices, the capacity increases rapidly, the cost is greatly reduced, and the control circuit is becoming more and more perfect. It greatly promotes the control of all kinds of motors. In the 1970s, a series of products of general frequency converter appeared, which can convert the power frequency power supply into the frequency conversion power supply with continuous adjustable frequency, which created conditions for the frequency conversion and speed regulation of AC motors. These frequency converters have a soft start function after the frequency is set. The frequency will rise from zero at a certain rate, and the rising rate can be adjusted arbitrarily in a large range, which solves the starting problem for synchronous motors. For the latest self synchronous permanent magnet synchronous motor, the inverter circuit composed of high-performance power semiconductor switches is an essential power link of its control system

3. The development of scale integrated circuit and computer technology has completely changed the modern yongruo. If you want to extend the application life of equipment, the control integrated circuit and computer technology of magnetic synchronous motor are the representatives of the development of electronic technology. It is not only the core of high-tech electronic information industry, but also the transformation foundation of many traditional industries. Their rapid development has promoted the development and innovation of motor control technology

In the 1970s, people put forward the concept of vector control for AC motor. The main idea of this theory is to decompose the three-phase current of the armature winding of AC motor into excitation current component and torque current component through coordinate transformation, so that the AC motor can be simulated as a DC motor to control, which can obtain the same good dynamic speed regulation characteristics as the DC motor. This control method has been mature and has been successfully applied in AC servo system. Because this method adopts coordinate transformation, it puts forward high requirements for the operation speed, data processing ability, real-time control and control accuracy of the controller, and the single chip microcomputer often cannot meet the requirements. In recent years, various integrated digital signal processors (DSPS) have developed rapidly, their performance has been continuously improved, and there are more and more software and development tools. There are high-performance and low-cost DSPs specially used for motor control. The development of integrated circuit and computing technology has played an important role in promoting the control technology of permanent magnet synchronous motor

second, the operation control method of permanent magnet synchronous motor

the operation of permanent magnet synchronous motor can be divided into external synchronization and self synchronization. An independent variable-frequency power supply is used to supply power to the permanent magnet synchronous motor, and the speed of the synchronous motor strictly changes with the power frequency, which is the operation of the external synchronous permanent magnet synchronous motor. External synchronous operation is often used in open-loop control. Due to the strict relationship between speed and frequency, this operation mode is suitable for occasions where multiple motors require strict synchronous operation. For example, in the textile industry, spindle drive, conveyor belt roller drive and other occasions. Therefore, a large capacity frequency converter can be selected to supply power to multiple permanent magnet synchronous motors at the same time. Of course, the frequency converter must be able to soft start, and the output frequency can gradually rise from low to high to solve the starting problem of synchronous motor

for the so-called self synchronous permanent magnet synchronous motor, the position of the rotating magnetic field generated by its stator winding is determined by the position of the permanent magnet rotor, which can automatically maintain a space angle of 900 with the rotor magnetic field to produce the maximum motor torque. The speed of rotating magnetic field is strictly determined by the speed of permanent magnet rotor. In addition to the inverter switching circuit, the permanent magnet synchronous motor running in this way also needs a sensor that can detect the rotor position. The switching operation of the inverter, that is, the polyphase current obtained from the stator winding of the permanent magnet synchronous motor, is completely controlled by the signal given by the rotor position detection device. This operation mode of stator rotating magnetic field is determined by the stator position, that is, the operation mode of self synchronous permanent magnet synchronous motor, which is a new mode developed from the late 1960s. The operation mode of self synchronous permanent magnet synchronous motor is analyzed in principle. It has the characteristics of DC motor, has stable starting torque, can start by itself, and can be similar to DC motor for closed-loop control of motor. Self synchronous permanent magnet synchronous motor has become the main application mode of permanent magnet synchronous motor

self synchronous permanent magnet synchronous motors can be divided into square wave motors and sine wave motors according to the form of current added to the motor stator winding. The current type square wave current in the winding of square wave motor, and the analysis of its working principle shows that its working principle is exactly the same as that of brush DC motor. The difference is that it uses electronic switch circuit and rotor position sensor to replace the commutator and brush of brush DC motor, so as to realize the brushless of DC motor and maintain the good control characteristics of DC motor. Therefore, this kind of square wave motor is used to be called brushless DC motor. This is the most widely used and promising self synchronous permanent magnet synchronous motor at present

the stator winding of sine wave self synchronous permanent magnet synchronous motor gets symmetrical three-phase AC, but the frequency, phase and amplitude of three-phase AC are determined by the position signal of the rotor. Photoelectric encoder is usually used for rotor position detection, which can accurately obtain the instantaneous rotor position information. Its control usually adopts single chip microcomputer or digital signal processor (DSP) as the core unit of the controller. Because its control performance, control accuracy and torque stability are better than brushless DC motor control system, it is mainly used in modern high-precision AC servo control system

III. application of permanent magnet synchronous motor in modern industry

AC difference is commonly used in drive motors in modern industry and agriculture

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