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Overview of rare earth permanent magnet motor development

Motor is an electromagnetic equipment which takes the magnetic field as the preface to transform mechanical energy and electric energy into each other. There are two methods to establish the air gap magnetic field necessary for electromechanical energy conversion in the motor. One is to generate current in the magnetic tile of motor winding, which not only requires special magnetic tile group of DC motor and corresponding equipment, but also needs to continuously supply energy to adhere to the current flow, such as general DC motor and synchronous motor; The other is to generate magnetic field by permanent magnet, which can not only simplify the motor structure, but also save energy. This is permanent magnet motor.

2 development overview of permanent magnet motor

The development of permanent magnet motor is closely related to the development of permanent magnet data. China is the first country in the world to discover the magnetic characteristics of permanent magnet data and apply it to practice. More than 2000 years ago, China made a compass by using the magnetic characteristics of permanent magnet data, which has played a great role in navigation, military and other fields, and has become one of the four great creations in ancient China.

The first motor in the world presented in the 1920s is a permanent magnet motor with an excitation magnetic field generated by a permanent magnet. However, the permanent magnet material used at that time was natural magnet ore (Fe3O4), and the magnetic energy density was very low. The motor made of it had a huge volume and was soon replaced by electrically excited motor.

With the rapid development of various motors and the creation of current magnetizers, people have conducted in-depth research on the mechanism, composition and manufacturing skills of permanent magnet materials, and found a variety of permanent magnet materials such as carbon steel, tungsten steel (maximum magnetic energy product is about 2.7kj/m3), cobalt steel (maximum magnetic energy product is about 7.2kj/m3) and so on. In particular, the aluminum nickel cobalt permanent magnet (the maximum magnetic energy product can reach 85kj / m3) presented in the 1930s and the ferrite permanent magnet (the maximum magnetic energy product can now reach 40kj / m3) presented in the 1950s have made great progress in magnetic function, and various micro and small motors are excited by permanent magnets. The power of permanent magnet motor is as small as several milliwatts and as large as tens of kilowatts. It has been widely used in military, industrial and agricultural production and daily life, and the output value has increased sharply. Accordingly, during this period, the planning theory, accounting method, magnetization and manufacturing skills of permanent magnet motor have also been broken, and a set of analysis and research methods represented by the graphic method of permanent magnet working diagram have been formed.

However, the coercivity of alnico permanent magnet is low (36 ~ 160kA / M) and the residual magnetic density of ferrite permanent magnet is not high (0.2 ~ 0.44t), which restricts their application scale in motor. Until the 1960s and 1980s, rare earth cobalt permanent magnet and neodymium iron boron permanent magnet (both collectively referred to as rare earth permanent magnet) came out one after another. Their excellent magnetic functions of high remanence density, high coercivity, high magnetic energy product and linear demagnetization curve are especially suitable for making motors, so that the development of permanent magnet motors has entered a new historical period.

The development of rare earth permanent magnet data can be roughly divided into three stages. Samarium cobalt permanent magnet discovered by Professor K.J. strnat in 1967 is the first generation of rare earth permanent magnet. Its chemical formula can be expressed as rco5, referred to as 1:5 rare earth permanent magnet for short. The maximum magnetic energy product of the product exceeds 199kj / m3 (25mg · OE). In 1973, the second generation of rare earth permanent magnet with better magnetic function was presented. Its chemical formula is r2co17, which is referred to as 2:17 rare earth permanent magnet for short. The maximum magnetic energy product of the product reaches 258.6kj/m3 (32.5mg · OE). In 1983, Sumitomo Special Metals Co., Ltd. of Japan and General Motors Co., Ltd. of the United States respectively developed neodymium iron boron (NdFeB) permanent magnet, which is called the third generation rare earth permanent magnet. Because the magnetic function of NdFeB permanent magnet is higher than that of other permanent magnet materials, and the price is lower than that of rare earth cobalt permanent magnet materials. The content of Neodymium in rare earth ore is more than ten times that of samarium, and does not contain strategic material cobalt, it has attracted great attention from the magnetic and electrical circles at home and abroad, and has invested a lot of human and material resources in research and development. Now we are discussing new and higher functional permanent magnet materials, such as samarium iron nitrogen permanent magnet and nanocomposite rare earth permanent magnet, hoping to make a new and greater breakthrough.

Correspondingly, the research and development of rare earth permanent magnet motor can be divided into three stages.

The first stage: in the late 1960s and 1970s, due to the high price of rare earth cobalt permanent magnet, the key points of research and development are aviation and aerospace motors and high-tech areas requiring high function rather than price.

The second stage: in the 1980s, especially after the relatively low price of NdFeB permanent magnet was presented in 1983, the key points of research and development at home and abroad were transferred to industrial and civil motors. The excellent magnetic function of rare earth permanent magnet, coupled with the rapid development of power electronic devices and microcomputer skills, not only makes many traditional electric excitation motors replaced by rare earth permanent magnet motors, but also can complete the high functions that are difficult to reach by traditional electric excitation motors.

The third stage: in the 1990s, with the continuous progress and improvement of permanent magnet data function, especially the improvement of thermal stability and corrosion resistance of NdFeB permanent magnet and the gradual decline of price, as well as the further development of power electronic devices, and the gradual sophistication of research and development experience of permanent magnet motor, in addition to vigorously promoting and applying the existing research results, permanent magnet motor will be used in national defense In addition to the more and more extensive application in industrial and agricultural production and daily life, the research and development of rare earth permanent magnet motor has entered a new stage. On the one hand, it is developing in the direction of high power (high speed, high torque), high function and miniaturization. Now, the single capacity of rare earth permanent magnet motor has exceeded 1000kW, the maximum speed has exceeded 300000 R / min, the minimum speed is less than 0.01r/min, and the minimum motor outer diameter is only 0.8mm and 1.2mm long. On the other hand, the research on the planning theory, accounting methods, structural technology and control skills of permanent magnet motor presents a new situation. Many relevant academic papers and scientific research achievements are presented, forming a set of analysis and research methods and accounting machine aided planning software based on the combination of electromagnetic field numerical accounting and equivalent magnetic circuit analytical solution.

China is rich in rare earth resources and rare earth is not rare. It is called "Rare Earth Kingdom". The output value of rare earth ore and rare earth permanent magnet ranks in the forefront of the world. The scientific research level of rare earth permanent magnet materials and rare earth permanent magnet motors has reached the international advanced level. Therefore, giving full play to the advantages of China's rich rare earth resources and vigorously studying and popularizing the use of various permanent magnet motors represented by rare earth permanent magnet motors have important theoretical meaning and practical value for the completion of China's socialist modernization.

3 main features and application of permanent magnet motor

Compared with traditional electric excitation motor, permanent magnet motor, especially rare earth permanent magnet motor, has simple structure and reliable operation; Small volume and light weight; Low loss and high power; The shape and scale of the motor can be flexible and diverse. Therefore, it is widely used in various fields, such as aerospace, national defense, industrial and agricultural production and daily life. The main features and application occasions of several typical permanent magnet motors are introduced below.

3.1 rare earth permanent magnet generator

Compared with traditional generators, permanent magnet synchronous generators do not need collector rings and brush equipment. The structure is simple and reduces the failure rate. The selection of rare earth permanent magnet can also increase the air gap flux density, improve the motor speed to the best value and improve the power mass ratio. Today's aviation and aerospace generators almost all use rare earth permanent magnet generators. Its typical products are 150KVA 14 pole 12000r / min ~ 21000r / min and 100kVA 60000r / min rare earth cobalt permanent magnet synchronous generators made by GE. The first rare earth permanent magnet motor developed in China is a 3kw20000r / min permanent magnet generator.

Permanent magnet generator is also used as the auxiliary exciter of large steam turbine generator. In the 1980s, China successfully developed the 40KVA ~ 160KVA rare earth permanent magnet auxiliary exciter with the largest international capacity. After being equipped with 200MW ~ 600MW steam turbine generator, the reliability of power station operation is greatly improved.

Now, small generators driven by internal combustion engines with independent power supply, permanent magnet generators for vehicles and small permanent magnet wind turbines directly driven by wind wheels are gradually popularized.

3.2 high efficiency permanent magnet synchronous motor

Compared with induction motor, permanent magnet synchronous motor does not need reactive excitation current, which can significantly improve the power factor (up to 1, even capacitive), reduce the stator current and stator resistance loss, and there is no rotor copper consumption during safe and stable operation, so it can reduce the electric fan (small capacity motor or even remove the electric fan) and the corresponding wind friction loss, and the power can be improved by 2 ~ 8 percentage points compared with the same standard induction motor. Moreover, the permanent magnet synchronous motor can adhere to high power and power factor within 25% ~ 120% of the rated load scale, which makes the energy-saving effect more remarkable when operating under light load. This kind of motor is generally equipped with starting winding on the rotor, which can be started directly at a certain frequency and voltage. Now it is mainly used in oil fields, textile and chemical fiber industry, ceramic and glass industry and fans and pumps with long annual operation time.

The high efficiency and high starting torque Nd-Fe-B permanent magnet synchronous motor independently developed in China can deal with the problem of "big horse pulling small car" in oilfield application. The starting torque is 50% ~ 100% larger than that of induction motor, which can replace the induction motor with a larger frame number, and the power saving rate is about 20%.

In the textile and chemical fiber industry, the load moment of inertia is large and requires high pull in torque. Reasonable planning of no-load flux leakage coefficient, salient pole ratio, rotor resistance, permanent magnet scale and stator winding turns of permanent magnet synchronous motor can improve the pull in function of permanent magnet motor and promote its application in new textile and chemical fiber industry.

The motors used for hundreds of kilowatts and megawatts of fans and pumps in large-scale power stations, mines, petroleum, chemical industries and other occupations are large energy consumers. However, the power and power factor of the motors used now are low. After switching to Nd-Fe-B permanent magnet, it not only improves the power and power factor and saves energy, but also has a brushless structure, which improves the reliability of operation. At present, 1120kw permanent magnet synchronous motor is the most powerful asynchronous starting high-efficiency rare earth permanent magnet motor in the world. The power is higher than 96.5% (the power of the same standard motor is 95%) and the power factor is 0.94. It can replace the general motor with 1 ~ 2 power levels higher than it.

3.3 communication between servo permanent magnet motor and brushless DC permanent magnet motor

Now more and more variable frequency power supply and communication motor are used to form communication speed regulation system to replace DC motor speed regulation system. In the communication motor, the speed of permanent magnet synchronous motor maintains a constant relationship with the power frequency during safe and stable operation, so that it can be directly used in the open-loop variable frequency speed regulation system. This kind of motor is generally started by the gradual increase of the frequency of the frequency converter. The starting winding can not be set on the rotor, and the brush and commutator are omitted, which is convenient for maintenance.

Permanent magnet synchronous motor powered by frequency converter and rotor orientation closed-loop control system constitute self synchronous permanent magnet motor, which not only has the excellent speed regulation function of electrically excited DC motor, but also completes brushless. It is mainly used in occasions with high control accuracy and high reliability, such as aviation, aerospace, CNC machine tools, machining centers, robots, electric cars, accounting machine peripherals, etc.

The Nd-Fe-B permanent magnet synchronous motor and drive system with wide speed regulation scale and high constant power speed regulation ratio have been developed. The speed regulation ratio is as high as 1:22500 and the limit speed reaches 9000r / min. Permanent magnet synchronous motor has the characteristics of high efficiency, small oscillation, low noise and high torque density. It is the most ideal motor in electric vehicles, machine tools and other driving equipment.

With the continuous progress of people's living standards, the requirements for household appliances are higher and higher. For example, household air conditioner is not only a large power consuming part, but also the primary source of noise. Its development trend is to use permanent magnet brushless DC motor with stepless speed regulation. It can not only actively adjust to the appropriate speed for long-term operation according to the change of room temperature, reduce noise and oscillation, make people feel more comfortable, but also save 1 / 3 of power than the air conditioner without speed regulation. Gradually switch to DC fans, washing machines, etc.

3.4 permanent magnet DC motor

After using permanent magnet excitation, the DC motor not only retains the excellent speed regulation characteristics and mechanical characteristics of the electrically excited DC motor, but also has the characteristics of simple structure and technology, small volume, less copper consumption and high power because it eliminates the excitation winding and excitation loss. Therefore, permanent magnet DC motors are widely used in household appliances, portable electronic equipment, electric tools and fine speed and azimuth transmission systems requiring outstanding dynamic functions. Among micro DC motors below 500W, permanent magnet motors account for 92%, while permanent magnet motors below 10W account for more than 99%.

Now, China's car industry is developing rapidly. The car industry is the largest user of permanent magnet motors, and motors are the key components of cars. In an ultra luxury car, there are more than 70 motors for different purposes, most of which are low-voltage permanent magnet DC micro motors. The quality of starter motor for cars and motorcycles can be reduced by half after selecting Nd-Fe-B permanent magnet and reducing planetary gear.

3.5 permanent magnet motors with several new structures

3.5.1 coreless NdFeB permanent magnet motor

Using the excellent characteristics of high coercivity of Nd-Fe-B permanent magnet data, the coreless motor is made without or less silicon steel sheet, and the quality is greatly reduced. The Coreless Permanent magnet motor adopts the magnetic gathering structure and sine cosine magnetization, and the generated magnetic field is sinusoidal dispersion. Therefore, it can not be inclined, and the centralized winding can be selected, which is convenient for AC control. The winding end is short, the loss is small, the torque density is high, and the oscillation noise is significantly reduced. It is used in many aspects, such as car steering wheel drive, robot, elevator and DVD drive.

3.5.2 transverse flux NdFeB permanent magnet motor

In order to deal with the opposition between the width of the slot where the coil is placed and the width of the tooth through which the magnetic flux flows, and improve the power density and torque density of the motor, people continue to explore new magnetic circuit structures and present the structural thinking of transverse flux machine. The stator cogging structure and armature coil of the motor are perpendicular to each other in space, the main magnetic flux flows along the axial direction of the motor, and there is no competition between current and magnetic load in space. Therefore, the size of the stator and the size of the energized coil are independent of each other, which can be arbitrarily selected within a certain scale, improving the power density. At present, the international research and development of this structure motor has just started, and has a broad development prospect.

3.6 permanent magnet special motor

There are many kinds of control motors and special motors. One of their common development trends is permanent magnetization, and the permanent magnet excitation with high power gradually replaces the electric excitation.

Because rare earth permanent magnet has the characteristics of high remanence density, high coercivity and high magnetic energy product, it can allow the motor to have large air gap length and air gap density. Therefore, it has great flexibility in the placement of permanent magnet and magnetic circuit structure planning. It can be made into structural shapes and scales different from traditional motors according to the application occasions, especially the needs of cars, accounting machines and aerospace engineering, such as disc motor, slot less motor, etc. This can further reduce the mass and moment of inertia of the motor and improve the response sensitivity of the motor; It can also reduce the ripple of motor torque and add the stability of operation; It can also simplify the structure and process of the motor. Therefore, it is widely used in the peripheral equipment of accounting machine, office equipment and occasions requiring precision positioning and control.

The voice coil motor, a moving coil linear motor used to drive the read-write head to move back and forth in the disk drive of the accounting machine, requires a high-power magnet to ensure the sensitivity, reduce the volume and reduce the mass. Nd-Fe-B permanent magnet can meet this requirement. In the 1960s, the voice coil motor for 14 inch disk drive was developed with ferrite permanent magnet. Since the selection of Nd-Fe-B permanent magnet, the size of the driver continues to shrink and the memory