Development of Rare Earth Permanent Magnet Motor

- Nov 01, 2017 -

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1 Introduction

The motor is a magnetic field for the medium for mechanical energy and electrical energy conversion of electromagnetic devices. There are two methods for establishing an air gap magnetic field necessary for electromechanical energy conversion within a motor. One is in the motor winding current generation, both the need for a special winding and the corresponding device, but also need to continue to supply energy to maintain current flow, such as ordinary DC motor and synchronous motor; the other is produced by the permanent magnet Magnetic field, can simplify the motor structure, but also save energy, which is permanent magnet motor.




2 Development of permanent magnet motor

The development of permanent magnet motor is closely related to the development of permanent magnet material. China is the world's first discovery of magnetic properties of permanent magnet materials and it applied to the practice of the country, more than two thousand years ago, China's use of magnetic properties of permanent magnetic materials made compass, in the maritime, military and other fields to play a huge Role, as one of China's four major inventions.

The first motor in the world in the 1820s was the permanent magnet motor that produced the field magnetic field by the permanent magnet. But the permanent magnet material used at that time was natural magnetite (Fe3O4), the magnetic energy density was very low, and the motor made with it was bulky and was soon replaced by the electric field motor.

With the rapid development of various motors and the invention of the current magnetizer, people on the permanent magnet material mechanism, composition and manufacturing technology carried out in-depth study, have found the carbon steel, tungsten steel (maximum energy product of about 2.7 kJ / m3 ), Cobalt steel (the largest magnetic energy product of about 7.2 kJ / m3) and other permanent magnetic materials. Especially in the 1930s, the occurrence of alnico permanent magnet (maximum energy product up to 85 kJ / m3) and 50 years of ferrite permanent magnet (maximum magnetic energy can reach up to 40 kJ / m3), magnetic properties are Greatly improved, a variety of micro and small motor and have used permanent magnet excitation. Permanent magnet motor power as small as several milliwatts, up to tens of kilowatts, in the military, industrial and agricultural production and daily life has been widely used, the output increased dramatically. Correspondingly, this period has made breakthrough progress in the design theory, calculation method, magnetization and manufacturing technology of permanent magnet motor, and formed a set of analysis and research methods represented by permanent magnet diagram.

However, the coercivity of alnico permanent magnet is low (36 ~ 160 kA / m), and the residual magnetic density of ferrite permanent magnet is not high (0.2 ~ 0.44 T), which limits their application in motor. Until the 1960s and 1980s, rare earth cobalt permanent magnet and NdFeB permanent magnet (both referred to as rare earth permanent magnet) have come out, their high residual magnetic density, high coercivity, high magnetic energy product and linear demagnetization curve Of the excellent magnetic properties are particularly suitable for the manufacture of motors, so that the development of permanent magnet motor into a new historical period.

The development of rare earth permanent magnet materials is broadly divided into three stages. In 1967, the United States KJStrnat Professor found that the samarium cobalt permanent magnet for the first generation of rare earth permanent magnet, the chemical formula can be expressed as RCo5, referred to as 1: 5 rare earth permanent magnet, the product of the maximum energy product of more than 199 kJ / m3 (25MG · Oe ). In 1973, there was a second generation of rare earth permanent magnet with better magnetic properties. The chemical formula was R2Co17, referred to as 2:17 rare earth permanent magnet, and the maximum energy product of the product reached 258.6 kJ / m3 (32.5MG · Oe). 1983 Japan's Sumitomo Specialty Metal Company and the United States General Motors Corporation successfully developed NdFeB (NdFeB) permanent magnet, known as the third generation of rare earth permanent magnet. As the magnetic properties of NdFeB permanent magnet is higher than that of other permanent magnetic materials, the price is lower than that of rare earth cobalt permanent magnet material. The content of neodymium in rare earth ore is ten times that of samarium, and it does not contain the strategic substance - cobalt Has aroused great concern at home and abroad the magnetic sector and the motor sector, have invested a lot of manpower and resources for research and development. Is currently studying the new higher performance permanent magnet materials, such as samarium iron nitrogen permanent magnet, nanocomposite rare earth permanent magnet, hoping to have a new and greater breakthrough.

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

The first stage: the late 1960s and 70s, due to the rare earth cobalt permanent magnet is expensive, research and development focus on aviation, aerospace motor and require high performance and the price is not the main factor in the high-tech field.

The second stage: the 20th century, 80 years, especially in 1983, the relatively low price of NdFeB permanent magnet, the domestic and foreign research and development focus shifted to industrial and civilian motor. Rare earth permanent magnet excellent magnetic properties, coupled with the rapid development of power electronic devices and microcomputer technology, not only many of the traditional electric field motor with rare earth permanent magnet motor to replace, and can achieve the traditional electric field motor is difficult to achieve high performance.

The third stage: into the 20th century, 90 years, with the continuous improvement of the performance of permanent magnet materials and improve, in particular, NdFeB permanent magnet thermal stability and corrosion resistance improvement and the gradual reduction of prices and further development of power electronic devices Development, coupled with the permanent magnet motor research and development experience gradually mature, in addition to vigorously promote and apply the existing research results, so that permanent magnet motor in the national defense, industrial and agricultural production and daily life and other aspects of more and more widely used, rare earth Permanent magnet motor research and development into a new stage. On the one hand, positive high power (high speed, high torque), high functionality and miniaturization direction. At present, the rare earth permanent magnet motor has a single capacity of more than 1 000 kW, the maximum speed has exceeded 300 000 r / min, the minimum speed of less than 0.01 r / min, the smallest motor diameter only 0.8 mm, 1.2 mm long. On the other hand, the research on the design theory, calculation method, structural process and control technology of permanent magnet motor has led to a new situation. The academic papers and scientific research achievements have appeared in large numbers, and the electromagnetic field numerical calculation and equivalent magnetic Road analysis and analysis of a combination of analytical research methods and computer-aided design software.

China's rare earth rich in resources, rare earth is not thin, known as "rare earth kingdom." Rare earth ore and rare earth permanent magnet production are in the forefront of the world. Rare earth permanent magnet materials and rare earth permanent magnet motor research level have reached the international advanced level. Therefore, it is of great theoretical and practical value to realize the socialist modernization of our country by giving full play to the advantages of rare earth resources in our country and vigorously studying and popularizing various permanent magnet motors represented by rare earth permanent magnet motors.


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3 permanent magnet motor of the main features and applications

Compared with the traditional electric field motor, permanent magnet motor, especially rare earth permanent magnet motor has a simple structure, reliable operation; small size, light weight; low loss, high efficiency; motor shape and size can be flexible and diverse and other significant advantages. And thus a wide range of applications, almost all over the aerospace, defense, industrial and agricultural production and daily life in various fields. The following describes the typical characteristics of several typical permanent magnet motor and its main application.


3.1 rare earth permanent magnet generator

Permanent magnet synchronous generator compared with the traditional generator does not require the collector ring and brush device, simple structure, reducing the failure rate. After the use of rare earth permanent magnet can also increase the air gap flux, and the motor speed to the best value, improve power quality ratio. Contemporary aviation, aerospace generators are almost entirely using rare earth permanent magnet generators. Typical products are rare earth cobalt permanent magnet synchronous generators of 150 kVA 14 pole 12 000 r / min to 21 000 r / min and 100 kVA 60 000 r / min manufactured by General Electric Company of the United States. Domestic research and development of the first rare earth permanent magnet motor is 3 kW 20 000 r / min permanent magnet generator.

The permanent magnet generator is also used as a sub-exciter for large-scale turbo-generator. In the 1980s, the world's largest 40 kVA ~ 160 kVA rare earth permanent magnet sub-exciter was equipped with 200 MW ~ 600 MW turbo-generator Greatly improve the reliability of power plant operation.

At present, the independent power supply with the internal combustion engine driven small generators, automotive permanent magnet generators, wind wheel direct drive small permanent magnet wind generators are gradually being promoted.


3.2 High efficiency permanent magnet synchronous motor

Compared with the induction motor, the permanent magnet synchronous motor can increase the power factor (up to 1 or even capacitive) without reducing the reactive current, reducing the stator current and the stator resistance loss, and there is no rotor copper consumption during stable operation , Which can reduce the fan (small capacity motor can even remove the fan) and the corresponding wind friction loss, efficiency than the same specifications induction motor can increase 2 to 8 percentage points. Moreover, the permanent magnet synchronous motor in the 25% ~ 120% rated load range can maintain a high efficiency and power factor, so that the light load operation energy saving effect is more significant. This type of motor is generally set on the rotor starting winding, with a frequency and voltage directly under the ability to start. At present, the main application in the oil field, textile and chemical fiber industry, ceramic glass industry and the long running time of the fan pumps and other fields.

China's independent development of high efficiency high starting torque NdFeB permanent magnet synchronous motor in the oil field application can solve the "big horse car" problem, the starting torque than the induction motor 50% to 100%, can replace a large seat Of the induction motor, saving rate of about 20%.

Textile fiber industry in the load moment of inertia, requiring high pull-in torque. Reasonable design of permanent magnet synchronous motor no-load magnetic flux leakage coefficient, salient pole ratio, rotor resistance, permanent magnet size and stator winding turns can improve the penetration of permanent magnet motor performance, prompting it to apply to the new textile and chemical industry.

Large power plants, mines, petroleum, chemical industry and other industries used hundreds of kilowatts and megawatt-class fan, pump motor is a large energy consumption, and the current motor efficiency and power factor is low, switch to NdFeB permanent magnet Improve the efficiency and power factor, save energy, and for the brushless structure, improve the reliability of the operation. The current 1 120kW permanent magnet synchronous motor is the world's most powerful asynchronous starting rare earth permanent magnet motor, the efficiency is higher than 96.5% (with the specifications of the motor efficiency of 95%), the power factor of 0.94, can replace it 1 to 2 power Level of ordinary motor.


3.3 AC servo permanent magnet motor and brushless DC permanent magnet motor

Now more and more use of variable frequency power supply and AC motor composed of AC speed control system to replace the DC motor speed control system. In the AC motor, the permanent magnet synchronous motor speed in the stable operation and power frequency to maintain a constant relationship, making it can be directly used for open-loop frequency control system. This type of motor is usually started by the frequency of the inverter to start, the rotor can not set the starting winding, and eliminates the brush and commutator, easy maintenance.

Frequency converter power supply permanent magnet synchronous motor plus rotor position closed-loop control system constitutes a self-synchronous permanent magnet motor, both with the excitation DC motor excellent speed performance, but also to achieve a brushless, mainly used in high control accuracy and high reliability Sexual occasions, such as aviation, aerospace, CNC machine tools, machining centers, robots, electric vehicles, computer peripherals and so on.

Has been developed into a wide speed range, high constant power ratio of NdFeB permanent magnet synchronous motor and drive system, speed ratio up to 1:22 500, the limit speed of 9 000 r / min. Permanent magnet synchronous motor high efficiency, small vibration, low noise, high torque density characteristics in the electric car, machine tools and other drive device is the best motor.

With the continuous improvement of people's living standards, the requirements of household appliances is getting higher and higher. For example, home air conditioners, both power consumption large, but also the main source of noise, its development trend is to use stepless speed regulation of permanent magnet brushless DC motor. It can be based on changes in room temperature, automatically adjust to the appropriate speed of long-term operation, reduce noise and vibration, make people feel more comfortable, but also less than the energy-saving air conditioner 1/3. Others such as refrigerators, washing machines, dust collectors, fans, etc. are also gradually switch to brushless DC motors.


3.4 permanent magnet DC motor

DC motor with permanent magnet excitation, both to retain the electric field DC motor good speed characteristics and mechanical properties, but also eliminates the need for excitation and excitation losses and the structure of a simple process, small size, with less copper, high efficiency Features. Thus, from the home appliances, portable electronic equipment, power tools to require a good dynamic performance of the precision speed and position of the transmission system are a large number of permanent magnet DC motor. 500 W below the miniature DC motor, permanent magnet motor accounted for 92%, and 10 W following the permanent magnet motor accounted for more than 99%.

At present, China's auto industry is developing rapidly, the automotive industry is the largest permanent magnet motor users, the motor is a key component of the car, a super-luxury cars, a variety of different uses of the motor up to 70 Taiwan, most of which is low pressure Magnetic DC micro motor. Automotive, motorcycles with starter motor, the use of NdFeB permanent magnet and the use of slowdown planetary gear, the starter motor can reduce the quality of half.


3.5 Several new structures of permanent magnet motor

3.5.1 non-core NdFeB permanent magnet motor

The use of NdFeB permanent magnet material high coercivity of the excellent features without or less use of silicon steel, made of non-core motor, the quality greatly reduced. Non-core permanent magnet motor with magnetic structure and cosine cosine magnetization, the resulting magnetic field was sinusoidal distribution, it can not chute, you can use the concentrated winding, easy AC control. Winding short end, low loss, high torque density, vibration noise significantly reduced. Used in automotive steering wheel drive, robots, elevators and DVD drive and many other aspects.

3.5.2 transverse magnetic flux NdFeB permanent magnet motor

In order to solve the contradiction between the width of the groove in which the coil is placed and the width of the tooth portion through which the magnetic flux flows, the power density and the torque density of the motor are improved, and a new magnetic circuit structure is continuously explored, and a transverse magnetic flux Flux Machine. The stator stator and the armature coil are spatially perpendicular to each other, and the main flux flows in the axial direction of the motor. There is no competition between the current and the magnetic load in space. Therefore, the stator size and the size of the energizing coil are independent of each other. Range can be arbitrarily selected to improve the power density. At present, the international research and development of this structure of the motor has just started, there is a broad development prospects.


3.6 permanent magnet special motor

Control motor and special motor types are many, one of its common trend is the permanent magnetization, high-performance permanent magnet excitation gradually replace the electric excitation.

Because the rare earth permanent magnet has the characteristics of high residual magnetic density, high coercivity and high magnetic energy product, it can tolerate the large air gap length and air gap density of the made motor. Therefore, in the design of permanent magnet and magnetic circuit structure There is a great deal of flexibility in making structural and dimensions different from traditional motors, such as disc motors, slotless motors, and the like, depending on the application, especially for automotive, computer and aerospace engineering. This can further reduce the motor quality and moment of inertia, improve the motor response sensitivity; also can reduce the motor torque pulsation, increase the smooth running; also can simplify the motor structure and process. And thus in the computer peripherals, office equipment and the need for precision positioning control occasions are widely used.

A moving coil linear motor for a reciprocating motion in a computer disk drive - a voice coil motor requires a high performance magnet to ensure sufficient sensitivity, reduced size and reduced quality. NdFeB permanent magnet just to meet this requirement. In the 1960s, the ferrite permanent magnet was developed with a 14-inch disk drive with a voice coil motor. Since the use of NdFeB permanent magnet, driven


Article from NdFeB Industry Network

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