What is permanent magnet motor?

What is permanent magnet motor?

Permanent magnet motor uses permanent magnet to generate the magnetic field of the motor, without excitation coil or excitation current, with high efficiency and simple structure, it is a good energy-saving motor. With the advent of high-performance permanent magnet materials and the rapid development of control technology, the application of permanent magnet motor will become more extensive.

20210209005540 53969 - What is permanent magnet motor?

Development history of permanent magnet motor

The development of permanent magnet motor is closely related to the development of permanent magnet materials. China is the first country in the world to discover the magnetic properties of permanent magnet materials and apply them to practice. More than 2000 years ago, China made compass from the magnetic properties of permanent magnet materials, which played a great role in navigation, military and other fields, and became one of the four great inventions in ancient China.
In the 1920s, the world’s first motor was the permanent magnet motor which generated excitation magnetic field by permanent magnet. However, the permanent magnet material used at that time was natural magnetite (Fe3O4), and the magnetic energy density was very low. The motor made of it was huge, and it was soon replaced by electric excitation motor.
With the rapid development of all kinds of motors and the invention of current magnetizer, the mechanism, composition and manufacturing technology of permanent magnet materials have been deeply studied, and many kinds of permanent magnet materials have been found, such as carbon steel, tungsten steel (the maximum magnetic energy product is about 2.7 kJ / m3), cobalt steel (the maximum magnetic energy product is about 7.2 kJ / m3). In particular, the alnico permanent magnet appeared in the 1930s (the maximum energy product can reach 85kj / m3) and the ferrite permanent magnet appeared in the 1950s (the maximum energy product can reach 40kj / m3). The magnetic properties have been greatly improved, and various micro and small motors have used permanent magnet excitation. The power of permanent magnet motor is as small as several milliwatts, as large as tens of kilowatts. It has been widely used in military, industrial and agricultural production and daily life, and the output has increased dramatically. Accordingly, during this period, breakthroughs have been made in the design theory, calculation method, magnetization and manufacturing technology of permanent magnet motor, and a set of analysis and research methods represented by the working diagram method of permanent magnet have been formed. However, the coercivity of alnico permanent magnets is low (36 ~ 160kA / M), and the remanence density of ferrite permanent magnets is low (0.2 ~ 0.44t), which limits their application in motor. Until the 1960s and 1980s, rare earth cobalt permanent magnet and neodymium iron boron permanent magnet (both referred to as rare earth permanent magnet) came out one after another. Their excellent magnetic properties of high remanence density, high coercivity, high magnetic energy product and linear demagnetization curve are especially suitable for manufacturing motor, which makes the development of permanent magnet motor enter a new historical period.

Characteristics and application of permanent magnet motor

Compared with traditional electrically excited magnetic motors, permanent magnetic motors, especially rare earth permanent magnetic motors, have the advantages of simple structure and reliable operation; Small volume and light weight; Small loss and high efficiency; the shape and size of the motor can be flexible and diverse. Therefore, the scope of application is extremely wide, almost in all fields of aerospace, national defense, industrial and agricultural production and daily life. The following introduces the main characteristics and main application occasions of several typical permanent magnet motors.
1. Rare earth permanent magnet generator permanent magnet synchronous generator does not need collector ring and brush device compared with traditional generator, and its structure is simple, thus reducing failure rate. The use of rare earth permanent magnet can also increase the air gap magnetic density, and increase the motor speed to the best value, improve the power mass ratio. Almost all generators used in contemporary aviation and aerospace use rare earth permanent magnetic generators. Its typical product is 150kVA 14-pole 12000r/min ~ 21000r/min and 100kVA 60000r/min Rare Earth cobalt permanent magnet synchronous generator manufactured by the General Electric Company of the United States. The first rare earth permanent magnet motor researched and developed in China is 3kW, r/min permanent magnet generator.
The permanent magnetic generator is also used as the auxiliary Exciter of the large steam turbine generator. In the 1980 s, our country successfully developed the rare earth permanent magnetic auxiliary exciter with the capacity of 40kVA ~ 160kVA in the world at that time, the reliability of the operation of the power station is greatly improved after equipped with 200MW ~ 600MW turbogenerator.
At present, small generators driven by internal combustion engines for independent power sources, small permanent magnet generators for vehicles, and small permanent magnet wind generators directly driven by wind wheels are being gradually promoted.
2. High-efficiency permanent magnet synchronous motor compared with induction motor, permanent magnet synchronous motor does not need reactive power excitation current, which can significantly improve the power factor (can reach 1, or even be capacitive), and reduce the stator current and stator resistance loss, moreover, there is no rotor copper consumption in stable operation, thus the fan (small capacity motors can even remove fans) and corresponding wind friction loss can be reduced, the efficiency of induction motors with the same specification can be increased by 2~8 percent age points. Moreover, the permanent magnet synchronous motor can maintain high efficiency and power factor in the range of 25%~ 120% rated load, which makes the energy saving effect of light load operation more obvious. Such motors are generally equipped with starting winding on the rotor, with the ability to directly start at a certain frequency and voltage. At present, it is mainly used in oil fields, textile and chemical fiber industries, ceramic glass industries and fan pumps with long years of operation.
The highly efficient and high starting torque NdFeB permanent magnet synchronous motor independently developed by our country can solve the problem of “big horse pull car” in the oil field application, and the starting torque is 50% to 100% larger than the induction motor, it can replace the induction motor with one larger base number, and the power saving rate is about 20%.
In textile and chemical fiber industry, the moment of inertia of load is large, which requires high pull-in torque. Reasonable design of no-load magnetic flux leakage coefficient, saliency ratio, rotor resistance, size of permanent magnet and number of turns in stator winding of permanent magnet synchronous motor can improve the pulling performance of permanent magnet motor, making it applied in new textile and chemical fiber industry.
The motor of several hundred kilowatts, megawatts-level draught fans and pump motor used in the industries of large power stations, mines, petroleum and chemical industries and so on are large energy-consuming customers, but the efficiency and power factor of the motor used are relatively low at present, after switching to neodymium iron boron permanent magnet, it not only improves the efficiency and power factor, saves energy, but also improves the reliability of operation for brushless structure. Currently, 1120kW permanent magnet synchronous motor is the most powerful asynchronous starting efficient rare earth permanent magnet motor in the world, whose efficiency is over 96.5% (the efficiency of motors with the same specification is 95%), and the power factor is 0.94, ordinary motors with 1-2 power levels larger than it can be replaced.
3. AC servo permanent magnet motor and Brushless DC permanent magnet motor now increasingly use frequency conversion power supply and AC motor to form ac speed regulation system to replace DC motor speed regulation system. In the AC motor, the speed of the permanent magnet synchronous motor maintains a constant relationship with the power supply frequency during stable operation, so that it can be directly used for open-loop frequency conversion speed regulation system. This kind of motor is usually started by the gradual increase of the frequency converter, and the starting winding can be set on the rotor, and the brush and commutator are eliminated, and the maintenance is convenient.
The permanent magnet synchronous motor powered by the frequency converter and the rotor position closed-loop control system constitute the self-synchronous permanent magnet motor, which not only has the excellent speed regulation performance of the electrically excited DC motor, but also realizes the brushless, it is mainly used in high control precision and high reliability occasions, such as aviation, aerospace, CNC machine tools, machining centers, robots, electric vehicles, computer peripherals, etc.
The neodymium iron boron permanent magnet synchronous motor with wide speed regulation range and high constant power speed regulation ratio and the drive system speed regulation ratio are as high as 1:22500, and the limit speed reaches 9000r/min. Permanent magnet synchronous motor is the most ideal motor in driving devices such as electric vehicles and machine tools, with the characteristics of high efficiency, small vibration, low noise and high torque density.
With the continuous improvement of people’s living standards, the requirements for household appliances are getting higher and higher. For example, household air conditioners are not only large pieces of electricity consumption, but also the main source of noise, and their development trend is to use permanent magnet brushless DC motors that can provide stepless speed regulation. It can automatically adjust to the appropriate speed to run for a long time according to the change of room temperature, reduce noise and vibration, and make people feel more comfortable, it also saves electricity 1/3 than the non-adjustable air conditioner. Others such as refrigerators, washing machines, dust collectors, fans, etc. are also gradually switching to brushless DC motors.
4. Permanent magnet DC motor after DC motor adopt permanent magnet excitation, it not only retains good speed regulation characteristics and mechanical characteristics of electrically excited DC motor, but also has the advantages of simple structure process and small volume due to eliminating excitation winding and excitation loss, it has the characteristics of small amount of copper and high efficiency. Therefore, permanent magnet direct current motors are widely used in household appliances, portable electronic devices, electric tools, and precision speed and position drive systems requiring good dynamic performance. Among the micro DC motors below 500W, permanent magnet motors account for 92%, while permanent magnet motors below 10W account for more than 99%.
At present, China’s automobile industry is developing rapidly. The automobile industry is the largest user of permanent magnet motors. Motors are the key components of automobiles. In an ultra-luxury car, there are more than 70 motors for different purposes, most of them are low-voltage permanent magnet DC micromotors. Starter motor for automobile and motorcycle, using Nd-Fe-B permanent magnet and adopting reduction gear, can reduce the weight of starter motor by half.

Classification of permanent magnet motor

There are many kinds of permanent magnet motors. According to the function of motor, it can be divided into permanent magnet generator and permanent magnet motor.

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Permanent magnet motor can be divided into permanent magnet DC motor and permanent magnet AC motor. Permanent magnet AC motor refers to the multi-phase synchronous motor with permanent magnet rotor, so it is often called permanent magnet synchronous motor (PMSM).
Permanent magnet DC motor can be divided into permanent magnet brushless DC motor (BLDCM) and permanent magnet brushless DC motor (BLDCM).
In today’s world, the theory and technology of modern power electronics are developing greatly. With the advent of power electronic devices, such as MOSFET, IGBT, MCT, etc., fundamental changes have taken place in control devices. Since f. blasceke proposed the principle of AC motor vector control in 1971, the development of vector control technology has opened a new era of AC servo drive control. The continuous introduction of various high-performance microprocessors has further accelerated the development of AC servo system replacing DC servo system. It is an inevitable trend that ac-i servo system will replace DC servo system. However, permanent magnet synchronous motor (PMSM) with sine wave back EMF and brushless DC motor (Blix) with trapezoidal wave back emf will become the mainstream of high performance AC servo system because of their excellent performance.

Matters needing attention of permanent magnet motor

Magnetic circuit structure and design calculation

In order to give full play to the magnetic properties of all kinds of permanent magnet materials, especially the excellent magnetic properties of rare earth permanent magnet, and to produce a cost-effective permanent magnet motor, we can not simply apply the structure and design calculation method of traditional permanent magnet motor or electric excitation motor. We must establish a new design concept, re analyze and improve the magnetic circuit structure. With the rapid development of computer hardware and software technology, as well as the continuous improvement of modern design methods such as electromagnetic field numerical calculation, optimization design and simulation technology, through the joint efforts of the motor academic and engineering circles, breakthrough progress has been made in the design theory, calculation method, structure technology and control technology of permanent magnet motor, and the electromagnetic field numerical simulation technology has been formed A complete set of analysis and research methods and computer-aided analysis and design software, which combines calculation with analytical solution of equivalent magnetic circuit, are being improved.

Control problems

Permanent magnet motor can maintain its magnetic field without external energy, but it is very difficult to adjust and control its magnetic field from the outside. Permanent magnet generator is difficult to adjust its output voltage and power factor from the outside. Permanent magnet DC motor can not adjust its speed by changing the excitation. These make the application of permanent magnet motor limited. However, with the rapid development of MOSFET, IGBT and other power electronic devices and control technology, most permanent magnet motors can only control the armature without magnetic field control. It is necessary to combine the rare earth permanent magnet materials, power electronic devices and microcomputer control in the design to make the permanent magnet motor run in a new working condition.

Irreversible demagnetization

If the design or use is improper, permanent magnet motor may produce irreversible demagnetization or loss of excitation when the temperature is too high (NdFeB permanent magnet) or too low (ferrite permanent magnet), under the action of armature reaction caused by impulse current, or under severe mechanical vibration, so that the motor performance is reduced or even can not be used. Therefore, it is necessary to study and develop the methods and devices suitable for motor manufacturers to check the thermal stability of permanent magnet materials, and analyze the anti demagnetization ability of various structural forms, so as to adopt corresponding measures to ensure that the permanent magnet motor does not lose its excitation during design and manufacturing.

Cost

Ferrite permanent magnet motor, especially micro permanent magnet DC motor, has been widely used because of its simple structure, low quality and low total cost. Because the price of rare earth permanent magnet is still relatively expensive, the cost of rare earth permanent magnet motor is generally higher than that of electric excitation motor, which needs to be compensated by its high performance and saving operation cost. In some cases, such as the voice coil motor of computer disk drive, the performance is improved, the volume and mass are significantly reduced, and the total cost is reduced. In the design, it is necessary to compare the performance and price according to the specific application situation and requirements, and to innovate the structure technology and optimize the design to reduce the cost.
Source: China Permanent Magnet Manufacturer – www.ymagnet.com

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