Development of permanent magnet motor and its application in various fields

Development history of permanent magnet motor

The development of permanent magnet motors 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 two thousand years ago, China made a compass using 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 of ancient China.

The first motor in the world, which appeared in the 1820 s, was a permanent magnet motor with an excitation magnetic field generated by permanent magnets. However, the permanent magnet material used at that time was natural magnetite (Fe3O4), which had a very low magnetic energy density. The motor made of it was bulky and was soon replaced by an electrically excited motor.

With the rapid development of various motors and the invention of the current magnetizing device, people have conducted in-depth research on the mechanism, composition and manufacturing technology of permanent magnet materials, and successively discovered 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) and other permanent magnet materials.

In particular, the aluminum-nickel-cobalt permanent magnet (the maximum magnetic energy product can reach 85 kJ/m3) that appeared in the 1930 s and the ferrite permanent magnet (the maximum magnetic energy product can reach 40 kJ/m3) that appeared in the 1950 s have greatly improved their magnetic properties, and various micro and small motors have used permanent magnet excitation. The power of permanent magnet motors is as small as a few milliwatts and as large as tens of kilowatts. They are widely used in military, industrial and agricultural production and daily life, and their output has increased sharply. Accordingly, during this period, breakthroughs have been made in the design theory, calculation method, magnetizing and manufacturing technology of permanent magnet motors, and a set of analysis and research methods represented by the graphic method of permanent magnet working diagram has been formed.

However, the coercivity of AlNiCo permanent magnets is low (36~160 kA/m), and the remanence density of ferrite permanent magnets is not high (0.2~0.44 T), which limits their application in motors. Until the 1960 s and 1980 s, rare earth cobalt permanent magnets and neodymium iron boron permanent magnets (collectively referred to as rare earth permanent magnets) came out one after another. Their excellent magnetic properties of high remanence density, high coercivity, high magnetic energy product and linear demagnetization curve are particularly suitable for manufacturing motors, thus making the development of permanent magnet motors enter a new historical period.

 

Characteristics and Application of Permanent Magnet Motor

Compared with traditional electrically excited motors, permanent magnet motors, especially rare earth permanent magnet motors, have simple structure, reliable operation; small size, light weight; low loss, high efficiency; and the shape and size of the motor can be flexible and diverse. Therefore, the scope of application is extremely wide, almost all over the aerospace, national defense, industrial and agricultural production and every field of daily life. The following describes the main features of several typical permanent magnet motors and their main applications.

Compared with the traditional generator, the permanent magnet synchronous generator does not need the collector ring and brush device, the structure is simple, and the failure rate is reduced. The use of rare earth permanent magnets can also increase the air gap flux density, and increase the motor speed to the best value, improve the power quality ratio. Modern aviation and aerospace generators almost all use rare earth permanent magnet generators. Its typical products are 150 kVA 14 pole 12 000 r/min ~ 21 000 r/min and 100 kVA 60 000 r/min rare earth cobalt permanent magnet synchronous generators manufactured by General Electric Company of the United States. The first rare earth permanent magnet motor developed in China is a permanent magnet generator with a 3 kW of 20 000 r/min.

 

Permanent magnet generators are also used as auxiliary exciter for large steam turbine generators. In the 1980 s, China successfully developed the 40 kVA ~ 160 kVA rare earth permanent magnet auxiliary exciter with the largest capacity in the world at that time. Equipped with 200 MW ~ 600 MW steam turbine generators, the reliability of power station operation was greatly improved.

At present, independent power supply with internal combustion engine driven small generator, vehicle permanent magnet generator, wind wheel direct drive small permanent magnet wind generator is gradually promoted.

 

The important role of permanent magnet motors in various application fields

1 energy-saving rare earth permanent magnet motor to consumption as the main object, such as textile and chemical fiber industry with rare earth permanent magnet synchronous motor, oil, mining, rare earth permanent magnet synchronous motor in coal mine transportation machinery, rare earth permanent magnet synchronous motor drive all kinds of pumps and fans.

2 Various rare earth permanent magnet motors are used by various types of vehicles (cars, motorcycles, trains), and rare earth permanent magnet motors are a big market for zui. According to statistics, about 70% of the vehicle rare earth permanent magnet motor. For luxury cars, the motor for various applications has reached more than 70 sets. Since the requirements of various automotive motors are different, the choice of permanent magnet materials is different. Motor magnets are used in air conditioners, fans, and power windows. From a price point of view, the advantages of ferrite will continue in the future. Ignition coils, drives, sensors, still use Sm-Co sintered magnets. In addition, auto parts, also electric vehicles can not be ignored, as an environmentally friendly (EV) and hybrid electric vehicles (HEV).

3 rare earth permanent magnet motor ac servo system a set of electronic, high performance, speed control system of mechatronics machinery. The system is a self-controlled permanent magnet synchronous motor body. The system is used for the development of CNC machine tools, flexible manufacturing technology; also used for electric vehicles, rather than traditional thermal power vehicles, vehicle emissions free. Rare earth permanent magnet motor is a kind of high-tech industry with great development prospects.

4 new areas are mainly for the support of low-power rare earth permanent magnet synchronous motor variable frequency speed control system for new air conditioners and refrigerators, wireless electric gadgets for various rare earth permanent magnet DC micromotors, rare earth permanent magnet brushless DC motors are different power instruments. The demand for such motors is also great.

5 in aerospace applications for the advantages of rare earth permanent magnet materials, making it very suitable for aircraft engine applications. Although there are some applications of rare earth permanent magnet motors in the air (such as generator voltage and short circuit protection, etc.), experts at home and abroad agree that rare earth permanent magnet motors are an important development direction of the new generation of aero engines.

 

Cost issues

Ferrite permanent magnet motors, especially miniature permanent magnet DC motors, have been widely used because of their simple structure, reduced mass, and generally lower total cost than electrically excited motors. 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 operating cost savings.

In some cases, such as the voice coil motor of computer disk drive, the performance is improved after using NdFeB permanent magnet, 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 and requirements, and to make the choice, but also to innovate the structural process and optimize the design to reduce the cost.