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The “central” of permanent magnet motors – permanent magnets

The development of permanent magnet motors is closely related to the development of permanent magnet materials.

The “central” of permanent magnet motors – permanent magnets插图

The world’s first motor that appeared in the 1920s was a permanent magnet motor that generated an excitation magnetic field from a 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 of it was bulky, and was soon replaced by the electric excitation motor.

With the needs of the rapid development of various motors and the invention of current magnetizers, people have conducted in-depth research on the mechanism, composition, and manufacturing technology of permanent magnet materials, and have successively discovered carbon steel and tungsten steel (the maximum magnetic energy product is about 2.7 kJ/m3 ), cobalt steel (maximum energy product about 7.2 kJ/m3) and other permanent magnet materials. Especially the AlNiCo permanent magnets that appeared in the 1930s (the maximum energy product can reach 85 kJ/m3) and the ferrite permanent magnets that appeared in the 1950s (the maximum energy product can now reach 40 kJ/m3), the magnetic properties are excellent. It has been greatly improved, and various micro and small motors have used permanent magnet excitation.

1. Permanent magnet material

The “central” of permanent magnet motors – permanent magnets插图1

Motor magnets: The permanent magnet materials commonly used in motors include sintered magnets and bonded magnets. The main types are alnico, ferrite, samarium cobalt, and neodymium iron boron.

Alnico: Alnico permanent magnet material is the earliest and most widely used permanent magnet material, and its preparation process and technology are relatively mature. At present, there are factories in Japan, the United States, Europe, Russia, and China.

Permanent magnet ferrite material: In the 1950s, ferrite began to develop vigorously, especially in the 1970s, strontium ferrite with better performance in terms of coercive force and magnetic energy machine was put into production in large quantities, and the use of permanent magnet ferrite was rapidly expanded. the use of. As a non-metallic magnetic material, ferrite does not have the disadvantages of easy oxidation, low Curie temperature, and high cost of metal permanent magnet materials, so it is very popular.

Samarium cobalt material: a permanent magnet material with excellent magnetic properties that emerged in the mid-1960s, and its performance is very stable. Samarium cobalt is especially suitable for manufacturing motors in terms of magnetic properties, but because of its high price, it is mainly used in the research and development of military motors such as aviation, aerospace, weapons, and high-tech motors with high performance but not price.

NdFeB material: NdFeB magnetic material is an alloy of neodymium, iron oxide, etc., also known as the magnet. It has extremely high magnetic energy products and coercive force, and the advantages of high energy density make NdFeB permanent magnet materials widely used in modern industry and electronic technology. Miniaturization, weight reduction, and thinning are possible. Because it contains a lot of neodymium and iron, it is easy to rust. Surface chemical passivation is one of the best solutions at present.

2. The relationship between magnet performance and motor performance

2.1 The influence of residual magnetism

For DC motors, under the same winding parameters and test conditions, the higher the residual magnetism, the lower the no-load speed, and the smaller the no-load current; the greater the maximum torque, the higher the efficiency of the highest efficiency point. In the actual test, the level of no-load speed and the maximum torque is generally used to judge the residual magnetism standard of the magnet.

For the same winding parameters and electrical parameters, the reason why the higher the remanence, the lower the no-load speed, and the smaller the no-load current is that the motor in operation produces enough reverse inductance at a relatively low speed. The generated voltage reduces the algebraic sum of the electromotive forces applied to the windings.

2.2 The influence of coercive force

During the operation of the motor, there is always the influence of temperature and reverse demagnetization field. From the perspective of motor design, the higher the coercive force, the smaller the thickness direction of the magnetic steel, and the smaller the coercive force, the larger the thickness direction of the magnetic steel. But after the magnetic steel exceeds a certain coercive force, it is useless, because other components of the motor cannot work stably at that temperature. It is sufficient that the coercive force meets the requirements, and the standard is to meet the requirements under the recommended experimental conditions, and there is no need to waste resources.

2.3 The influence of squareness

The squareness only affects the straightness of the efficiency curve of the motor performance test. Although the straightness of the motor efficiency curve has not been listed as an important indicator standard, it is very important for the continued distance of the hub motor under natural road conditions.

Due to different road conditions, the motor cannot always work at the maximum efficiency point, which is one of the reasons why the maximum efficiency of some motors is not high and the continuation distance is longer. For a good in-wheel motor, not only the maximum efficiency should be high, but also the efficiency curve should be as horizontal as possible. The smaller the slope of the efficiency reduction, the better. As the market, technology, and standards of in-wheel motors mature, this will gradually become an important standard.

2.4 The impact of performance consistency

Inconsistent residual magnetism: Even if there are individual ones with particularly high performance, it is not good. Due to the inconsistency of the magnetic flux in each unidirectional magnetic field section, the vibration occurs due to the asymmetry of the torque.

Inconsistent coercive force: especially if the coercive force of individual products is too low, it is easy to produce reverse demagnetization, resulting in the inconsistent magnetic flux of each magnet and vibrating the motor. This effect is more pronounced for brushless motors.

3. Precautions for permanent magnet motors

The “central” of permanent magnet motors – permanent magnets插图2
3.1 Magnetic circuit structure and design calculation

In order to give full play to the magnetic properties of various permanent magnet materials, especially the excellent magnetic properties of rare earth permanent magnets, and manufacture cost-effective permanent magnet motors, the structure, and design calculation methods of traditional permanent magnet motors or electric excitation motors cannot be simply applied, 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 electrical academia and engineering circles, the design theory of permanent magnet motors. Breakthroughs have been made in calculation methods, structural technology, and control technology, and a complete set of analysis and research methods and computer-aided analysis and design software combining electromagnetic field numerical calculation and equivalent magnetic circuit analytical solutions have been formed, and are being continuously improved.

3.2 Control problems

After the permanent magnet motor is made, it can maintain its magnetic field without external energy, but it also makes it extremely difficult to adjust and control its magnetic field from the outside. It is difficult for permanent magnet generators to adjust their output voltage and power factor from the outside, and permanent magnet DC motors can no longer adjust their speed by changing the excitation. These limit the application range of permanent magnet motors.

However, with the rapid development of power electronic devices such as MOSFETs and IGBTs and control technologies, most permanent magnet motors can be used without magnetic field control but only with armature control. The design needs to combine the three new technologies of rare earth permanent magnet materials, power electronic devices, and microcomputer control so that the permanent magnet motor can operate under brand-new working conditions.

3.3 Irreversible demagnetization problem

If the design or use is improper, the permanent magnet motor is under the action of the armature reaction generated by the impact current when the temperature is too high (NdFeB permanent magnet) or too low (ferrite permanent magnet), or under severe mechanical vibration. It may cause irreversible demagnetization, or reduce the performance of the motor, or even make it unusable. Therefore, it is necessary to research and develop methods and devices for checking the thermal stability of permanent magnet materials suitable for motor manufacturers, and to analyze the anti-demagnetization capabilities of various structural forms, so that corresponding measures can be taken to ensure Permanent magnet motors do not lose magnetism.

3.4 Cost issues

Ferrite permanent magnet motors, especially miniature permanent magnet DC motors, are widely used because of their simple structure and process, lightweight, and generally lower total cost than electric excitation motors. Since rare earth permanent magnets are still relatively expensive at present, the cost of rare earth permanent magnet motors is generally higher than that of electric excitation motors, which needs to be compensated by their high performance and operating cost savings. In some occasions, such as the voice coil motor of the computer disk drive, the performance of NdFeB permanent magnets 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 use occasions and requirements to decide the choice to innovate the structure and process and optimize the design to reduce the cost.

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