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Do you really understand the design of permanent magnets of permanent magnet motors?

Date:2022-07-06   Author:XINDA MOTOR
The development of permanent magnet motors is closely related to the development of permanent magnet materials. my country 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, our country used the magnetic properties of permanent magnet materials to make a compass, which has played a huge role in navigation, military and other fields. It has become one of the four great inventions in ancient my country.

The world's first motor that appeared in the 1820s was 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 magnetite (Fe3O4), which has a very low magnetic energy density, and the motor made from it was bulky, and was soon replaced by an electric excitation motor.

With the rapid development of various motors and the invention of current magnetizers, people have carried out 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 (the maximum magnetic energy product is about 7.2 kJ/m3) and other permanent magnet materials. In particular, the AlNiCo permanent magnets that appeared in the 1930s (the maximum magnetic energy product can reach 85 kJ/m3) and the ferrite permanent magnets that appeared in the 1950s (the maximum magnetic energy product can now reach 40 kJ/m3) have various magnetic properties. With the great improvement, 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 the output has increased sharply. Correspondingly, during this period, breakthroughs have been made in the design theory, calculation method, magnetization and manufacturing technology of permanent magnet motors, and a set of analysis and research methods represented by the working diagram of permanent magnets 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 1960s and 1980s, rare earth cobalt permanent magnets and neodymium iron boron permanent magnets (both collectively referred to as rare earth permanent magnets) came out one after another, with their high remanence density, high coercivity, high magnetic energy product and linear demagnetization curve. The excellent magnetic properties of the permanent magnet motor are especially suitable for the manufacture of electric motors, so that the development of permanent magnet motors has entered a new historical period.

Permanent magnet material

Motor magnets: Permanent magnet materials commonly used in motors include sintered magnets and bonded magnets, the main types are AlNiCo, ferrite, samarium cobalt, NdFeB, etc.

AlNiCo: AlNiCo permanent magnet material is the earliest 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. Among the large-scale production enterprises, the output of Hangzhou permanent magnet is currently the first in China, with an annual production capacity of 3,000 tons.

Permanent magnet ferrite material: In the 1950s, ferrite began to flourish. Especially in the 1970s, strontium ferrite with better performance in coercive force and magnetic energy machine was put into production in large quantities, and the 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 particularly suitable for the manufacture of motors in terms of magnetic properties, but due to its high price, it is mainly used in the research and development of aviation, aerospace, weapons and other military motors and high-performance motors in high-tech fields where price is not the main factor.

NdFeB material: NdFeB magnetic material is an alloy of neodymium, iron oxide, etc., also known as magnetic steel. It has extremely high magnetic energy product and coercive force, and at the same time, the advantages of high energy density make NdFeB permanent magnet materials widely used in modern industry and electronic technology. It is possible to reduce the size, weight and thickness. 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.

Corrosion resistance, maximum working temperature, processing performance, shape of demagnetization curve, and price comparison of permanent magnet materials commonly used in motors (Figure)

The relationship between magnetic steel performance and motor performance 

1. Influence of residual magnetism 

For a DC motor, under the same winding parameters and test conditions, the higher the remanence, the lower the no-load speed and the lower the no-load current; the greater the maximum torque, the higher the efficiency at the highest efficiency point. In the actual test, the level of no-load speed and the size of the maximum torque are generally used to judge the remanence standard of the magnetic steel. 

For the same winding parameters and electrical parameters, the reason why the higher the remanence is, the lower the no-load speed and the lower the no-load current is, is because the running motor generates enough reverse inductance at a relatively low speed. Generates a voltage that causes the algebraic sum of the electromotive force applied to the winding to decrease.

2. The influence of coercivity 

In the process of motor operation, there is always the influence of temperature and reverse demagnetization. From the perspective of motor design, the higher the coercive force, the smaller the thickness direction of the magnetic steel can be, and the smaller the coercive force, the larger the thickness direction of the magnetic steel. However, after the magnetic steel exceeds a certain coercive force, it is useless, because other components of the motor cannot work stably at that temperature. The coercivity only needs to meet the requirements, and the standard is to meet the requirements under the recommended experimental conditions, and there is no need to waste resources.

3. The effect of squareness 

The squareness only affects the straightness of the motor performance test efficiency curve. Although the straightness of the motor efficiency curve has not been listed as an important index standard, it is very important for the continuation distance of the in-wheel motor under natural road conditions. important. Because of 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 long. A good in-wheel motor should not only have a high maximum efficiency, but also the efficiency curve should be as level as possible, and the smaller the slope of the efficiency reduction, the better. This will gradually become an important standard as the market, technology and standards of in-wheel motors mature.  

4. The impact of performance consistency 

Inconsistent remanence: Even if there are individual ones with particularly high performance, it is not good. Due to the inconsistency of the magnetic flux of each unidirectional magnetic field segment, the torque is asymmetric and the vibration occurs.

Inconsistent coercive force: In particular, the coercive force of individual products is too low, which is prone to reverse demagnetization, resulting in inconsistent magnetic flux of each magnetic steel and causing the motor to vibrate. This effect is more pronounced for brushless motors.

Influence of Magnet Shape and Tolerance on Motor Performance 

1. The influence of the thickness of the magnetic steel

When the inner or outer magnetic circuit is fixed, when the thickness increases, the air gap decreases and the effective magnetic flux increases. The obvious performance is that the no-load speed and no-load current decrease under the same residual magnetism, and the maximum efficiency of the motor is reduced. improve. However, there are also negative aspects, such as the increased commutation vibration of the motor and the relatively steeper efficiency curve of the motor. Therefore, the thickness of the motor magnets should be as consistent as possible to reduce vibration.

2. The influence of the width of the magnetic steel

For the close-packed brushless motor magnets, the total cumulative gap cannot exceed 0.5 mm. If it is too small, it will not be able to be installed. If it is too large, it will lead to motor vibration and reduced efficiency. This is because the position of the Hall element and the magnetic The actual position of the steel does not correspond, and the consistency of the width must be ensured, otherwise the motor has low efficiency and large vibration. 

For brushed motors, there is a certain gap between the magnets, which is reserved for the mechanical commutation transition zone. Although there is a gap, most manufacturers have strict magnetic steel installation procedures to ensure the installation accuracy in order to ensure the accurate installation position of the motor magnet. If the width of the magnetic steel exceeds, it will not be installed; if the width of the magnetic steel is too small, the positioning of the magnetic steel will be misaligned, the vibration of the motor will increase, and the efficiency will decrease.
3. The influence of magnetic steel chamfering size and non-chamfering

If the angle is not chamfered, the change rate of the magnetic field at the edge of the magnetic field of the motor is large, causing the pulse vibration of the motor. The larger the angle of chamfering, the smaller the vibration. However, chamfering generally has a certain loss of magnetic flux. For some specifications, when the chamfering reaches 0.8, the magnetic flux loss is 0.5~1.5%. When the remanence of the brushed motor is low, appropriately reducing the size of the chamfer is beneficial to compensate the remanence, but the pulse vibration of the motor increases. Generally speaking, when the remanence is low, the tolerance in the length direction can be appropriately enlarged, which can improve the effective magnetic flux to a certain extent and make the performance of the motor basically unchanged.

Precautions for permanent magnet motors

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 to 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. , a new design concept must be established, and the magnetic circuit structure must be re-analyzed and improved. 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 electric motor academia and engineering circles, it has been widely used in the design theory, Breakthrough progress has been made in calculation methods, structural technology and control technology, etc., and a set of analysis and research methods and computer-aided analysis and design software combining electromagnetic field numerical calculation and equivalent magnetic circuit analytical solution have been formed, and are constantly improving. .

2. Control issues

The permanent magnet motor 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 the permanent magnet generator to adjust its output voltage and power factor from the outside, and the permanent magnet DC motor can no longer adjust its speed by changing the excitation method. These limit the application range of permanent magnet motors. However, with the rapid development of power electronic devices and control technologies such as MOSFETs and IGBTs, most permanent magnet motors can be used without magnetic field control and only with armature control. When designing, it is necessary to combine three new technologies of rare earth permanent magnet materials, power electronic devices and microcomputer control, so that the permanent magnet motor can run under new working conditions.

3. The problem of irreversible demagnetization

If the design or use is improper, the permanent magnet motor will be under the action of the armature reaction caused by the inrush current when the temperature is too high (NdFeB permanent magnet) or too low (ferrite permanent magnet), or when there is severe mechanical vibration It is possible to cause irreversible demagnetization, or loss of magnetization, which will reduce the performance of the motor and 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 as to take corresponding measures to ensure that during design and manufacture. Permanent magnet motors do not lose their magnetism.

4. Cost issues

Ferrite permanent magnet motors, especially miniature permanent magnet DC motors, have been widely used due to their simple structure and process, reduced weight, 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 its high performance and operating cost savings. In some occasions, such as voice coil motors of computer disk drives, 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, but also to innovate the structural process and design optimization to reduce the cost.