Performance characteristics of permanent magnet materials and permanent magnet motors

Main properties of permanent magnet materials used in motor

Permanent magnet materials commonly used in motors

Permanent magnet materials commonly used in motor include sintered magnet and bonded magnet, mainly including aluminum nickel cobalt, ferrite, samarium cobalt and Nd-Fe-B.
Aluminum nickel cobalt materials were used more before 1980s. It has the advantages of excellent temperature stability, time stability and applicable ultra-high temperature environment conditions. It is only used in some special use motors such as military or instrument with high temperature requirements and very good magnetic stability.
Ferrite material is a non-metallic permanent magnet material, which is cheap. It is mainly used in the economic series micro motor products with low demand for performance and volume and wide quantity. Such as toy motor, daily electrical motor, audio-visual motor, office equipment and general instrument motor, motor of automobile motorcycle and low power drive motor for industrial use.
Samarium cobalt material is a kind of permanent magnet material with excellent magnetic properties which has been developed in the mid 1960s, and its properties are very stable. Samarium cobalt is particularly suitable for manufacturing motors in magnetic properties, but because of its high price, SmCo is mainly used in the research and development of aviation, aerospace, weapons and other military motors and high-performance motors, which are not the main factors. NdFeB is a third generation of high performance permanent magnet material which appeared in 1980s. It has higher magnetic properties than samarium cobalt, poor thermal stability and easy corrosion. It must be treated with surface protection, but it is cheap, so it has been widely used. With the continuous update of NdFeB materials and the improvement of temperature performance, especially since 1990s, the NdFeB materials with low temperature coefficient and high temperature resistance have been developed successfully. The working temperature of high-performance NdFeB can reach 200 ℃, And the price is also decreasing, so that most industrial and civil motors adopt NdFeB materials, and will replace most of the raw ferrite materials and be used in low-cost economic motors.
Bonded permanent magnet material is a composite permanent magnet material made by the method of compression, injection or extrusion molding after mixing binder and permanent magnet material, including bonded ferrite, bonded aluminum nickel cobalt, samarium cobalt and bonded NdFeB. The bonding Nd-Fe-B is the best bonding permanent magnet material at present. Compared with sintered permanent magnet materials, it has the advantages of good mechanical processing performance, easy forming, various complex shapes, good uniformity of magnetic properties and easy to conduct multi pole magnetization. But the magnetic energy of bonded permanent magnet is lower than that of the same type of sintered magnet, and the magnetic energy product is about 40% – 70% of the sintered magnet of the same material. In the bonded permanent magnet materials, bonding Nd-Fe-B has the best prospect. If the process problems and quality are improved, bonded NdFeB will become the most widely used permanent magnet material. It is mainly used in small BLDCM and stepper motor.

Main properties of permanent magnet materials

  • (1) Residual magnetic induction strength. When the magnetic field is zero, the magnetic induction strength of the permanent magnet material is obtained. This index data is directly related to the air gap magnetic density in the motor. The higher the magnetic induction intensity, the higher the air gap magnetic density of the motor, the best value of the main indexes of the motor such as torque constant and counter potential coefficient. The value relationship between the electric load and the magnetic load of the motor can be the most reasonable and the efficiency can be the best.
  • (2) Coercive force HC (magnetoinduction coercive force HC b). The reverse magnetic field intensity required when the residual magnetic induction intensity br drops to zero when the permanent magnet is saturated magnetized. This index is related to the anti demagnetization capability of motor, namely overload multiple and air gap magnetic density. The higher the value of HC, the stronger the anti demagnetization ability of the motor, the greater the overload multiple, the stronger the adaptability to the dynamic working environment of strong demagnetization. At the same time, the air gap magnetic density of the motor will be improved.
  • (3) The maximum magnetic energy product BHmax. The maximum value of magnetic energy provided by permanent magnet materials to the magnetic circuit. The higher BHmax is, the greater the magnetic energy provided by the permanent magnet material is, that is, the less permanent magnet materials are used in the motor at the same power.
  • (4) Intrinsic coercive force HC I. This index refers to the magnetic field strength value when the residual magnetization m drops to zero. The corresponding value of HC B on demagnetization curve B = 0 only means that the permanent magnet can not provide energy to the magnetic circuit at this time, and does not mean that the permanent magnet does not have energy. But when m = 0, the corresponding value of HC I indicates that the permanent magnet has been demagnetized and has no magnetic energy storage. Although HC I is not directly related to the working point of the motor, it is the true coercive force of permanent magnet material, which represents the ability of permanent magnet material to possess magnetic field energy and anti magnetic field resistance. The intrinsic coercive force is closely related to the temperature stability of permanent magnet. The higher the intrinsic coercivity, the higher the working temperature of permanent magnet materials.
  • (5) Temperature coefficient a. Temperature is one of the main factors that affect the magnetic properties of permanent magnet materials. When the temperature changes at 1 ℃, the reversible change of magnetic properties is called the temperature coefficient of magnetic materials. The temperature coefficient can be divided into remanence induced temperature coefficient and coercive force temperature coefficient. The higher the temperature coefficient, the greater the change of the motor from cold to hot, it directly limits the range of the motor temperature. The power volume ratio of the motor is affected indirectly.

Permanent magnet motor and its characteristics

The maximum power in permanent magnet motor has reached 1000kW and the minimum diameter
φ 0.8mm, maximum speed 30000r/min, minimum speed 0.01r/min. Compared with the electric excitation motor, permanent magnet motor has the following characteristics

Simple structure and high reliability

The pole shoe and excitation coil used in the excitation of the original electric excitation motor can be replaced by one or more permanent magnets by using permanent magnet excitation material. The parts are reduced in a large amount and the structure is greatly simplified. At the same time, the collector ring and brush used for excitation are eliminated, which not only improves the process of the motor, but also improves the mechanical reliability of the motor operation and increases the service life.

Excellent performance

The air gap magnetic density of permanent magnet motor, especially the motor with rare earth permanent magnet material, can be greatly improved, and the motor index can achieve the best design. Its direct effect is the motor volume reduction and weight reduction. In addition, compared with other motors, permanent magnet motor has excellent control performance. This is because: first, because of the high performance of rare earth permanent magnet materials, the torque constant, torque inertia ratio and power density of the motor are greatly improved. The design can reduce the inertia, electrical and mechanical time constant greatly, and it has been improved as the main index of servo control performance. Secondly, in modern permanent magnet motor, the design of permanent magnet circuit has been perfect, and the coercive force of rare earth permanent magnet material is high, so the armature resistance and other demagnetization ability of PM motor are greatly strengthened, and the control parameters of the motor are greatly reduced with the external disturbance. Thirdly, the design of excitation winding and excitation field is reduced because the permanent magnet material is used instead of electric excitation, so many parameters such as excitation flux, excitation winding inductance and excitation current are reduced, thus directly reducing controllable variables or parameters. The permanent magnet motor has excellent controllability by combining the above factors.
For example, the speed regulation performance of all digital permanent magnet AC servo motor is very excellent, and the speed ratio of sine wave AC servo motor can reach up to 1:100000. After the permanent magnet materials are used, the output torque and dynamic response characteristics of the stepper motor and the low speed synchronous motor are improved and improved obviously. Compared with the same specification motor, the dynamic performance index, steady-state performance index, control performance index and reliability index of permanent magnet motor are greatly improved than that of ordinary motor.

High efficiency and energy saving

The permanent magnet motor can not only reduce the resistance loss, but also improve the power factor effectively. If permanent magnet synchronous motor can maintain high efficiency and power factor in the range of 25% – 120% of rated load. The efficiency of the micro permanent magnet DC motor within 16 of the 7th phase of 2006 of the special topic of 300W micromotor is 10% to 20% higher than that of the same specification. After the large number of fans and pumps load motor are changed to permanent magnet motor, the comprehensive energy saving effect is very significant. The greater the power is, the greater the excitation loss accounts for the total loss ratio, so the more prominent the efficiency of the permanent magnet motor is.

Problems to be studied in the design of permanent magnet motor

Utilization of permanent magnet materials

In permanent magnet motor, the cost of permanent magnet material accounts for a large proportion of the total cost of the motor. Therefore, how to save materials and improve the utilization ratio of materials is one of the most concerned problems of permanent magnet motor manufacturers. Theoretically, the maximum accumulated point of permanent magnet indicates that the magnet can provide the largest energy to the outside, and the maximum working point can be obtained from demagnetization curve. But it is not so simple in practical application. To study the application of motor, analyze the predetermined function of motor and find out the corresponding key index, so as to determine the best position of motor working point selection, and reasonably determine the shape and volume of permanent magnet, and also consider the influence of its processing technology. After considering all kinds of factors, the best design of motor in function, performance, cost and so on is achieved.

Overload and demagnetization

Demagnetization of magnetic materials includes temperature demagnetization, time demagnetization and environmental demagnetization. It can be divided into two types: reversible demagnetization and irreversible demagnetization. The relationship between coercive force and intrinsic coercive force of permanent magnet material and stable working temperature is studied in depth; the influence degree of temperature coefficient on motor performance index and demagnetization safety coefficient; definition of maximum working temperature of motor based on magnetic energy change; proportion of reversible demagnetization and irreversible demagnetization in the range of motor temperature and its influence on motor performance; permanent magnet material It is necessary to recharge and reuse after demagnetization.

Analysis and design

The theory and design of modern permanent magnet motor are mature, not only many design programs and methods based on magnetic circuit analysis and calculation, but also the numerical analysis method of permanent magnet magnetic field has been widely used in engineering practice. But in permanent magnet motor, permanent magnet is the source of field excitation or magnetic circuit, and it is also a part of magnetic field and magnetic circuit. Meanwhile, the manufacturing technology, shape and size of permanent magnet materials, magnetizing tools and magnetizing methods will make the consistency and uniformity of permanent magnet materials not ideal, sometimes with large dispersion, Even the performance data of the same grade and batch of permanent magnet materials may be different. Therefore, the dispersion of permanent magnet also brings some difficulties to the design analysis and numerical calculation of permanent magnet magnetic field, and the accuracy of the design will be affected. For example, the establishment of permanent magnet model and the equivalent problem in the field theory and numerical analysis, and many problems such as leakage coefficient, local demagnetization and armature response accurate calculation in engineering magnetic circuit calculation are more error than that of the analysis and calculation of electric excitation motor.

Magnetization and magnetic measurement

The design of permanent magnet motor is based on the saturated magnetization of permanent magnet materials. So, whether the magnet used in the motor has been fully magnetized and saturated. If the permanent magnet is supplied by the magnetic material manufacturer and supplied with magnetic, there is no problem. However, if it is the whole magnetization on the motor, it is worth studying how to ensure the permanent magnet is fully magnetized, and how to maintain the uniformity and consistency of magnetic properties at the same time.
In the same way, there are still many problems worth studying for the magnetic energy detection. For example, how to effectively and easily and accurately inspect the magnet supplied with magnetic field in the motor manufacturer. However, most motor manufacturers can not measure the magnetic properties of permanent magnet materials effectively in the parts stage. Only when the performance of the machine is unqualified, the problems of magnetic materials can be found.

Corrosion resistance

The corrosion of NdFeB has a great influence on the quality of the motor. At present, the surface protection of NdFeB has not been solved well in China. The plating methods often appear the phenomenon that the surface coating falls off and causes the motor to fail. At the same time, the ability to withstand special environmental conditions (such as humidity, salt fog and special gas) is limited, which affects the adaptability of the motor to special environmental conditions. It is hoped that permanent magnet materials will be further improved in the surface protection capability.

Magnetic performance stability, uniformity and consistency

In order to ensure that the performance of the motor does not change greatly in its life cycle, especially for some military motors with special requirements, its reliable working life is required to be more than 15 years. It is hoped that the magnetic properties of permanent magnet materials can be kept stable for a long time.
The magnetic properties uniformity and consistency of permanent magnet materials are required by motors, especially high precision motors. The inhomogeneity of magnetic energy will lead to the nonuniformity of the magnetic field, the increase of torque fluctuation, the increase of output voltage ripple, the poor linearity, and the decrease of precision index of the control motor. In addition, the magnetic properties of permanent magnet materials of the same brand are inconsistent in different batches, sometimes leading to the unqualified motor batch. Therefore, the high precision motor requires that the magnetic properties of permanent magnet materials meet the requirements of the error within 5%, and the uniformity error is within 3%.

Processing process

There are many processing problems in the manufacturing process of permanent magnet motor. For example, how to master the process parameters in the bonding process, whether it will affect the magnetic properties. Whether the impact, vibration and processing environment in machining process have an impact on magnetic energy, how to take protective measures in the process of process turnover and assembly of magnetic parts.
The permanent magnet in permanent magnet motor is supplied with magnetic. Although it reduces the difficulty of magnetizing in the motor factory, it greatly increases the manufacturing process difficulty. For example, permanent magnet BLDCM and PMSM are often made of surface mounting structure with multi pole and multi magnet directly sticking to the rotor surface. The bonding process of sintered Nd-Fe-B with high performance is more complex and difficult to operate. When the speed is high, the bonding reliability is not ideal. If sintered magnet can be made into a multi polar ring structure with radial crystal magnetization, the manufacturing process of motor rotor will be greatly simplified. At present, the bonding magnet can be achieved, but the magnetic properties are not high. If sintered magnet can be realized and can be supplied in batches and the price is not high (it is reported that the research is being carried out by the manufacturers), it will have a promising future.

Concluding remarks

In conclusion, the development of permanent magnet materials promotes the development of permanent magnet motor, and the widespread application of PM and the requirement for permanent magnet materials promote the further improvement of permanent magnet materials. With the application of permanent magnet in aviation, aerospace, national defense, industry and agriculture and daily life, the application prospect of permanent magnet materials in motor will be quite extensive.
Source: China Permanent Magnet Manufacturer www.ymagnet.com

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