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Why do permanent magnet motor magnets fall off and what are the serious consequences?

Date:2023-10-13   Author:XINDA MOTOR

The permanent magnet motor uses permanent magnets to generate the magnetic field of the motor. It does not require an excitation coil or excitation current. It has 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 motors will become more widespread. Compared with traditional electric excitation motors, permanent magnet motors, especially rare earth permanent magnet motors, have significant advantages such as simple structure, reliable operation, small size and light weight, low loss and high efficiency, and the shape and size of the motor can be flexible and diverse. . Therefore, it has an extremely wide range of applications, covering almost all fields of aerospace, national defense, industrial and agricultural production, and daily life.

The picture below is a simple working principle model of a permanent magnet DC motor. Two permanent magnets generate a magnetic field at the central coil. The coil is passed through current, and the electromagnetic force (left-hand rule) is generated by the magnetic field, which then rotates. The motor The rotating part is called the rotor, and the non-moving part is called the stator. Obviously, the permanent magnet in the picture below belongs to the stator, and the coil belongs to the rotor.

For rotating electrical machines, when the permanent magnet is the stator, the shape of the tile-shaped magnet is usually an outer arc surface-mounted type and is attached to the casing. When the permanent magnet is the rotor, the shape is usually an inner-arc surface-mounted tile-shaped magnet that is attached to the rotor core. , or embedded in the rotor core in a square shape, as shown below.

For linear motors, the permanent magnets are mainly square and parallelogram. Cylindrical linear motors also use axially magnetized ring magnets.

Permanent magnet motor magnets have the following characteristics :

1. The shapes are not too complicated (except for some micromotors, such as VCM motors), and are mostly rectangular, tile-shaped, sector-shaped, and bread-shaped. Especially under the premise of reducing the cost of motor design, many will use embedded square magnets. steel;

2. Magnetization is relatively simple, basically unipolar magnetization, and a multi-pole magnetic circuit is formed after assembly. If the entire ring is made, such as bonding a NdFeB magnetic ring or a hot-pressed magnetic ring, multi-pole radiation magnetization is generally used;

3. The core technical requirements are mainly high temperature stability, magnetic flux consistency and adaptability . Surface-mounted rotor magnets will require good glue affinity. Linear motor magnets will have relatively strict requirements for salt spray. Wind power electromagnetic steel will require The requirements for salt spray will be more stringent, and the drive motor magnets will require very good high-temperature stability;

4. Magnetic energy products are used in both medium and low grades, but the coercivity is mostly at the mid-to-high level. At present, the magnetic steel grades of drive motors of electric vehicles are mainly high magnetic energy products and high coercive forces, such as 45UH, 48UH, 50UH, and 42EH. , 45EH, etc., mature diffusion technology is essential;

5. Segmented bonded magnets have been widely used in the field of high-temperature motors. The purpose is to improve the segmented insulation of the magnets and reduce the eddy current loss of the magnets when the motor is running. Some magnets will also appear on the surface. Add an epoxy coating to increase its insulation.

Key testing items for motor magnets:

1. High temperature stability. Some customers will require open-circuit magnetic attenuation measurement, and some customers will require semi-open circuit magnetic attenuation measurement. When the motor is running, the magnets must not only withstand high temperatures but also withstand alternating reverse magnetic fields, so the finished product’s magnetic attenuation and base material high-temperature demagnetization curves It must be tested and monitored ;

2. Magnetic flux consistency. Magnetic steel is the magnetic field source of the motor rotor or stator. If there is a consistency difference, it will cause motor vibration and power reduction, which will affect the overall motor function. Therefore, motor magnets generally have requirements for magnetic flux consistency, and some require 5%. Within, some require within 3% or even 2%. Factors affecting the consistency of the magnetic flux must be taken into consideration, such as the consistency of the residual magnetism, the consistency of the tolerance, and the consistency of the chamfer coating, etc.

3. Adaptability. Surface-mounted magnets are mostly in the shape of tiles. Conventional two-dimensional testing methods for angles and radians have large errors or are difficult to test. At this time, its adaptability needs to be considered. Some closely arranged magnets require To control the accumulated gap, some dovetail groove surface-mounted magnets need to consider the tightness of the assembly. It is best to make a self-made profiling jig according to the user's assembly method to test the suitability of the magnets .

       In the actual production and manufacturing process of permanent magnet motors, the magnets are fixed on the stator or rotor using adhesives. Theoretically, the magnets and matching parts can be tightly fixed by the magnetic force of the two. During the assembly process of the magnets, it can be found that the suction force between the two is particularly strong. In principle, there should not be separation between the two. However, in the actual application process of the permanent magnet motor, there is still the fact that the magnets fall off.

In order to ensure a good matching relationship between the two, the magnetic steel should have a good isomorphic relationship with the fixed surface, that is, the matching surfaces of the two should be as consistent as possible, and adhesive should be used to strengthen the fixation. When the temperature of the permanent magnet motor is too high and the magnet loses its magnetism during operation, the original magnetism of the magnet weakens or disappears, causing the magnet to lose attraction with the mating surface. The two can only be fixed with a better adhesive, and the motor is running The vibration during the process is also very likely to cause the magnet to shift or fall off from the mating parts, causing frictional collision between the stator and the rotor of the motor, that is, causing the stator and rotor parts to be swept or even damage the windings.

Precautions related to 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 create a cost-effective permanent magnet motor, we cannot simply apply the structure and design calculation methods of traditional permanent magnet motors or electric excitation motors. , a new design concept must be established, and the magnetic circuit structure must be reanalyzed 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 electrical academic and engineering circles, the design theory and design of permanent magnet motors have been Breakthrough progress has been made in calculation methods, structural technology and control technology, and a complete set of analysis research methods and computer-aided analysis and design software that combine electromagnetic field numerical calculations and equivalent magnetic circuit analytical solutions have been formed, and are being continuously improved. .

2. Control issues

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

3. Irreversible demagnetization problem

If improperly designed or used, the permanent magnet motor will operate at too high (neodymium iron boron permanent magnet) or too low (ferrite permanent magnet) temperature, under the action of armature reaction caused by inrush current, or under severe mechanical vibration. Irreversible demagnetization, or loss of magnetization, may occur, which may reduce the performance of the motor or even make it unusable. Therefore, it is necessary to not only research and develop methods and devices for checking the thermal stability of permanent magnet materials suitable for motor manufacturers, but also to analyze the anti-demagnetization capabilities of various structural forms, so that corresponding measures can be taken to ensure that during design and manufacturing Permanent magnet motors do not lose magnetism.

4. Cost issue

Ferrite permanent magnet motors, especially miniature permanent magnet DC motors, are 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 currently relatively expensive, 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 cases, such as the voice coil motor 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. During design, it is necessary to compare performance and price before deciding on a choice based on specific use occasions and requirements. It is also necessary to innovate structural processes and design optimization to reduce costs.