Permanent magnet motors use permanent magnets to generate the magnetic field of the motor. They do not require excitation coils or excitation currents. They are highly efficient and simple in structure. They are very energy-saving motors. 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 extensive. 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, light weight, low loss, high efficiency, and flexible and diverse shapes and sizes of motors. Therefore, the scope of application is extremely wide, almost covering all fields of aerospace, national defense, industrial and agricultural production and daily life.
The figure below is a simple working principle model of a permanent magnet DC motor. Two permanent magnets generate a magnetic field at the coil in the center. When current is passed through the coil, the magnetic field will generate electromagnetic force (left-hand rule), causing it to rotate. The rotating part of the motor is called the rotor, and the stationary part is called the stator. It is obvious that the permanent magnet in the figure below belongs to the stator, and the coil belongs to the rotor.
For rotating motors , when the permanent magnet is the stator, it is mostly in the shape of an outer arc surface-mounted tile-shaped magnet attached to the casing. When the permanent magnet is the rotor, it is mostly in the shape of an inner arc surface-mounted tile-shaped magnet attached to the rotor core, or embedded in the rotor core in the shape of a square piece, as shown in the figure below.
For linear motors, permanent magnets are mainly square and parallelogram in shape. Cylindrical linear motors also use axially magnetized annular magnets.
Permanent magnet motor magnetic steel has the following characteristics:
1. The shape will not be too complicated (except for some micro motors, such as VCM motors), mostly rectangular, tile, fan, and bread shapes. Especially under the premise of reducing the cost of motor design, many will use embedded square magnetic steel;
2. Magnetization is relatively simple, basically single-pole magnetization, and a multi-pole magnetic circuit is formed after assembly. If a complete 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 of technical requirements lies in high temperature stability, magnetic flux consistency and adaptability . Surface mounted rotor magnets will require good adhesive affinity, linear motor magnets will have relatively stricter requirements for salt spray, wind turbine magnets will have stricter requirements for salt spray, and drive motor magnets will require very good high temperature stability;
4. The magnetic energy product has applications in both high, medium and low grades, but the coercivity is mostly at the medium and high level. At present, the magnetic steel grades of the drive motors of electric vehicles are mainly high magnetic energy product and high coercivity, such as 45UH, 48UH, 50UH, 42EH, 45EH, etc., and a mature diffusion process 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 to reduce the eddy current loss of the magnets when the motor is running. Some magnets will also have an epoxy coating added to the surface to increase their insulation.
Key inspection items for motor magnetic steel:
1. High temperature stability.Some customers will require open circuit magnetic decay measurement, while others will require semi-open circuit magnetic decay measurement. When the motor is running, the magnetic steel must not only withstand high temperatures but also the alternating reverse magnetic field. Therefore, the finished product magnetic decay and substrate high temperature demagnetization curves must be tested and monitored .
2. Magnetic flux consistency.Magnets are the source of the magnetic field of the motor rotor or stator. If there are consistency differences, it will cause motor vibration and power reduction, which will affect the function of the entire motor. Therefore, motor magnets generally have requirements for flux consistency. Some require it to be within 5%, some require it to be within 3% or even 2%. Factors that affect the flux consistency must be taken into account, such as the consistency of residual magnetism, the consistency of tolerances, and the consistency of chamfer plating .
3. Adaptability.Surface-mounted magnets are mostly tile-shaped. Conventional two-dimensional testing methods have large errors or are difficult to test for angles and curvatures. At this time, its adaptability needs to be considered. Some tightly arranged magnets need to control the accumulated gaps, and some dovetail slot surface-mounted magnets need to consider the tightness of the assembly. It is best to make a self-made profiling fixture according to the user’s assembly method to test the adaptability of the magnet .
In the actual production and manufacturing process of permanent magnet motors, the magnets are fixed to the stator or rotor with adhesives. Theoretically, the magnets and mating parts can be tightly fixed by the magnetic force of the two. In the assembly process of the magnets, it can be found that the attraction between the two is particularly large. In principle, the two should not be separated. However, in the actual application process of permanent magnet motors, the fact that the magnets fall off still exists.
In order to ensure a good fit between the two, the magnet should have a good isomorphic relationship with the fixed surface, that is, the mating surfaces of the two should be as consistent as possible, and adhesives should be used for enhanced fixation. When the permanent magnet motor loses its magnetism due to high temperature during operation, the original magnetism of the magnet weakens or disappears, resulting in a loss of attraction with the mating surface. The two can only be fixed by a good adhesive. The vibration during the operation of the motor is also very likely to cause the magnet to shift or fall off from the mating parts, resulting in frictional collision between the motor stator and rotor, that is, the stator and rotor parts are swept or even the winding is damaged.
Permanent magnet motor related matters
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, it is not possible to simply apply the structure and design calculation methods of traditional permanent magnet motors or electromagnetic excitation motors. It is necessary to establish new design concepts and 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 motor academic and engineering communities, breakthroughs have been made in the design theory, calculation methods, structural processes and control technologies of permanent magnet motors, forming a complete set of analysis and research methods and computer-aided analysis and design software that combines electromagnetic field numerical calculation and equivalent magnetic circuit analytical solution, and is being continuously improved.
After the permanent magnet motor is manufactured, it can maintain its magnetic field without external energy, but it is also 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 have limited the application scope 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 but only armature control. When designing, it is necessary to combine the three new technologies of rare earth permanent magnet materials, power electronic devices and microcomputer control to make the permanent magnet motor operate under brand new working conditions.
3. Irreversible demagnetization problem
If improperly designed or used, permanent magnet motors may experience irreversible demagnetization, or demagnetization, when the temperature is too high (NdFeB permanent magnets) or too low (ferrite permanent magnets), under the armature reaction caused by the impact current, or under severe mechanical vibration, which will reduce the performance of the motor and even make it unusable. Therefore, it is necessary to research and develop methods and devices suitable for motor manufacturers to check the thermal stability of permanent magnet materials, and to analyze the anti-demagnetization capabilities of various structural forms, so that corresponding measures can be taken during design and manufacturing to ensure that permanent magnet motors do not lose magnetism.
Ferrite permanent magnet motors, especially micro permanent magnet DC motors , are widely used due to their simple structure and process, reduced mass, and generally lower total cost than electric excitation motors. Since rare earth permanent magnets are still 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 their high performance and savings in operating costs. In some cases, such as voice coil motors for computer disk drives, the use of neodymium iron boron permanent magnets improves performance, significantly reduces volume and mass, and reduces total cost. When designing, it is necessary to make a comparison between performance and price based on the specific application and requirements, and to innovate the structure and process and optimize the design to reduce costs.
Post time:
Feb-28-2025