01 Characteristics of Magnet Steel in Permanent Magnet Motors
The magnet steel used in permanent magnet motors possesses the following characteristics:
- Its shape design is usually relatively simple (except for Voice Coil Motors, VCMs), with common shapes including rectangular, trapezoidal (tile-shaped), fan-shaped, and bread-shaped. Guided by the principle of cost reduction in motor design, many designs tend to adopt embedded square magnet steel.
- The magnetization process of magnet steel is relatively simple, mainly involving single-pole magnetization, which forms a multi-pole magnetic circuit after assembly. For integral ring structures, such as bonded NdFeB magnetic rings or hot-pressed magnetic rings, multi-pole radial magnetization is commonly used.
- The key technical requirements lie in high-temperature stability (HTS), consistency of magnetic flux density (MFD), and form, fit, and function (FIT). Surface-mounted rotor magnet steel needs to have good glue affinity, while linear motor magnet steel has high requirements for salt spray resistance. Wind turbine magnet steel has even stricter requirements for salt spray resistance, and drive motor magnet steel requires excellent HTS.
- Magnetic energy product (BH)max is applied at all levels, but coercivity (Hc) is mostly in the medium to high range. Currently, electric vehicle drive motor magnet steel is mainly characterized by high (BH)max and high Hc, with common grades including 45UH, 48UH, 50UH, 42EH, and 45EH. Mature grain boundary diffusion (GBD) technology plays an indispensable role in this process.
- Segmented bonded magnet steel is widely used in high-temperature motor fields to enhance the segmented insulation performance and reduce eddy current losses during motor operation. Some magnet steels also have an epoxy coating on the surface to enhance their insulation properties.
02 Key Testing Items for Motor Magnet Steel
- HTS: Some customers require testing for open-circuit magnetic decay, while others focus on semi-open-circuit magnetic decay. Considering that the magnet steel needs to withstand high temperatures and alternating reverse magnetic fields during motor operation, the finished product’s magnetic decay and the HTS curve of the base material are critical parameters that must be tested and monitored.
- MFD Consistency: As the source of the magnetic field for motor rotors or stators, the consistency of the MFD of magnet steel is crucial. Any differences in consistency can lead to motor vibration, power reduction, and affect the overall performance of the motor. Therefore, there are strict requirements for the consistency of the MFD of motor magnet steel, with different customers having requirements within 5%, 3%, or even 2%. Factors affecting MFD consistency include the consistency of remanence, the consistency of dimensional tolerances, and the consistency of chamfer coatings.
- FIT: Surface-mounted magnet steel is mainly trapezoidal in shape. For details such as angles and radii, traditional two-dimensional testing methods may have large errors or be difficult to test. Therefore, the FIT of magnet steel must be considered. For tightly arranged magnet steel, cumulative gaps need to be controlled; for dovetail groove surface-mounted magnet steel, the tightness of the assembly needs to be considered. To ensure FIT, it is best to make custom-made profiling fixtures according to the user’s assembly method to verify whether the FIT of the magnet steel meets the requirements.
03 Can Motor Magnet Steel Fall Off?
In the actual manufacturing process of permanent magnet motors (PMMs), magnet steel is fixed to the stator or rotor using adhesive. Theoretically, the magnet steel and the mating components can be tightly fixed by their magnetic force. During the assembly process of the magnet steel, it can be observed that the suction force between them is particularly strong, and in principle, there should be no separation between them. However, in practical applications of PMMs, there are still instances of magnet steel falling off. To ensure a good mating relationship, the magnet steel should have a good conformal relationship with the fixing surface, meaning that the mating surfaces of the two should be as consistent as possible, and adhesive should be used to reinforce the fixation. When the operating temperature of the PMM is too high, causing the magnet steel to lose its magnetism, the magnetism of the magnet steel weakens or disappears, resulting in a loss of attraction with the mating surface. In this case, the two can only rely on good adhesive for fixation. Vibrations during motor operation are also likely to cause the magnet steel to shift or fall off from the mating components, leading to frictional collisions between the stator and rotor, i.e., the stator and rotor may rub against each other or even damage the windings, causing severe consequences.