There are many types of magnetic materials with greatly different properties, but they all have problems with magnetic stability. The stability of permanent magnet direct drive motors is mainly affected by temperature, vibration, time, and current shock. The reasons for permanent magnet demagnetization are very complex. It may be caused by one reason, or it may be the result of a combination of multiple reasons. However, generally speaking, the magnetic loss of permanent magnets is mainly caused by material, mechanical, chemical, and electromagnetic properties.
1) The reason for the material itself
At present, when designing permanent magnet direct drive motors in my country, the performance differences of permanent magnets are generally not considered. However, in the manufacturing process of NdFeB permanent magnets, the content of alloy elements, the smelting process, the crushing process, the molding process, and the sintering and tempering process will all affect the performance of the permanent magnets.
2) Mechanical and chemical reasons
NdFeB permanent magnets contain a large amount of iron and neodymium. Iron is easily oxidized, so its surface should be spray-coated. At present, the widely used coatings are epoxy resin spraying, electrophoresis, and electroplating. In terms of solvent resistance, electroplating resistance > epoxy coating resistance > electrophoresis resistance. In terms of impact resistance, electroplating resistance > electrophoresis resistance > epoxy resin coating resistance. In terms of salt spray resistance, electrophoresis performance > epoxy coating performance > electroplating performance. Different coatings have different corrosion resistance and should be sprayed according to usage.
During the production process of permanent magnet direct drive motors, improper assembly results in damage to the permanent magnet coating; during use, the motor will produce violent vibrations, damaging and breaking the permanent magnets, resulting in reduced or even loss of permanent magnet performance.
3) Electromagnetic and temperature reasons
NdFeB permanent magnet material has the characteristics of high remanence density, high magnetic energy product, and high coercive force. Its magnetic properties are higher than rare earth cobalt permanent magnets. It is a permanent magnet material with high magnetic properties and has been widely used in rare earth permanent magnet direct drive motors. The disadvantages of NdFeB permanent magnet materials are low Curie temperature, high-temperature coefficient, and large magnetic loss when used at high temperatures. Stator cogging, non-sinusoidal distribution of winding magnetomotive force, and harmonic magnetomotive force generated by harmonic currents in the windings can also cause eddy current losses in the rotor permanent magnets, resulting in higher temperature rise. High temperature causes the already regularly arranged magnetic domains and magnetic domain walls to move (the magnetic moments of the magnetic domains rotate and the magnetic domain walls move), causing the “band-oriented” magnetic domains to be arranged disorderly, and the magnetic fields cancel each other out. They do not show magnetism on the outside, so demagnetization will occur.
4) Causes of motor fault conditions
When the permanent magnet direct drive motor is in working conditions such as a short circuit and locked rotor, the armature current increases instantaneously. When the reaction potential of the demagnetized direct-axis armature is large enough to a certain extent, the permanent magnet will be demagnetized.
Measures to prevent demagnetization of permanent magnet direct drive motors:
With the improvement of the sintered permanent magnet production process and the upgrading of production equipment, defective products with insufficient magnetization and uneven magnetic field generally do not appear. According to the use environment and conditions of the permanent magnets, targeted spraying is used to avoid corrosion of the permanent magnets; during the motor production process, the permanent magnets can be reinforced with screws to prevent damage to the permanent magnets caused by vibration. The above measures overcome the material, mechanical, and chemical shortcomings of permanent magnets.
