What is the cause of permanent magnet direct drive motor demagnetization?

There are many kinds of magnetic materials and their performance is very different, but they all have the problem of magnetic stability. The stability of permanent magnet direct drive motor is mainly affected by temperature, vibration, time and current impact. The cause of permanent magnet demagnetization is very complicated, which may be caused by one reason or the result of multiple reasons. However, in general, the magnetic loss of permanent magnets is mainly due to material, mechanical, chemical and electromagnetic characteristics.

1) Reasons for the material itself

At present, the performance difference of permanent magnets is generally not considered when designing permanent magnet direct drive motors in China. However, in the manufacturing process of NdFeB permanent magnets, the content of alloying elements, the melting process, the crushing process, the forming 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 sprayed. At present, the widely used coatings are epoxy resin spraying, electrophoresis and electroplating. In terms of solvent resistance, plating resistance> epoxy coating resistance> electrophoresis resistance. In terms of impact resistance, plating resistance> electrophoresis resistance> epoxy resin coating. 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 the use.

In the production process of permanent magnet direct drive motor, due to improper assembly, the permanent magnet coating is damaged; in the process of use, the motor will produce severe vibration, damage and break the permanent magnet, resulting in the performance of the permanent magnet is reduced or even lost.

3) Electromagnetic and temperature reasons

NdFeB permanent magnetic material has the characteristics of high remanence density, high magnetic energy product and high coercivity. Its magnetic properties are higher than those of rare earth cobalt permanent magnets. It is a permanent magnet material with high magnetic performance 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. The stator cogging effect, the non-sinusoidal distribution of the winding magnetomotive force, and the harmonic magnetomotive force generated by the harmonic current in the winding will also cause eddy current losses in the rotor permanent magnets, resulting in higher temperature rise. The high temperature makes the magnetic domains and magnetic domain walls that have been regularly arranged move (the magnetic moments of the magnetic domains rotate, and the magnetic domain walls move), causing the "band orientation" magnetic domains to be arranged and disordered, and the magnetic fields cancel each other. They do not show magnetism outside, so Demagnetization will occur.

4) Causes of motor fault conditions

When the permanent magnet direct drive motor is in short circuit and locked rotor, the armature current increases instantaneously. When the reaction potential of the direct axis armature is large to a certain extent, it will cause the permanent magnet to demagnetize.

Measures to prevent permanent magnet direct drive motor demagnetization:

With the improvement of the sintered permanent magnet production process and the upgrading of production equipment, generally there will be no defective products with insufficient magnetization and uneven magnetic field. According to the use environment and conditions of the permanent magnet, targeted spraying is used to avoid the corrosion of the permanent magnet; in the motor production process, the permanent magnet can be reinforced by screws to prevent vibration from causing damage to the permanent magnet. The above measures overcome the material, mechanical and chemical disadvantages of permanent magnets.