Will permanent magnet motors demagnetize at high temperature?

The temperature on the end plate of the permanent magnet motor's rotor has reached 121°C or more. It is speculated that the temperature at the rotor magnet should be above 140°C.

 

Based on these limited data, a possible failure mechanism has gradually formed:

 

The positive feedback process of permanent magnet motor is gradually deteriorating. First, the temperature of the rotor rises, which causes the magnet steel to produce slight demagnetization locally, which weakens the permanent magnetic field, which makes the current larger, and makes the permanent magnet motor bear greater demagnetization current. The loss of the rotor and the magnet is further increased, so that the temperature of the rotor is further increased, the area of demagnetization becomes larger and the depth of demagnetization deepens. As shown in the figure below, the magnet works on the blue line. This line is called the recovery curve. Generally, the recovery curve consists of two parts: a straight part and a non-straight part. The junction of the two sections is called the knee point (the brain fills a bent knee). Under the action of the magnetic circuit and current load of the permanent magnet motor, the working point (Y magnetic density-X coercivity) coordinate of the magnetic steel is a specific point P, when the point P is higher than the inflection point (as shown on the left in the figure below) , Although the magnetic field is weakened, when the current is removed, the magnetic steel can return to its original performance, that is, the demagnetization curve will not change. But when the operating point P is lower than the inflection point (as shown on the right in the figure below), even if the load is removed, the performance of the magnet will not be fully restored. Instead, it starts from point P and returns to point Br1 (the blue dashed line in the figure) along the line parallel to the original demagnetization. Obviously Br1 is smaller than the original Br point, which means that the newly formed magnetic steel demagnetization curve has poor performance. In the original scheme, this degradation is irreversible, that is, irreversible demagnetization.

 

The permanent magnet motor makes the magnet work above the inflection point, but the inflection point of the magnet is a function of temperature. The higher the temperature, the higher the inflection point of the magnet, which means that the probability of irreversible demagnetization will increase. When large currents appear at the same time, the positive feedback demagnetization mechanism we mentioned above is prone to appear. Based on the above theory and the collected data, our speculation is formed: "Because the temperature is too high, at least part of the permanent magnet motor's magnetic steel operating point is below the inflection point, irreversible demagnetization has occurred, resulting in an increase in current, which gradually deepens the non-reversible demagnetization. The scope of the project forms a vicious circle".