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Thermal Stability and Magnet Flux Loss: Insights for Motor Engineers

In the field of bonded neo magnets, Magnequench has steadily built a reputation for offering robust and specialized expertise. Our success lies in our team’s comprehensive understanding of magnetic materials and their behaviour under various temperature conditions. Such knowledge is invaluable when addressing the challenges of magnet flux loss – a pivotal focus for motor engineers.

We believe in a collaborative approach. Working closely with customer teams across diverse applications, we can gain a deep understanding of their unique requirements. This two-way knowledge sharing allows us to develop effective magnetic solutions tailored to the specific challenges faced by our customers.

Understanding the thermal stability of magnets is vital in enhancing the lifespan and efficiency of motor designs. It is important for motor designers to understand the impact of short- and long-term magnet exposure to high temperatures. This is the thermal stability of the magnet and is usually expressed as the percent loss in flux over a specific time duration and at a certain temperature. Three types of losses can occur in a magnet once it is exposed to certain temperatures:

  1. Reversible loss,
  2. Irreversible loss, and
  3. Structural loss.

(i) Reversible loss: The loss that occurs when a magnet is taken from room temperature to some elevated temperature but is recovered when the magnet is returned to room temperature.

(ii) Irreversible loss: The loss that occurs when a magnet is taken from room temperature to some elevated temperature for some period but is not recovered when the magnet is returned to room temperature.

Figure 1 shows the magnet flux at room temperature and after the magnet exposed to high temperature for very short time.

Fig. 1 Magnet flux at room and elevated temperature

The flux from magnet at room temperature is A. The increase in temperature results in reduction in flux to C. When the temperature is again reduced to original the flux doesn’t go back to A, but moves to B. In Fig. 1, the total loss is represented by AC, irreversible loss is AB and reversible loss is BC.

We can further categorize irreversible loss into recoverable and non-recoverable losses. Recoverable loss occurs when the magnet experiences partial demagnetization due to exposure to excessively high temperatures. Although we can remedy this loss by re-magnetizing the magnet, it’s generally impractical. The reason is that re-magnetization is typically unfeasible once the magnet is incorporated into a component. Consequently, it’s essential to consider this loss during the design phase of the application.

(iii) Structural loss: This irreversible loss cannot be recovered by re-magnetization. It is a permanent loss resulting from the degradation of magnetic materials due to excessive operating temperature. The situation worsens in the presence of moisture.

Figure 2 shows the percentage flux loss from a magnet exposed to high temperature for a longer duration. Fig. 2 represents the irreversible flux loss component of Fig. 1. In this figure, the total loss or the irreversible loss is represented by A’ & C’. Once the magnet is re-magnetized, the part of the total loss B’ & C’ is recovered and hence termed as a recoverable loss. The portion of the total loss not recovered (i.e. A’ & B’) is known as the structural loss or permanent loss that happened due to the oxidation of the magnet.

Fig. 2 Magnet flux loss at elevated temperature

It can also be seen from Figure 2 that there is an initial flux loss in the initial 1-2 hours of the aging test. This is typical in Neo bonded magnets, mainly due to the thermal relaxation of domain structure at high temperatures. For an isotropic magnet, at its thermally demagnetized state, the domains are randomly oriented, and hence, the net magnetization is zero. When an external field is applied to the magnet, all domains are forced to align to the direction of the applied field, resulting in max net magnetization (saturation state). When the external field is removed, some domains will move back to their original preferred direction, resulting in slightly lower net magnetization (remanence state). Further, when such magnetized magnet is exposed to high temperature (such as in aging test in Figure 2), more domains will move back to their original preferred direction in the first 1-2 hours of high temperature exposure, resulting in further lower net magnetization. The last part of domain movement caused by high temperature is called domain relaxation, it leads to the observed initial flux loss in Figure 2.

3(a) Irreversible flux loss at different temperatures


3(b) Initial flux loss at different temperatures

Figure 3 (a) shows the flux loss curve for the same magnet when exposed to different temperatures. From this figure it is observed that when the magnet is exposed to higher temperature the total irreversible loss increases. Figure 3 (b) shows the initial flux loss at different temperatures, from which it is observed that with the increase in the temperature the domain relaxation increases resulting in higher initial flux loss.

The magnet flux loss depends on the shape of the magnet. The magnet shape offering higher permeance co-efficient offers lower flux loss as shown in Fig. 4.

Fig. 4 Impact of magnet shape on flux loss

The magnet composition is one of the important factors in achieving the desired flux loss at the temperature of interest. Generally, the magnet with higher coercivity (Hci) offers lower flux loss. Magnequench’s experience and understanding the impact of composition on the magnetic property, flux loss including the structural loss and cost for the magnet allows them to developed magnets with lower cost composition, offering lower coercivity but with the flux loss like magnet offering higher coercivity due to the use of relatively higher cost composition. Magnequench helps customers in arriving at the cost optimal magnets for their respective applications and its working environment/condition.

In conclusion, understanding magnet behavior under different temperatures and selecting the right magnet with regard to composition and shape are crucial in motor design. At Magnequench, we strive to provide our expertise to ensure the best cost, efficiency, and performance outcomes.

Our commitment to research, knowledge sharing, and the application of innovative solutions sets us apart in the industry. With a keen focus on the environmental impact and the performance of our customers’ applications, we consistently aim to align with the fundamental principles of motor design. This approach allows us to offer solutions that meet the changing demands of the industry, effectively bridging the gap between theoretical concepts and practical applications.

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