Magnet Selection for Cost-Optimized Motor Designs

September 11, 2023

Isotropic bonded NdFeB magnets, colloquially termed MQ1, offer several unique advantages in various motor applications. These pluses include being free of heavy rare-earth materials, providing high yield in

near-net-shape magnet production, allowing tailored magnetization profiles for optimal magnet performance due to their isotropic nature and exhibiting high resistivity to eliminate eddy current loss in the magnet.

MQ1 magnets also exhibit excellent thermal characteristics, maintaining their performance across a wide temperature range. These features make them particularly suitable for demanding applications such as automotive accessories and home appliances, where motors can either experience significant temperature fluctuations and/or require superior energy efficiency.

Which grade is optimal?

There is a wide range of MQ1 magnet grades; hence for any new design, the first question a designer has is which grade of magnet should be chosen to develop a cost-optimal motor?

Several aspects should be considered when choosing the magnet grade for a new design: 1) affordability and dependability of raw materials; 2) the magnetic properties at both room and high temperatures and 3) the magnet’s stability, specifically the potential for flux loss at high temperatures. All are important factors to consider.

Raw material impact: Fluctuations in the pricing of raw materials inevitably impact the cost stability of NdFeB magnets. As the originator of the raw material for the MQ1 magnet, Magnequench possesses a profound understanding of rare-earth materials and their consequential effects on total magnet costs. Technical teams are constantly at work to innovate new technologies and optimize compositions to mitigate the impact of these price fluctuations. Moreover, we forge strategic partnerships with various tiers of automotive suppliers and home-appliance manufacturers. The goal is to ensure that designers attain maximum performance from the MQ1 magnet within a specified budget, thereby creating a harmonious balance between cost and efficiency.

Figure 1 illustrates the magnetic properties that MQ1 magnets can achieve at room temperature. Magnequench created cost-optimized MQ1 magnet grades that replace higher-priced light rare earth (LRE) elements such as Nd/NdPr with lower- and stable-priced LREs such as Ce/La – without a significant sacrifice in magnetics in both room and high-temperature thermal properties. Table 1 presents the representative MQ1 magnet grades. For an example, replacing almost 60% of total rare earth (TRE) by Ce/La results in only 18% reduction in magnet Br.

Table-1: Representative MQ1 magnet grades

Magnet grade Ce or La/TRE (%) Br (kG) Hc (kOe) Hci (kOe) (BH)max (MGOe)

MQ1-10 0 7.04 5.73 9.26 10.17

MQ1-9 20 6.67 5.36 8.98 8.97

MQ1-8 39 6.29 4.99 8.08 7.93

MQ1-7 58 5.80 4.75 8.50 6.94

Figure 2 illustrates the flux loss at 120°C (248°F) for representative MQ1 magnet grades, indicating that magnets with Ce/La as part of the total rare earth (TRE) exhibit similar or even slightly better thermal stability compared to magnets with only Nd/NdPr as TRE.

Using a magnet with a slightly lower Br depends on the allowable motor size and weight. The use of the MQ1-7 magnet in comparison to the MQ1-10 magnet may lead to 10% to 20% increase in motor volume and weight. A magnet with Ce/La has slightly lower Hc, so to avoid the magnet operating point falling below the knee during the worst-case magnet operating condition (when the magnet sees high temperature and the maximum demagnetization current), a slightly thicker magnet is required. Even with a thicker and heavier magnet, the magnet with Ce/La content still will be the cost-optimal design because of the lower and stable price of LREs like Ce/La.

Maximum allowable flux loss

It also is important to understand the maximum allowable flux loss for a motor under design. The flux loss data for magnets are measured with specific dimensions of a magnet – mostly Permeance Co-efficient (PC) = 2. However, the actual PC in motors is much higher than 2. Figure 3 shows the flux loss for magnets with different PC values. It can be observed that the magnet with higher PC offers much better thermal resistance and hence lower flux loss. The use of magnets that offer less than target flux loss (measured for PC = 2) will lead to an unintended but significantly higher safety margin and an increased motor cost. Instead, selecting a magnet grade with slightly higher flux loss (for PC = 2) than target will meet the flux-loss target, helping to achieve a cost-optimal design.

Linearity of room- and high-temperature magnetic characteristics

The PC of the magnetic circuit should be as high as possible to achieve the highest airgap flux. The characteristics of a magnet with PC of magnetic circuit form critical parameters in evaluating the potential for partial demagnetization at the maximum temperature that a magnet will experience during the operation of the motor. Figure 4 shows the magnet characteristics at room temperature and at elevated temperatures. The magnet has non-linear B-H characteristics at higher temperatures. Point A represents the magnet operating point at no-load and room temperature.

At high temperatures, the no-load operating point moves to point A′. When the normal load is applied to a motor, armature/stator current induced field, or mmf, moves the operating point to B at room temperature or at B′ at high temperature. Point B′ is on the linear region of the high-temperature B-H characteristics of the magnet. When the load is removed and the magnet is at high temperature, the magnet recoils and the operating point moves to A′. As the motor cools back to room temperature, it will move back to A’. For the worst load conditions.

As this point is below the knee point, when the load is removed the magnet recoils based on the recoil permeability and the normal load the magnet operating point will be B′′ instead of expected B′. When the motor cools down to room temperature, the magnet operating point will be B′′, the load removal will bring back the magnet Br to Br′ due to irreversible demagnetization of the magnet. To avoid such irreversible demagnetization, it is important to select the magnet with linear magnetic characteristics at the maximum temperature a magnet can

When determining the optimal magnet grade for motor designs, the process can be quite challenging. Considering the wide variety of magnet grades falling under the MQ1 category, making an informed decision requires technical acumen and industry expertise. In such a context, businesses can benefit immensely from aligning themselves with experienced and reputable magnet manufacturers. These specialized manufacturers, with their extensive understanding and innovation in isotropic bonded NdFeB magnets, notably the versatile MQ1 series, are well-positioned to offer guidance and support.

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