MATERIALS FEATURE Magnet Ce or La/ grade MQ1-10 MQ1-9 MQ1-8 MQ1-7 TRE (%) 20 39 58 Br (kG) 7.04 6.67 6.29 5.80 Table 1: Representative MQ1 magnet grades Hc (kOe) 5.73 5.36 4.99 4.75 Hci (BH)max (kOe) (MGOe) 9.26 8.98 8.08 8.50 10.17 8.97 7.93 6.94 Figure 1: Envelope for possible MQ1 magnet property. Figure 2 illustrates the flux loss at 120°C 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 a 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. 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 magnetic characteristics of a magnet with PC of magnetic circuit form critical parameters in evaluating the potential for partial demagnetization at the AUTOMOTIVE ENGINEERING Figure 2: Flux loss at 120° C for representative MQ1 magnet grades. Figure 3: Impact of Magnet dimension or PC on flux loss. 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'. October 2023 27 ALL IMAGES: MAGNEQUENCH