How Do High-Temperature Grades Improve Ring Magnet Reliability?

A motor engineer is testing a new servo motor design. The prototype uses an N42 Ring Magnet with a 45 mm outer diameter and a 10 mm radial thickness. At 25°C, the flux output is within 2 percent of the design target. At 120°C, the torque drops by 18 percent, and after the motor cools down, the torque does not fully recover. The engineer removes the magnet and measures the flux. It has lost 6 percent of its original magnetization. The magnet has suffered irreversible loss. The root cause is the low coercivity of the N42 grade. At 120°C, the operating point of the magnet has moved below the knee of the demagnetization curve, and the magnet has partially demagnetized. This guide explains how high-temperature grades prevent this failure.

1. What Is the Difference Between Reversible and Irreversible Loss in a Ring Magnet?

When a Ring Magnet is heated, the magnetization decreases. This decrease is called reversible loss. When the magnet cools down, the magnetization returns to its original value. The reversible loss is a function of the temperature coefficient of the material. For NdFeB, the reversible loss is approximately 0.11 percent per degree Celsius. An N42 magnet at 120°C loses about 10.5 percent of its remanence. This loss is reversible and is accounted for in the motor design. The irreversible loss is different. It occurs when the magnet is exposed to a combination of high temperature and an opposing magnetic field that pushes the operating point below the knee of the demagnetization curve. The magnet loses magnetization permanently. The table below compares the two loss mechanisms.

In our factory, we measure the irreversible loss of every Ring Magnet batch by exposing a sample to the rated temperature and a controlled opposing field, then measuring the flux before and after. The irreversible loss must be less than 2 percent for a motor application. Xiamen Zhaobao Magnet Co., Ltd. has been manufacturing high-temperature ring magnets for over 15 years.

2. How Do High-Temperature Grades Increase Coercivity and the Knee Point?

The high-temperature grades of NdFeB, such as H, SH, UH, and EH, have higher coercivity than the standard grades (N, M). The coercivity is increased by adding heavy rare earth elements such as dysprosium (Dy) and terbium (Tb), and by optimizing the grain boundary structure. The higher coercivity pushes the knee of the demagnetization curve to a higher opposing field, which means the magnet can withstand a higher demagnetizing field before suffering irreversible loss. The table below compares the coercivity and maximum operating temperature of different grades.

The coercivity of N42 is 955 kA/m. At 120°C, the coercivity drops to approximately 600 kA/m. The operating point of the magnet in the motor must be above this value to avoid irreversible loss. The N42H grade has a coercivity of 1,353 kA/m at room temperature, which drops to about 850 kA/m at 120°C. This provides a sufficient margin above the operating point. In our factory, we recommend the H grade for motors operating at 120°C, the SH grade for 150°C, and the UH grade for 180°C. Xiamen Zhaobao Magnet Co., Ltd. produces all of these grades.

3. How Does the Operating Point of the Motor Affect Grade Selection?

The operating point is determined by the permeance coefficient (Pc), which is the ratio of the magnet's length to its cross-sectional area, and the demagnetizing field from the motor current. A motor with a thin magnet and a large air gap has a low Pc, which means the operating point is closer to the knee of the demagnetization curve. A motor with a thick magnet and a small air gap has a high Pc, which means the operating point is further from the knee. The table below shows the recommended grade for different permeance coefficients and temperatures.

4. What Are the Cost and Supply Considerations for High-Temperature Grades?

The high-temperature grades cost more than the standard grades because they contain dysprosium and terbium, which are expensive and subject to supply volatility. The cost premium increases with the grade: H is 5 to 10 percent more than N, SH is 15 to 25 percent more, and UH is 30 to 50 percent more. The table below shows the relative cost and supply risk for each grade.

In our factory, we work with our customers to optimize the grade selection and the magnet geometry to minimize the cost while maintaining the reliability margin. For example, increasing the magnet thickness can raise the Pc and allow the use of a lower grade, which may reduce the total cost. We also offer grain boundary diffusion (GBD) grades that achieve high coercivity with lower Dy content, which reduces the cost and the supply risk. Xiamen Zhaobao Magnet Co., Ltd. provides grade selection and cost optimization support for motor applications.

Summary for Motor Engineers

High-temperature grades improve Ring Magnet reliability by increasing the coercivity and pushing the knee of the demagnetization curve to a higher opposing field. The H, SH, UH, and EH grades provide maximum operating temperatures of 120°C, 150°C, 180°C, and 200°C, respectively. The grade selection must be based on the permeance coefficient, the operating temperature, and the worst-case demagnetizing current. The cost premium of the high-temperature grades is justified by the prevention of irreversible loss and the resulting motor failure. The grain boundary diffusion process offers a cost-effective alternative with lower Dy content. Xiamen Zhaobao Magnet Co., Ltd. has been manufacturing high-temperature ring magnets for over 15 years and provides full grade selection and cost optimization support for motor applications.

Xiamen Zhaobao Magnet Co., Ltd. manufactures Ring Magnet units in N, H, SH, UH, and EH grades, with grain boundary diffusion options. We provide demagnetization curves, irreversible loss data, and grade selection support for all of our products.