Experimental Study on Remanence Variation of Permanent Magnets for High-Speed Machines

Abstract

High-speed permanent magnet synchronous machines (PMSM) are widely used. However, there are challenges in the machine design [1-3]. To protect the magnets against the centrifugal force, a retaining ring is often employed on the rotor with interference fit, applying a strong compressive stress on the magnets. When the machine runs at high speed, the centrifugal force further enlarges the compressive stress on the magnets. On the other hand, high-speed machines have a disadvantage of high loss density which results in high temperature rise [1] [4]. Since different materials on the rotor have different thermal expansion, the high temperature rise produces more stress on the magnets. Therefore, the magnets in the high-speed PMSM suffer from high stress which may be up to 90 MPa, as well as high temperature such as 130 °C or higher.It has been well studied that the property of the permanent magnets varies (usually deteriorates) at high temperature, and such variation of property is usually considered in the machine design. However, it is hardly reported in existing literatures how the magnet property changes under high compressive stress.Therefore, an experimental study on this issue is carried out, by measuring the magnet property under various stress and temperature. A test rig is designed and built, and 7 types of NdFeB and 2 types of SmCo magnets are tested.Fig. 1 shows the test results of the NdFeB 42SH magnet at 24°C. When the compressive stress is applied on the magnet, its remanence increases slightly and then stays almost constant, as the curve of test-1 shows. However, when the pressure is released, it is found that the magnet remanence decreases. Then, compressive stress is applied again, and the remanence increases more remarkably, as shown as the curve of test-2. If the pressure is released again, the remanence returns with no extra decrease. Pressure is applied and released repeatedly, and the magnet property variation also repeats (see the curves of test-3, 4, and 5). Moreover, the same property variation trend is observed for the other NdFeB magnets and the SmCo magnets, i.e., the magnet property under the second application of compressive stress is lower than that under the first application of stress, but, the property will not become worse when the stress is applied more times. Therefore, it can be said that the magnet property is stabilized by the compressive stress. Fig. 2 shows the stabilized property variation versus the compressive stress for the other NdFeB magnets at room temperature. Each grade of NdFeB magnet has its own significance of property variation, and so is the case of the SmCo magnets. Similarly, the property variation versus the compressive stress at other temperature is also measured, demonstrating the negative thermal coefficient of the magnet remanence.In high-speed PMSM, the magnets suffer from significant compressive stress and temperature rise, therefore, their property of remanence is different from the data given by the manufacturer, as the manufacturer’s data does not take into account the stress influence. Clearly, the stress influence is negligible in regular-speed machines, but should be considered in the high-speed machine designs.AcknowledgementThis work was supported by the Natural Science Foundation of China under the grants **