Temperature properties of passive magnetic bearing

Abstract

The practical interest in passive magnetic bearings is due to the fact that they are not subject to wear and have insignificant dissipative energy losses. However, when the magnets are heated during operation, a reversible and irreversible decrease in the residual magnetization occurs and, consequently, the force of interaction between them changes. The thermal properties of passive magnetic bearings are adversely affected by the large demagnetizing fields from the coupled magnets. To predict the performance of passive magnetic bearings with regard to external temperature, an assessment of temperature changes in the bearing capacity of bearings with optimally shaped magnets is necessary. The optimum magnets were considered to be those with sizes, whose interaction force, reduced to a single volume of magnetic material, was the maximum. A magnetic system of two cylindrical magnets with equal radii and heights located coaxially or with some radial displacement was considered. The studies were carried out on cylindrical magnets made of SmCo5 alloy, capable of maintaining magnetically hard properties at temperatures up to 250°C. It was confirmed that irreversible changes in the magnetic force for bearings with optimally sized magnets are close to each other. The destabilizing (radial) magnetic force at small radial displacements of one of the magnets also irreversibly decreased. A temperature of 250°C defines the upper limit of the temperature range for the normal operation of magnetic bearings with SmCo5 magnets. It was found that as a result of the action of a demagnetizing field on magnets during heating, they develop a significant inhomogeneity of magnetization over the volume. After demagnetization of the magnets kept at a temperature of 260 °C, the force of their interaction reaches only 85 - 90% of the original, which is associated with microstructural changes in the magnets. Theoretically and experimentally, the numerical values of the temperature coefficients of changes in the bearing capacity and stiffness of bearings are determined.