Disputation Anar Westerberg: Nonlinear models of permanent magnets for electrical machines

Abstract

The increased demand for electricity, environmental problems related to mining of rare earth metals and to usage of fossil fuels, national security issues, and monopolization of the rare earth metals market forcing society to explore alternative options for conventional rare earth permanent magnets. Generally, the relationship between magnetic flux density and the magnetic field of permanent magnets has a nonlinear shape, but the operating region of conventional permanent magnets is linear. There is no convenient model of permanent magnets in existing commercial finite element method software that models partial demagnetization and remagnetization of nonlinear permanent magnets correctly. This thesis presents a model that is able to model an electrical machine with nonlinear permanent magnets and does not require a large computational power. A gradual improvement of the model resulted in four versions of it. It started with a simple linear model and ended with a model that is able to simulate partial demagnetization and remagnetization of the magnet and includes all four quadrants of the BH loop. Also, the last version limits the maximum magnetization depending on the angle of normalized magnetization. The magnetic characteristics of Alnico 8 LNGT40 and Alnico 9 LNGT72 magnets were measured in a vibrating sample magnetometer in order to obtain the material data required for this research work. A mathematical model of the MH loops of Alnico 8 LNGT40 in different directions was developed. The models of a synchronous permanent magnet generator with a spoke type topology with Alnico 5, 8 and 9 magnets were tested under normal and three phase short circuit conditions. The results were compared and discussed. It is found Alnico 5 is more sensitive to short circuit than Alnico 8 and 9. Electric steel clamps holding the permanent magnets and airgap above the magnets protect them from demagnetization.