Effect of Stress on Magnetic Properties of Electrical Steel Sheet and Core Losses in Electrical Machines

Abstract

This dissertation deals with the effect of stress on the magnetic properties of electrical steel sheets and, consequently, on the flux density distribution and the core loss of electrical machines. The stress effect on the permeability and the iron loss is measured using a modified single sheet tester, designed and custom built with the provision of unidirectional in-plane stressing. The measured stress dependent permeability and the developed stress dependent iron loss model are implemented in a 2D finite element (FE) machine model to investigate the effect of stress due to the shrink-fit, and the centrifugal forces on the flux and loss density distributions across the cross-section of the machine. From the simultaneous magnetic and magnetostriction measurements, a comprehensive stress dependent magnetostriction model has been proposed and later used in the magneto-mechanical coupling term of the well known Sablik-Jiles-Atherton static hysteresis model. Some modifications to the model parameters and the coupling term have been proposed to correctly model the BH-loop variation over a wide range of stress, both compressive and tensile. Similarly, the model parameters of the statistical iron loss model are made stress dependent, over a wide range of frequency, induction and stress, for further use in the estimation of the core losses. Finally, the measured permeability and the developed iron loss model, both stress dependent, are implemented in a 2D FE model of a synchronous reluctance machine. The stress distribution across the machine geometry is obtained from the static linear elasticity analysis for the shrink-fit and the rotation, and is coupled to the magnetic field formulation through the stress dependent permeability. The obtained magnetic field distribution is used in conjunction with the stress dependent iron loss model to post process for the core losses. The results obtained from the FE simulation with the inclusion of stress show a substantial difference in both the flux density distribution across the machine geometry and the loss density distribution across the stator core, when compared to the FE simulation result without the stress.