Energy-based magneto-mechanical model for electrical steel sheets

Within this dissertation an energy-based model for magneto-mechanical coupling in electrical steels has been developed, studied and implemented in finite element software. The method introduces a novel way of explicitly solving magnetic and mechanical fields. Starting from the knowledge of the dependence of magnetostriction on stress and magnetic field, and of the dependence of the magnetic field on the stress, a Helmholtz free energy is presented. The material constitutive equations are explicitly written. The parameters of the model are identified experimentally from a modified Epstein frame. This setup allows stressing a pack of electrical steel sheets and measuring the magnetostriction and B-H curves under pre-stresses. The resulting strains are acquired with a piezo-electric force transducer. The measurements are compared with data obtained from a vertical yoke system at no pre-stress with strain gauges and discussed. The method is then applied to study magneto-mechanical coupling in radial-flux rotating electrical machines. A test device is studied; it consists of a stack of round electrical steel sheets assembled with windings wound to obtain a magnetic flux density as in real electrical machines. This one enables the coupled method to be verified when there is no air gap. The last device is a sample machine wound in the same way as an asynchronous machine used for studying losses. Its advantage is that there is no outer frame that would prevent the placement of accelerometers for measuring the acceleration of the iron yoke when excited. The validity of the model for real electrical machines is verified by the good agreement between those measurements show good agreement with the computed results. The method appeared to be suitable and robust for the computation of displacements in rotating electrical machines. The tangential and radial displacements on the teeth of stators are obtained from the method. The influences of magnetic forces on the structures are studied and quantified.