Magnetization process in electrical steel sheets

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The energy losses in transformers, generators and electro-motors, where electrical steel sheets are utilized, are strongly determined by the magnetic domains in these sheets. The understanding of the loss mechanisms and the development of strategies for their reduction therefore requires fundamental studies of the domains and magnetization processes by microscopic imaging. To lower eddy current effects, however, the sheets are covered by non-transparent insulation coatings that make a direct observation of the domain structure by optical means impossible. For magneto-optical Kerr microscopy this cover layer has to be removed and the surface has to be carefully polished, which unfortunately leads to drastic changes of the magnetic properties and domain structure. By using a recently developed new generation of magneto-optical indicator films (MOIF), which are placed on top of coated sheets and which indirectly reveal the underlying domain distribution, the situation has considerably changed: the domains can now be imaged in the presence of insulation coatings with a sufficient resolution and even in a time-resolved manner by single-shot imaging at power frequencies. The indicator films furthermore allow for the imaging of magnetic poles at grain boundaries, making it possible to study the role of grain boundaries for the flux transport. In this project the potential of the MOIF films shall be used to obtain a deeper understanding of the quasistatic and dynamic magnetization processes in electrical steel under working conditions. The goal is a fundamental understanding of the microscopic sources of energy loss to provide a basis for a targeted minimization of loss in electrical devices and machines.