Novel techniques for characterisation and control of magnetostriction in G.O.S.S

The magnetostriction of the core laminations is one of the primary causes of transformer acoustic noise. The magnetostriction of grain oriented silicon steel is extremely sensitive to compressive stress applied along its rolling direction which increases the magnitude of magnetostriction drastically. A measurement system using piezoelectric accelerometers has been built and optimised for magnetostriction measurements under stress within the range of 10 MPa to -10 MPa. This system was used for characterisation of wide range of samples which were prepared and processed under different thermal and mechanical conditions. In this study the influence of factors such as strip thickness, coating stress, annealing under tension, cutting stress and rotational magnetisation on the magnetostriction of silicon steel under stress were investigated. It was observed that the increase of strip thickness leads to the decrease of the magnetostatic energy and therefore a reduction in the volume of closure domains in the stress patterns leading to magnetostriction under stress increasing in proportion with the thickness. Also a gradual increase in coating weight resulted in an effective increase of tensile stress introduced to the surface of the steel which was evaluated by analysis of the stress shift of the magnetostriction curve. An investigation of sample cutting techniques showed that the water jet cutting introduced an advantageous tensile stress along the cut edge in the RD of the steel. Measurement of the pk-pk magnetostriction under rotational magnetisation was shown to be significantly higher than due to uniaxial magnetisation under high compression. The acquired data was used to develop a new prediction model based on the Boltzmann function capable of evaluating the influence of those factors on magnetostriction in GO steel. The final model was able to accurately describe the effect of all studied aspects being present during the production of the steel and affecting the magnetostriction sensitivity of the final material