Study of effective methods of characterisation of magnetostriction and its fundamental effect on transformer core noise

Magnetostriction of core laminations is one of the main sources of transformer acoustic noise. The magnetostriction of grain oriented silicon steel is extremely sensitive to applied compressive stress. A measurement system using piezoelectric accelerometers has been designed and built. This was optimized for magnetostriction measurements under stress within the range of 10 MPa to -10 MPa on large as-cut sheets. This system was used for characterization of wide range of grain-oriented grades. Laboratories around the world are using many different methods of measurement of the magnetostrictive properties of electrical steel. In response to this level of interest, an international round robin exercise on magnetostriction measurement has been carried out and eight different magnetostriction-measuring systems have been compared. Results show a reasonable correlation between the different methods. In this study the influence of factors such as the domain refinement process, curvature, and geometry on the magnetostriction of 3% grain oriented silicon steel were investigated. The study shows that both laser scribing and mechanical scribing have a similar effect on the sample’s domain structure and would cause an increase in magnetostriction. A proposed domain model was used successfully to estimate the effect of scribing on magnetostriction. Correlation between magnetostriction of 3% grain oriented silicon steel with transformer vibration was investigated. It was shown that increasing the clamping pressure to 4Nm can decrease the out of plane vibration in the joint regions due to the increase of friction and reduction of air gap which reduces the air gap flux and consequently the Maxwell forces. Also it has been shown that the primary source for the differences between the vibration of the cores under the same magnetic excitation and clamping pressure in the measured cores is due to the differences in the magnetostriction characteristics of the grades of electrical steels. Correlations between the magnetostriction harmonics and the vibration of the cores have been determined