The use of domain observations in understanding and improving the magnetic properties of transformer steels

The observation of the domain pattern of 3% grain-oriented silicon-iron (transformer steel) by the Kerr magneto-optic effect has enabled a greater understanding of the basic magnetization processes of this material to be obtained. In this review paper, an attempt is made to describe recent progress in this field of study. The principal commercial criterion of transformer steel is the power loss and therefore many attempts have been made to correlate the domain observations with measured loss. The increased knowledge of the shape of the loss per cycle against frequency characteristic has shown that the anomalous loss is responsible for approximately 50% of the total loss. Experimental results are described which attribute the anomalous loss to many causes such as the occurrence of domain walls, domain wall angles and configuration, variation of wall spacing with lamination thickness, non-uniform and non-repetitive wall motion (pinning), grain size effects, lack of flux penetration (wall bowing) and domain nucleation. In addition, experimental domain observations are described which explain the optimum thickness of transformer steel for minimum loss and the variation of power loss with applied stress.