Effects of burrs on a three phase transformer core including local loss, total loss and flux distribution

This thesis examines the effects of burrs on performance of transformer cores through application of artificial burrs, measurements of overall specific total loss and local specific total loss, measurements of flux density redistribution caused by burrs and through development of eddy current loss model incorporating two dimensional eddy current losses and flux density distribution within a conductive sample. A clamping system was designed for application of artificial burrs in a completely repeatable and reversible manner. Various burr sizes and arrangements were investigated to confirm the effect and show the extent of the effect for different scenarios with the experimental core magnetised in the range of 0.1 T to 1.8 T. It was observed that for large number of laminations affected by the artificial burrs the overall core losses of the three phase, 350 kVA distribution transformer doubled while the local losses in the vicinity of the burred region reached over 1 kW/kg. Three eddy current loss models were developed in this investigation. One for a symmetrical case of burrs located on either edge of a stack of laminations, one for a non symmetrical case of burrs located on the edge or within the stack of laminations and finally, one taking into account flux density decay within a conductive material. A secondary objective of this thesis is a feasibility study of using thin film sensors within a commercial size transformer core. This was achieved by adapting a physical vapour deposition system to accommodate large laminations and depositing thin film needle sensors for flux density measurement and thin film thermocouples for loss measurement inside a stack of the core. The flux density measurement by thin film needle sensors within the core was successful. Thin film thermocouples did not provide successful results and the development of this part of the work is suggested as future work