Relevance of step-lap joints for magnetic characteristics of transformer cores

Abstract

It has been stated that multistep-lap (MSL) jointed cores show distinct improvements regarding power losses P in comparison to single-step-lap (SSL). On the other hand, model core experiments tend to yield contradictory results. In the paper, special emphasis was put on the underlying physical mechanisms, taking into consideration that model cores usually show 'perfect' stacking, while full-size cores exhibit considerably high air gap lengths g as well as shifts s between the overlap regions of adjoining packages. It is shown that an increase of the number N of steps reduces the sensitivity of the core in respect to variations of g and s. This is due to increasing 'critical induction' BC as of which the excitation VL of the overlap region steeply increases. Low VL corresponds to lower balancing interlaminar flux in the overlap/gap regions and thus to a generally more homogeneous flux distribution linked with lower P. The quantity BC can be assumed to be of high practical relevance for the design of transformer cores.