Behaviour of transformer cores with multi-step-lap joints

In recent time, no-load losses P and noise n of transformer cores have been reduced by replacing the former single-step-lap SSL joints by multi-step-lap joints MSL. On the other hand, the application of MSL in model cores was found not only to decrease P and n, but in some cases they increase. The aim of the paper is the investigation of the difference between SSL and MSL with respect to local magnetic conditions at joint regions and their relevance for global conditions in the core. The results show that the joint is the key region of the transformer core, affecting its dynamical behaviour in a distinct way. To minimise the iron losses and the noise of the core, the step lap number N should be chosen so that the projected induction value B falls below B c. B c characterises the joint design, marking the induction for which gap bridges, reach saturation and as a result gradually form virtual temporary air gaps. For very high B, N should be chosen in a careful way in order to avoid the disadvantageous case B> B' c. B' c marks an induction level where MSL becomes disadvantageous compared to SSL.