I think I know what you’re talking about. If the changes you are talking about are as shown by the diagram in Vulcan’s post #3 you can end up with problems in the windings under heavy loads. This is going back a long time and I could be wrong, but I seem to remember having dealt with this back in my university days. The problems are caused by the windings being rectangular rather than circular. As a result they are prone to failure at considerably lower currents than are normally seen if circular windings. The small radius curve in the windings, where they go around the corner in the core, causes considerably greater heating of the winding and as such reduces the tolerance of the transformer to high currents. I would expect that the result would be fairly similar to what you are experiencing with transformers being less tolerant to and failing sooner under high load or short circuit conditions. I seem to remember that there is a way round this problem. First off you need to design your core so that the cross sectional area of the rectangular core is slightly larger than the circular one. Then the former that you use to wind the core on need to be round rather than rectangular, with an internal diameter the same as the diagonal length of the cross section of the core. This is going to reduce the overall efficiency of the transformer but the effect will only be marginal and by removing the small radius curves in the windings where it goes round the square corner of the core you will get rid of the problems.