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Reply To: Transformer riddle no.2 – Power transformer with inferior core

#1159
BertHickman

    Actually if your teacher had replaced the core with an inferior one, the ring would not have gone as high, since core losses would have caused a reduction in the flux “seen” by the ring. The ring would also not go as high if the core had a lower permeability either since more flux would escape the core before it could pass through the ring (i.e., giving you a lower coupling coefficient and higher leakage inductance). The forces seen within your transformer arise from the same basic effect (Lenz’s Law), but the directions of the forces are considerably more complex due to the more complicated geometry of the windings and flux paths through, and outside of the core, especially if the core itself saturates during the current fault. Because of the closed magnetic path, the effective coupling between windings is considerably higher than the case above with the floating ring, so the fault current can be considerably greater – potentially sufficient to exceed the yield strength of the winding or the surrounding insulating materials. In what way(s) was the core material inferior? Did it have lower relative permeability, lower saturation flux, higher electrical conductivity, was it made from thicker laminations, or did the material have a larger area hysteresis loop than desired?? Simple core loss alone does not explain the winding damage problem. Lower permeability or lower saturation flux might…