I am learning about transformers, but one question that is bothering me, which no book seems to clearly answer, is whether or not there is flux in the core of an ideal transformer with a shorted secondary winding.

If current in a loop flows to oppose changes in flux, then in an ideal transformer with a shorted secondary, no flux should ever exist in the core, because as soon as flux increases from zero, opposing flux from the secondary winding would cancel it out.

Non-ideal transformers have at least some flux in their cores, because they have measurable reluctance, or even an air gap, which is why equivalent circuits have a parallel inductor and resistor. Assuming the transformer is well built, this amount of flux should fairly small as long as the transformer is driven at the proper frequency. So here is the question that has been bugging me for years:

Why does every textbook and core datasheet make it seem like the B-H curve is the be-all end-all of transformer core selection?! Unless the transformer is only lightly loaded there IS NO FLUX to worry about. What am I missing here?

 

Normally, the short circuit test (secondary) for a transformer is done at a much reduced power level to measure copper (I²R) loss because there is no output power or core loss. Does that answer your question?

Cheers, DPW [ Everyone's knowledge is in-complete...Albert Einstein]

 

Maybe this is more of a physics question. Can I move threads?

 

Sure, I'll close this one.