Why Do Transformer Voltage-Current Graphs Disagree?

[ChrisHarvey]

I have obtained the approximate equivalent circuit of a transformer at its rated supply voltage having carried out the open circuit and short circuit tests on it.

If I plot a graph of primary voltage against primary current for an open-circuited transformer I get a curve. If I use my equivalent circuit to give me predicted values for this, it gives me a straight line which intersects the curve at 2 places: one intersection is at the rated voltage.

Why is it that the 2 graphs do not agree?

I have thought about it and am wondering if it has anything to do with the fact that the relative permeability of the core changes as it reaches saturation. This would change my graphs in the 'right direction' but I have read nothing about this kind of thing in any textbook - but then I can't find this explained anywhere.

Is this idea correct, or is there some other reason for this?

Thank you very much to anyone who can give me any help,
it is much appreciated,
Chris

 

[rayjohn01]

Permeability of magnetic cores is not in general a constant and heads towards zero at saturation -- look up the topic of Hysteresis this deals with losses in cores in AC conditions . Since this is a non-linear effect it is often only represented as an equivalent resistance ( but recognizing this is an average value under the exciting conditions ) , in fact it would give rise to harmonics of a sine signal hence distortion in audio signals as well as pure losses. Some materials are far better than others hence their use in so called high'Q' coils or at RF. Try 'Wikipedia' for an explanation , search Yahoo.
Ray

 

[ChrisHarvey]

Thankyou for your reply - I've found it really helpful.