Transformer losses question

[Patrick]

This is a "sort of" homework question, but I think it's valid to ask.

I did an experiment with the objective of modelling transformer losses.
Naturally one of the tests performed was the open circuit test, so that we
can determine core losses (Rc) and magnetizing inductance (Xm).

I plotted Rc and Xm vs. Voltage and obtained a graph I can't explain. As
the voltage was increased, Rc decreased. I can't explain this - since Rc
models eddy currents and hysteresis losses (right?) I would have expected Rc
to increase as V increased (since there would be more current flowing
through this element).

The other graph showed Xm increasing with voltage. This makes sense, I
think, since as the voltage is increased more current goes through the
excitation branch, and hence, we have more leakage flux in the primary
inductor.

So I'm rather confused about the Rc results - is a decrease in Rc with
voltage, in fact, what we expect or does it look more like I have bad data?
(ie, screwed up the experiment :-o)

Thanks for your help

 

[Winfield Hill]

Patrick wrote...

I did an experiment with the objective of modelling transformer losses.
Naturally one of the tests performed was the open circuit test, so that
we can determine core losses (Rc) and magnetizing inductance (Xm).

I plotted Rc and Xm vs. Voltage and obtained a graph I can't explain.

How did you measure these parameters, exactly?