A method of measuring both the inductance and the paralleled capacitance of a transformer winding was presented some years ago at in this URL. That essay should be read to best follow the thesis of this one.
That essay’s target devices were high voltage transformers whose secondary windings and secondary loadings from long Cockcroft-Walton voltage multipliers yielded so much paralleled capacitance across the primary windings that conventional LC test instruments were rendered useless for making transformer winding inductance measurements. Instead, dual resonance testing methods overcame that problem.
It should be noted that the above essay referred to measurements being made on ferrite core transformers but is also applicable to low frequency transformers with laminated steel cores such as the following device in Figure 1. Although iron core properties are known to change markedly versus excitation level, this measurement technique itself is not inherently limited to ferrite core transformers which fact is demonstrated by the following example.
Figure 1 A line frequency power transformer with laminated steel cores.
Test results for this iron core transformer were analyzed using the same GWBASIC code as before and can be seen in Figure 2.
Figure 2 The test results for the line frequency transformer in Figure 1.
Averaging the multiple readings, the primary winding apparently exhibits 338 mHy, the full secondary exhibits 70.7 mHy, one side of the secondary exhibits 17.1 mHy, while the other side of the secondary exhibits 17.3 mHy; thus revealing some imbalance between two otherwise identical windings. But, with leakage inductance and measurement tolerances, nothing is ever perfect, right?
The shunt capacitance calculation results which are very small, and even negative, tell us that the shunt capacitance of this transformer is essentially negligible.
Just as a quick check, 70.7 / 17.1 = 4.134 and 70.7 / 17.3 = 4.087 which are close to, but somewhat above, the nominal 4.000 ratio that would nominally apply for the 2:1 turns ratio.
My suspicion is that we were seeing the effects of leakage inductances.
John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).
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