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Old 21st February 2013, 08:24 AM   #1
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Default Making sense of transformer measurements

I have just measured my power transformer with a LCR metre, a scope and a signal generator. The transformer is rated 625VAC, with 1 x 240V primary and 2 separate 45V secondaries.

When shorting the primary to measure the leakage inductance, on one secondary the reading was 28.8uH, on the other secondary it was 44.6uH. Is that normal? Is there anything wrong? Should they be identical?

When shorting the secondary windings to measure the leakage inductance from the primary side, the reading was 1.06mH. This is much higher. Should leakage inductance be the same when measuring from either the primary or secondary side? I must be somehow wrong, as the leakage inductance is unlikely that high.

One secondary winding gives a stable 0.15R reading, and the other jumps around between 0.14R to 0.15R. Does it mean there is some inbalance? The primary has 2.09R.

When I entered these numbers into PSU2, it works out that the transformer can deliver 10A on each of the secondary, with a Source Resistance = 0.225R.

I also connected the signal generator with a 1k resistor in series on the secondaries and measured the voltage developed across the coils and found the resonant frequency to be 910kHz for both secondary windings. So when I return to work tomorrow with a much better calculator I can easily work out the leakage capacitance very easily.

So if I am going to model this with LTSpice, should I model the transformer to be an ideal voltage source in series with the parallel of H=28.8uH (44.8uH for the other), R=0.15 (or 0.225R?) and C=? (to be worked out)? Please see an illustration in the attached picture.

Thanks.
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Last edited by HiFiNutNut; 21st February 2013 at 08:31 AM.
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Old 21st February 2013, 11:42 AM   #2
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Leakage inductance strongly depends on how the transformer has been winded. So, don't worry about that difference. The winding nearest the primary will have the lower leakage, and in a power trafo usually don't care too much.
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Old 21st February 2013, 11:48 AM   #3
DF96 is offline DF96  England
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Each winding will have its own leakage inductance. It may vary a bit between two nominally identical windings. Leakage inductance tells you how much of a winding's flux is not linked to other windings. It is a measure of flux linkage or (lack of) coupling.

Remember with a transformer any impedance will be transformed when measured from another winding.
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Old 21st February 2013, 12:24 PM   #4
DUG is offline DUG  Canada
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Were you measuring the inductance with a frequency close to the operating frequency?
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Old 21st February 2013, 06:53 PM   #5
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Quote:
Originally Posted by DUG View Post
Were you measuring the inductance with a frequency close to the operating frequency?
I think the LCR metre measures L at 500kHz. I think that should be good enough.
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Old 21st February 2013, 07:08 PM   #6
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Originally Posted by DF96 View Post
Remember with a transformer any impedance will be transformed when measured from another winding.
Thanks. So that is the reason I got different readings when measuring from the primary or secondary side.
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Old 21st February 2013, 07:14 PM   #7
JMFahey is offline JMFahey  Argentina
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500kHz

500 KHz and power transformer shouldn't be mentioned in the same phrase.

I think it must use a *much* lower test frequency.
Say, 400 Hz or something.
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Old 21st February 2013, 07:23 PM   #8
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I measured both secondary windings having a resonant frequency at 910kHz. If I use the formula F = 1 / (2 * Pi * SQRT(L * C)), I would have the leakage capacitance C = L * square(1 / f) / (2 * Pi). Here f = 910000. Should I use the leakage inductance for the L?

If so, since I have 28.8uH and 44.8uH leakage inductance on the 2 secondary windings respectively, the leakage capacitance turn up to be 1044pF and 672pH respectively. Does it sound right to you?

Last edited by HiFiNutNut; 21st February 2013 at 07:26 PM.
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Old 21st February 2013, 07:40 PM   #9
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I am still thinking about how an equivalent circuit looks like in LTSpice.

Looking from the power supply side, I presume the circuit sees the transformer's secondary leakage inductance, leakage capacitance and winding resistance.

For a good approximation only, can we ignore other parameters hence saving the efforts of modelling the primary side? Would this approximation be good enough to use to design a PSU?

If so, can I model the transformer as an ideal voltage source in series with the parallels of leakage inductance, leakage capacitance and winding resistance? or the inductance and resistance are in series, then in parallel with the capacitance?
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Old 21st February 2013, 08:40 PM   #10
DF96 is offline DF96  England
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Real transformers are usually modelled as ideal transformers with added parasitics, such as leakage inductance and capacitance.

A transformer is not a voltage source. Ultimately it is a pair of coupled inductors, so that is where you have to start from.
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