Split Secondary Transformer Voltage Equality

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wwhitney

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For a typical 50 kVA single phase transformer with a 120x240V secondary (two 120V coils that may be used either in series for 240V, or in parallel for 120V), when energized, if I measure the voltage across each 120V coil separately, how close are the voltages go to be? Within 1 V, or 1 mV, or 1 microvolt, or . . . ?

A related question is what the typical impedance for the two 120V secondary coils in series would be. Since if the impedance is say 10 milliohms, and the voltage discrepancy is 10 mV, then I believe that 1A would flow between the two two coils when connected in parallel for 120V output. Which presumably is not desireable.

Thanks,
Wayne
 
ptonsparky said:
.5 kva 12/24V .08 amp between parallel secondary coils.
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ptonsparky said:
Open circuit voltage avg 12.769 & 12.762
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Thanks for doing a test!

So if the transformer secondary(s) match the model of an idealized voltage source in series with a fixed effective internal impedance, and the measurements are accurate, we get that 7 mV drives 80 mA through the coils. Which means the sum of these internal impedances is 7/80 = 90 milliohms (to one significant figure).

Which suggests in the 24V (series) configuration, if you shorted the output leads together you'd get 24/0.9 = 300A (one significant figure). Except I imagine that at that current level, the 500 VA transformer would be well outside the linear portion of its operating range, and some concept I don't understand like magnetic saturation would kick in and increase the internal impedance.

Cheers, Wayne
 
It's been more years than most of you have lived, but while working during college, we (a 2-way radio OEM) specified bifilar winding on secondaries of various transformers. Turns count matches exactly, inductance almost so. I believe it is common now on toroidal transformers; EI core I don't know.
 
GeorgeB said:
It's been more years than most of you have lived, but while working during college, we (a 2-way radio OEM) specified bifilar winding on secondaries of various transformers. Turns count matches exactly, inductance almost so. I believe it is common now on toroidal transformers; EI core I don't know.
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Also a thing in the audio world.
 
LarryFine said:
Also a thing in the audio world.
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And RF, but I was sticking to possible NEC concerns. Do even 100V speaker runs need NEC compliance? The only issue I've ever heard is insulation type, and then mostly in plenums. PVC conduit is sometimes an issue, but TW is PVC; limits on its use?
 
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