Determining VA for unknown power transformer

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I have been told you can get an idea from the size and weight compare it to a known transformer. PRR anyone is this a good way to guess a VA rating.
 
Compare to known transformers of similar vintage.

For small (under 50VA) transformers: Measure one winding resistance and compute the current that causes a 10% drop of voltage.
 
[quote author="kubi"][quote author="StephenGiles"]PRR said:
For small (under 50VA) transformers: Measure one winding resistance and compute the current that causes a 10% drop of voltage

Please explain further so I learn and may begin to use them.
Stephen[/quote]

Oh, a chance to chime in with my limited knowledge :grin:

U1=Voltage of your trafo at a certain winding that you can meassure with a DMM with no load.
U2=Voltage under max theoretical load. Because we don't know the max load and would like to determinate it, we assume the voltage is usually 10% less than U1.
R=Resistance across the winding mentioned above.

Ohm's law: U=R*I -=> I=U/R

And that all means your trafo has I=(U1-U2)/R Amperes.[/quote]

Note quite, U1-U2 , you would end up with 10% of U1

Rewritten: I=V*0.9 /R
 
[quote author="kubi"][quote author="ebartlet"]Note quite, U1-U2 , you would end up with 10% of U1

Rewritten: I=V*0.9 /R[/quote]

Sorry Eric, your error in reasoning.
It's really the 10% we are calculating with. (U1-U2)/R is right.[/quote]

Hmmm... now I'm getting confused.

Aren't we trying to calculate output current with a load that would cause a 10% voltage drop or in other words 90% of the unloaded voltage?
 
Ah I get it, I uderstood it when I pretended that the winding resistance was in series with a voltage source (in a way it is, this way was easier for me to visualize), even though thery are one in the same. :grin:

Thanks
 
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