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Old 2nd December 2009, 04:25 PM   #1
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Default Trafo with unknown power rating

Hi,

I've got a transformer (220 V primary, 40 V secondary, center-tapped) for which I don't know the power ratings (somewhere between 40 and 80 VA I'd guess). So is where anyway of determine the power rating of this transformer.
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Old 2nd December 2009, 10:30 PM   #2
mjf is offline mjf  Austria
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hello.
is it a toroid or an EI transformer?
power rating depends on the dimensions of the core.........look into a table with core dimensions
or compare it with commercial products for a quick suggestion.
greets
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Old 3rd December 2009, 07:24 AM   #3
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It's an EI-tranfsormer, didn't know the power rating depended on the dimension of the core. The core is pretty big compered to the winding, which makes me wonder.

So do anyone know where I can find a such table for the core dimesnion then?

...compare it with comercial transformer wasn't a bad idea btw, think I'll do that if I don't find the dimension.
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Old 3rd December 2009, 07:33 AM   #4
Elvee is offline Elvee  Belgium
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There are some ways to guesstimate the power rating: one is the weight.
You take a known transformer of similar construction and power range as a reference, and you extrapolate the VA rating from their respective weight.
Alternatively, you can use the primary DC resistance instead of the weight (works in the opposite direction, obviously).
Another technique is to measure the core area A (not possible for toroids). The VA rating is then A², for iron of reasonably good quality (A is expressed in cm²).

PS:
Valid for 50Hz, which should apply to you.
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Old 3rd December 2009, 08:21 AM   #5
zvir is offline zvir  Croatia
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Hi
Can you help me with the same question?
I have a transformer with 220V primary, 85V center-tapped secondary, 50Hz
it weighs almost 10kgs
How do you get the power rating from the resistance of the colis?
mine has ~1ohm on primary and ~0.4ohm on secomdary. I have an analog meter so i cannot be more precise...
And about the core area, is that the area of the top part of the transformer? The part that I painted red in this picture? http://img686.imageshack.us/img686/4589/79223334.jpg

if it is, then its area is 96 cm^2, which would make its power rating something about 9kVA.
Are these calculations good?
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Old 3rd December 2009, 09:22 AM   #6
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by zvir View Post
Hi
Can you help me with the same question?
I have a transformer with 220V primary, 85V center-tapped secondary, 50Hz
it weighs almost 10kgs
How do you get the power rating from the resistance of the colis?
mine has ~1ohm on primary and ~0.4ohm on secomdary. I have an analog meter so i cannot be more precise...
And about the core area, is that the area of the top part of the transformer? The part that I painted red in this picture? http://img686.imageshack.us/img686/4589/79223334.jpg

if it is, then its area is 96 cm^2, which would make its power rating something about 9kVA.
Are these calculations good?
For a large transformer, you need to make a 4W resistance measurement. And you also need a transformer having a similar size for valid extrapolation: you can extrapolate from 250VA to 400VA, but not to 4KVA. The same applies for weight.
The core area is the effective area of the center leg: see picture
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File Type: jpg XformerArea.jpg (26.8 KB, 190 views)
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Old 3rd December 2009, 07:35 PM   #7
mjf is offline mjf  Austria
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hello.
found a table in an old textbook.........perhaps it helps (or confuses?)
the upper half is for EI cores.........e.g. M74 means the side length is around 74mm.
down half of the table shows dimensions of toroids,.........approximate diameter of the toroid.
you can compare this with commercial available transformers.

or try to measure it,with a few differnt resistors as a load and calculate......
greets
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