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2nd December 2009, 03:25 PM  #1 
diyAudio Member

Trafo with unknown power rating
Hi,
I've got a transformer (220 V primary, 40 V secondary, centertapped) 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. 
2nd December 2009, 09:30 PM  #2 
diyAudio Member
Join Date: Jan 2009
Location: Austria

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 
3rd December 2009, 06:24 AM  #3 
diyAudio Member

It's an EItranfsormer, 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. 
3rd December 2009, 06:33 AM  #4 
diyAudio Member
Join Date: Sep 2006

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. 
3rd December 2009, 07:21 AM  #5 
diyAudio Member
Join Date: Feb 2009

Hi
Can you help me with the same question? I have a transformer with 220V primary, 85V centertapped 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? 
3rd December 2009, 08:22 AM  #6  
diyAudio Member
Join Date: Sep 2006

Quote:
The core area is the effective area of the center leg: see picture 

3rd December 2009, 06:35 PM  #7 
diyAudio Member
Join Date: Jan 2009
Location: Austria

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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