transformers - core volume / size vs. KVA / power

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Opcom:
Found a nice big transformer. I was able to measure alot of things and even do some calculation, but I really don't know how much power it will handle. At least 1KW CCS certainly.

Is there a formula for E-I type transformers where I can supply the core volume (LxWxH) and guage the power rating? The ARRL handybook only shows two dimensions (what about the third!) and the total weight is really not a good indicator, depending how much pig-iron is present.

core inside the windings:
H=3.875"
W=1.875"
L=2.5"

volume = 18.16 cubic inches.
-where the windings are would around the height dimension (although it should not matter).

Also, secondaries are 6VAC/~10A, 10V/~10A, and 2240V with a tap at 1280V.

_______________o 2240V - 180 Ohms
\
/     104 Ohms
\______________o 1280V - 76 Ohms
/
\
/      76 Ohms
\
/______________o 0V (common)

Sort of an interesting secondary..

anyway, any ideas on core volume vs VA rating?

W3RSW:
There's a lot of literature on transformer weight vs. power; core size vs. power, etc. But as you probably already know, older ARRL handbooks had a section devoted to the capacity of old TV power transformers, economy power supplies and rewinding data. 

a.) Are the filament windings marked at 10 amps or does your tilde mean "about," or your guess? Curious that if fil. windings were marked why not the HV?

b.) If you have a 25k /100watt resistor, put it across the secondary and measure the voltage droop from the no load condition.
     Then take a 5k / 250 watt resistor and do the same quickly.  You do have those in your junk box? Convert the values to current.  Plot both on a piece of semi-log paper and that will show you current vs. voltage droop.  This is fun to do on about all your power transformers.  If you can measure more that two points, say four different loads it gets better.  You can see the 'knee', slope changes showing saturation, etc.

Sounds like you've had similar experiences; one of the suprises I learned as a novice was that a lot of big transfomers with hefty fil. windings coupled with a HV winding were for oscillosopes.  Lots of voltage, little current.  Oh, (thought) the DC resistance values you gave look like you might have a keeper.  An 800-0-800 ac volts plate transfomer I have rated at 500 ma. shows 140 ohms across the total secondary.

Yours looks like it would be good in a bridge circuit since it's not center tapped.  Curious to see what you finally measure.

Tom WA3KLR:
Just some quick off-the-top-of-the-head answers for  now:

I've always used a rule of thumb based on some transformers here, lower than 1 KW, but 15 - 30 Watts per pound (range due to duty cycle and how conservative/reliability) for 60 Hz. iron-laminated transformers.

An electrical emperical test would be to have high value power resistors on hand and see what load across the HV secondary it takes to produce a 10 - 15% drop from no load versus loaded.

I will take a look at what transformer design notes I have, but as I recall we need the actual core cross-sectional area.  Is this what your H & W dimensions are?

Another BIG clue is to measure the primary inductance, no loads.   This is a fundamental design parameter.

Tom WA3KLR:
From the Radiotron Designer's Handbook 4th Edition page 235:

core sized needed (for 60 Hz.)

 cross-sectional area in inches = (V*A)1/2 / 5.58


So, solving for Volt*Amps, knowing core cross-sectional area:

 (5.58 x core area)2 = Volt*Amps capacity

If your numbers are correct for the core cross-section, then your core area is 7.266 square inches yielding 1644 Volt*Amps.                                         

Tom WA3KLR:
To analyze backwards from the measured primary winding inductance, we need to know the magnetic path length (download the drawing shown below) and the primary voltage.

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