Procedure to determine unknown transformer rating?

[Windcrest77]

What is the good procedure to determine the power / ma rating of an unknown transformer? I know that when I apply 120v to the primary I get approx 200v unloaded on the secondary. But I have no clue of what the rating of this transformer is, I salvaged it years ago, no part numbers. Intuitively I think I need to apply a varying load while measuring the secondary current and voltage drop. Maybe plot that graph at different loads? But at what point would I determine if it is considered to be a 200ma transformer, a 300ma, a 100ma, etc? Maybe when I see the voltage drop by maybe 10%, 15% then maybe call it that current rating at that point?

 

[abraxalito]

Weight is normally a fair measure of the VA rating of a trafo.

Another way to go would be to compare primary resistance with the primary resistance of other similar trafos (say from a supplier like Mouser). You don't need to go to the lengths of plotting current vs voltage.

 

[Mark Johnson]

Assume your transformer manufacturer uses the exact same materials and the exact same ratio of copper to core, as the American transformer company "Antek". Then use this graph. Then get your 15 year old niece who's taken algebra, to help you interpret the linear equation which best-fits the experimental data. And Bob's your uncle.

_

 

[gorgon53]

If it is a standard size core, the asiest way is to look up the power ratings for your specific core dimension from core manufacturers datasheets.
Google is your friend.
Max rms current can be limited by duty cycle, regulation or/and temperature, but the max limit is always set by the wire and isolation properties, life expectance and intended use.
To be on the safe side, take a current density of 2.5A/ mm2 is ok for old transformers weigthing up to a couple of kg.
The bigger the transformer, the more current derating is needed, and vice versa.

 

[jjasniew]

One would think there would be a straight-forward methodology, accurate enough to be able to sell it to somebody without a return due to erroneous claims about its ratings. I for one would think the most common denominator is a voltage drop percentage at rated load; it's going to be something less than open circuit voltage. Like "10".

To follow up your "guess" on what the load is your unknown can do, one would think a iron temperature measurement would confirm. Run it overnight, put your probe on the laminations, read 50 C. Read 100 C maybe it's a bit too far...

I'm just guessing at all this, but for such a common component, one would think there'd be a definitive way using measurement to come up with the ratings on every output winding of an unknown tranny.

I recall my father had an old B&W TV where the power transformer had fins, like a heatsink. Just seeing that one time convinces me that final operating temperature at rated load has something to do with what it can handle - or what you could claim and rely on the thing can handle.

 

[Elvee]

Some ideas here:
https://www.diyaudio.com/community/threads/trafo-with-unknown-power-rating.156030/post-1999401
https://www.diyaudio.com/community/...multi-winding-transformer.247758/post-3745504
There are other threads on the subject, but the search function of the forum sucks, meaning you have to spend a lot of time to find them

 

[wg_ski]

Data of post 3 is going to be typical of 50-Hz-capable toroids. For 60-Hz-only you get a little more power to weight, as you can usually go down one core size since you drop the # of turns 20%.

EI’s depend so heavily on construction that your best bet is to measure the wire diameter and use 3.2 amps per square mm. If its all sealed up where you can’t get to one of the windings with calipers then weight is all you have to go on. But compare like for like - lamination stack size, paper or plastic bobbin, concentric vs. split-wound.

 

[JMFahey]

Important detail.
All methods above work fine for a single secondary winding type.

Calculations/comparisons/assumptions are fine because "all that gets into the primary appears at the secondary"

Now if various secondaries for different functions: main +V, filaments, low voltage SSc preamps or reverb, fixed bias are present, you do not know what designer choices were.

Was it intended for a tube organ with a 5W RMS power amp but 30 triodes?
A 100 W amp with a simple PI?

Admittedly extremes but yours might be anywhere in between.

Measuring actual wire diameter can give more accurate results ... IF accessible.

Measuring voltage drop only is driving too close to the precipice for me.

jm2c, ymmv, etc..

 

[Windcrest77]

Thanks I wasn't expecting to get so many suggestions about weighing the transformer, determining its displacement volume, temperature, etc. I would be confused on how I then convert ounces to ma! Every transformer company provides the amperage rating of every secondary coil when they sell you a unit. That is the measurement procedure I am seeking. I suspect the rating is a nominal spec determined by the relationship of load to voltage drop, then published by the mfg on the data sheet right? Should be simple to measure?

But at what point in the voltage drop curve does one stop and say "yeah I can sell this as a 150ma transformer but not a 200ma transformer". It has to be some kind of standardized test procedure that all mfg's use I would think. Using ohms law and reverse engineering a known transformer might give me the method. Lets say I used a unit that had a 100ma rated secondary and primary spec was 115v and secondary 100v. I can measure the open voltage, it should be a little over 100v. Then select 3 or 4 load resistors to pull/draw 50ma, 100ma, 125ma and 150ma. Use a clip lead to cycle through those 4 resistors, measuring the current and voltage for each. I should then see why the mfg is calling it a 100ma unit and not a 125ma unit then right? Whats the cutoff where they make the decision on what to print?

 

[Elvee]

Every transformer company provides the amperage rating of every secondary coil when they sell you a unit.
That is the measurement procedure I am seeking

There is no such measurement procedure: the ratings are decided by design and construction. The only measurements are quality control: insulation resistance, Hi-pot testing, etc.
It is only when you need to reverse-engineer a transformer that you have to resort to empiric methods, like the ones proposed above

 

[gorgon53]

VACUUMSCHMELZE double C cores, 50Hz

SM 42, 0.108kg, 1.75T, 5.3VA, 7.0A/mm2, 1.68
SM 55, 0.276kg, 1.76T, 21.1VA, 5.3A/mm2, 1.28
SM 65, 0.500kg, 1.78T, 45.7VA, 4.4A/mm2, 1.18
SM 85a, 1.122kg, 1.78T, 115VA, 3.8A/mm2, 1.10
SM102b, 2.64kg, 1.78T, 300VA, 3.2A/mm2, 1.06

SE 60, 0.30kg, 1.76T, 20.4VA, 6.0A/mm2, 1.28
SE 78, 0.684kg, 1.78T, 63VA, 4.5A/mm2, 1.14
SE84b, 1.280kg, 1.78T, 125VA, 4.0A/mm2, 1.09
SE92b, 0.922kg, 1.82T, 131VA, 3.0A/mm2, 1.14
SE130a, 2.24kg, 1.83T, 387VA 2.4A/mm2, 1.06
SE150a, 3.26kg, 1.83T, 590VA, 2.2A/mm2, 1.05
SE170a, 5.84kg, 1.83T, 1130VA, 1.9A/mm2, 1.04
SE231a, 13.74kg, 1.83T, 3000VA, 1.5A/mm2, 1.02

 

[gorgon53]

For more clarification of the above, S stands for schnittbandkern (wound cut core), M or E stands for core typeI, weigth of the total core (kg),
peak of Bmax in tesla (T), secondary power in voltamperes (VA), Cu-wire current density (A/mm2), and
The last number is the voltage factor, for example a SM42 has a voltage factor of 1.68. This means that the no load voltage is 1.68 x full load voltage.

 

[Mark Johnson]

1. Measure the no-load output voltage of each secondary winding when primary is connected to AC mains
2. Measure the DC resistance of each secondary winding when all other windings are open circuit
3. Estimate the total Volt-Ampere rating by weighing the transformer and interpolating on a weight-versus-VA plot
4. Calculate MY_SAFE_VA_Rating: multiply the estimate from step 3 above, by 0.75 {or some other conservative safety factor}
5. Estimate the GRUNT of secondary winding 1
6. Estimate the GRUNT of secondary winding 2
7. Estimate the GRUNT of secondary winding 3
8. Calculate the TOTAL_GRUNT of all secondary windings: TOTAL_GRUNT = GRUNT_1 + GRUNT_2 + GRUNT_3
9. Calculate the SHARE of secondary winding 1: SHARE_1 = GRUNT_1 / TOTAL_GRUNT
10. Calculate the SHARE of secondary winding 2: SHARE_2 = GRUNT_2 / TOTAL_GRUNT
11. Calculate the SHARE of secondary winding 3: SHARE_3 = GRUNT_3 / TOTAL_GRUNT
12. Calculate the VA of secondary winding 1: VA_1 = SHARE_1 * MY_SAFE_VA
13. Calculate the VA of secondary winding 2: VA_2 = SHARE_2 * MY_SAFE_VA
14. Calculate the VA of secondary winding 3: VA_3 = SHARE_3 * MY_SAFE_VA
15. Calculate the max RMS current of secondary winding 1: MAX_CURRENT_1 = VA_1 / NO_LOAD_VOLTAGE_1
16. Calculate the max RMS current of secondary winding 2: MAX_CURRENT_1 = VA_2 / NO_LOAD_VOLTAGE_2
17. Calculate the max RMS current of secondary winding 3: MAX_CURRENT_1 = VA_3 / NO_LOAD_VOLTAGE_3
18. done!

Now all that remains is to figure out how to estimate the GRUNT of a secondary winding. To start a lively discussion, I offer this suggestion

• GRUNT of secondary winding 1 = NO_LOAD_VOLTAGE_1 / DC_RESISTANCE_1

and I anticipate that one or more readers might disagree, perhaps saying there needs to be a term in the GRUNT calculation which is raised to the second power.

 

[wg_ski]

Thanks I wasn't expecting to get so many suggestions about weighing the transformer, determining its displacement volume, temperature, etc. I would be confused on how I then convert ounces to ma! Every transformer company provides the amperage rating of every secondary coil when they sell you a unit. That is the measurement procedure I am seeking. I suspect the rating is a nominal spec determined by the relationship of load to voltage drop, then published by the mfg on the data sheet right? Should be simple to measure?

But at what point in the voltage drop curve does one stop and say "yeah I can sell this as a 150ma transformer but not a 200ma transformer". It has to be some kind of standardized test procedure that all mfg's use I would think. Using ohms law and reverse engineering a known transformer might give me the method. Lets say I used a unit that had a 100ma rated secondary and primary spec was 115v and secondary 100v. I can measure the open voltage, it should be a little over 100v. Then select 3 or 4 load resistors to pull/draw 50ma, 100ma, 125ma and 150ma. Use a clip lead to cycle through those 4 resistors, measuring the current and voltage for each. I should then see why the mfg is calling it a 100ma unit and not a 125ma unit then right? Whats the cutoff where they make the decision on what to print?

But manufacturers know exactly what is in there, and how much the temperature rises, when they sell a unit. If you are selling an unknown transformer it will have to be sold as such. Just give the customer as much information as you HAVE (including the weight) and let him make his own decision on suitability for his purposes.

I used to buy them like that all the time, back in the day when there were outfits all over the place selling surplus electronics (and Skycraft ruled them all).

 

[JMFahey]

. I would be confused on how I then convert ounces to ma!

Proper ratio is weight (ounces - grams) to full VA
You still remain in doubt about individual windings.

Every transformer company provides the amperage rating of every secondary coil when they sell you a unit.

Yes. They designed it considering those parameters.

That is the measurement procedure I am seeking. I suspect the rating is a nominal spec determined by the relationship of load to voltage drop, then published by the mfg on the data sheet right? Should be simple to measure?

No.

But at what point in the voltage drop curve does one stop and say "yeah I can sell this as a 150ma transformer but not a 200ma transformer". It has to be some kind of standardized test procedure that all mfg's use I would think.

No, they design product to meet certain needs.
It's a horses before the carriage thing.

Using ohms law and reverse engineering a known transformer might give me the method. Lets say I used a unit that had a 100ma rated secondary and primary spec was 115v and secondary 100v. I can measure the open voltage, it should be a little over 100v. Then select 3 or 4 load resistors to pull/draw 50ma, 100ma, 125ma and 150ma. Use a clip lead to cycle through those 4 resistors, measuring the current and voltage for each. I should then see why the mfg is calling it a 100ma unit and not a 125ma unit then right?

Manufacturers do not wind first at random and then measure to know what they did.

Whats the cutoff where they make the decision on what to print?

They print their calculation results.

 

[Windcrest77]

Thanks all, I see it's not so simple! Especially if there is more than one winding.