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 Post subject: How to find used transformer current rating
PostPosted: Dec Fri 08, 2017 4:12 pm 
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I have a homebrew, well, something, that I got in a box of stuff a few years ago, which parts had been removed, so I finished parting it out. There's a small power transformer, and I know from the tubes it has a 12 volt filament line, but that's about it, and I know which wingdings are the primary and secondary. But the transformer has no markings on it.

The leads need replaced, so I opened it up and if I unsolder the leads, I'd be able to get my calipers on the wire from each winding just below the solder. What I'm thinking is, if I measure the wire and determine the gauge, I should be able to find the amapcity of each given size wire to determine it current rating.

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 Post subject: Re: How to find used transformer current rating
PostPosted: Dec Fri 08, 2017 4:23 pm 
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No.

The limiting factor in a transformer is the total heating, not the current capacity of individual wires.

You can get in the ballpark by simply looking at the core size...namely the cross-sectional area of the iron going through the winding bobbin. The total VA rating tends to scale with this area.

Post the core size, and I'll give you a starting point for testing.

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 Post subject: Re: How to find used transformer current rating
PostPosted: Dec Fri 08, 2017 4:45 pm 
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10W/LB is pretty safe.

DG

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 Post subject: Re: How to find used transformer current rating
PostPosted: Dec Fri 08, 2017 6:14 pm 
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Location: Lexington, KY USA
Michael,

The wire size can give you a rough upper limit for the current that might be safe for the particular winding. You still have to evaluate the transformer as a whole.

The DC resistances of the windings also offer a clue. It's pretty safe to operate any secondary at the same percent copper loss as the primary. It is a common transformer design practice to allot half the copper loss to the primary and half to the secondaries.

Ted


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 Post subject: Re: How to find used transformer current rating
PostPosted: Dec Fri 08, 2017 6:29 pm 
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Modern transformers have a rating stamped on them like Class 135 which is their insulation rating.
http://www.ecmweb.com/content/basics-tr ... ers-part-2
Based on that you can determine through trial and error how many watts you can get out of a transformer.

If your transformer is class 135 that means the insulation is rated for 135° C maximum temperature over a certain amount of time. So if you put it under a load and the external temperature of the transformer does not rise above 135° C minus about 10% or 121° C and stays there then you have found the transformers continuous duty rating.

Now if you intend to use a transformer for short periods of time I would not hesitate to go with the ampacity of the output windings minus about 25%.

I did that for the transformers I pulled from old UPS's to use to power cordless power tools that I did not want to continue buying batteries for, Since I only use them for a few minutes at a time I have yet to see them overheat. The transformers VA are 600 and 400, to get the maximum watts you need the power factor. I cheated and went with ampacity and voltage. The 600 watt one had output wires that could handle about 35 amps and output about 17 volts. 17 * 35 = 595. None of the devices I connect to it draws that much power for any length of time. The max I have seen them draw is about 10 amps. well within the max amperage of the wires in question. Of course I knew the output VA in this case.

If you plan on using your transformer for filaments I would test it on a load and see how hot it gets, if it gets hotter than 100° C (212° F) in less then an hour and keeps climbing your load is too high. Your looking for a stable temperature well below the melting temperature of the insulation (who knows what that is). keep in mind whatever the outside temp is, the inside core temp will be much higher.

Sorry there is no easy answer for this. I asked the question a few years ago and got similar information.

I am not an expert.

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 Post subject: Re: How to find used transformer current rating
PostPosted: Dec Fri 08, 2017 6:40 pm 
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Location: Albuquerque, NM 87123
TPAairman wrote:

The leads need replaced


1st off: STOP THAT!!! Use proper grammar.

You could determine current ratings of filament supplies by adding resistance until the voltage drops to 12.6, 6.3 or 5 volts AC.
Most high voltage windings were made in standard currents, like 50, 70 or 90 ma; you can make up a resistive load then check for overheating over time.


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 Post subject: Re: How to find used transformer current rating
PostPosted: Dec Fri 08, 2017 6:54 pm 
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ARRL Handbook has a handy graph.

Figure out the area of the core, if possible, and use the graph:
Attachment:
TransPower.jpg
TransPower.jpg [ 50.54 KiB | Viewed 802 times ]

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 Post subject: Re: How to find used transformer current rating
PostPosted: Dec Fri 08, 2017 7:22 pm 
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Flinx wrote:
Sorry there is no easy answer for this. I asked the question a few years ago and got similar information.


I figured there might be more to it than that. I'll have a chance to get the core size in the morning.

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 Post subject: Re: How to find used transformer current rating
PostPosted: Dec Fri 08, 2017 7:59 pm 
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W7TFO wrote:
10W/LB is pretty safe.
DG

But that does not tell you how the current ratings are distributed among multiple windings.

Winding voltage ratings are given with ALL windings loaded at rated current simultaneously.

You may be able to guess the filament current for the rectifier.
But you cannot guess any other ratings accurately.

- Leigh

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 Post subject: Re: How to find used transformer current rating
PostPosted: Dec Fri 08, 2017 9:03 pm 
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I really don't think this one is going to be rocket science.

1st, from the size, we'll have a ballpark number for the VA rating.

Next, simply by assuming the type of equipment, we can---as Leigh says--get a good guess for the filament. For the moment, let's assume 2 amps. For 12.6 volt filament, that's 25.2VA.

Another interim guess is that the total VA rating is 50. With some rounding, that leaves 25 VA for the HV winding. OK--try 300 volts at 60mA---that's 18VA, so we're still in the ballpark.

Next, measure the DCR of the HV winding---suppose it is 200 ohms. That means that the drop from no-load to full-load is going to be 120 volts......OOPS!!--check some other PTs and find how much the typical HV drop is at the rated current---maybe 20 - 30%? Maybe the conclusion is that that HV winding is only good for about 40mA. This means that the filament winding can be loaded a bit more.


Don't spend more than 15-20 minutes on the above---just come up with something that seems close, wire up the transformer with the loads, and test it. If it get's only barely warm after 1/2 hour, then you are done.

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"Measure voltage, but THINK current." --anon.


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 Post subject: Re: How to find used transformer current rating
PostPosted: Dec Sat 09, 2017 1:10 am 
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Location: Powell River BC Canada
If you apply a DC current to a winding it will heat up. A good question is, how hot?
and how much hotter is that temperature, than the temperature of the whole transformer
at the time of measurement ?

It is a bit of a probe into the transformer. Once the winding is hot, how fast does it
cool down to the whole transformer's temperature.

Copper wire has a resistance that changes as it heats up. Find the calculation, or better still,
find someone's spreadsheet. Then use your 4 or 5 digit ohm meter to get a valid value.

A dipped and solidly varnished winding will cool slower than a crumby old loose one
just waiting to short out.

This stuff is important in electric motor work, so just thrown in here for consideration.

We used DC welders to heat the coils of motors we wanted to warm up, quickly.

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