Salvaged Transformer Testing

[ncusack]

Hello,

I'm wondering if there is any further testing for a salvaged transformer aside from continuity check and applying an ac voltage to verify the different taps. I recently got 2 unknown transformers from a friend and I don't want to use them without verifying they will work. If anyone can shed some light on this situation it would be greatly appreciated.

Regards,

Neill

[kagliostro]

After you identify the output windings and voltages, you can try to establish the current disposable for each voltage using resistors as load and find at which load the voltage decrease of 5% and use that value as the max current you can withdraw from that secondary winding


K

[Paul1453]

Unless the PTs are clearly marked with the windings info you have to map them out with resistance measurements 1st.  I've got a picture of two I did in my junk gear thread.  Then I used my bench 6.6VAC supply to give me an idea of how high VAC my HV windings will put out when I put 120VAC on the primary.  I didn't know the voltage drop to get the current estimate part, that is helpful.

If they are not marked, without mapping and measuring voltages, they are only good as boat anchors.

[ncusack]

I've already mapped out the secondaries and have unloaded voltage readings. Ill load them with some resistors and see where things land. I'm also curious if there is any way of testing for noise in a power transformer. I've read you can hook up a current meter and measure noise but im not sure how that works.

[trobbins]

It sounds like you have already determined the transformers are mains power transformers?  How did you satisfy yourself about that?

[ncusack]

one of them is an old power transformer from an organ and the other I figured was one as well due to the wire colors. I hooked up a variac to what i believed to be the primaries and applied a small voltage and measured across what i believed to be the secondaries. Turned out to be ~660vac w/center tap and a 5 and 6.3v tap and the other works out to 580 with 5 and 6.3v taps. These were all unloaded voltages so ill have to gear up some power resistors and experiment to find the current ratings. The things I'm concerned with is what problems these might have. They may have winding issues which I think the current testing will reveal but I also know power transformers can be noisy but I'm not sure what the typical causes for that are or how to know without building an amp.

[eleventeen]

PTs....noisy? I would not worry.


Occasionally a PT, if some kind of gap has opened up in the laminations can induce a 60-Hz vibration into a chassis but this is really, really rare. As one who has salvaged many dozens of PTs in projects for 40+ years, I've never had that problem. I guess the bad news is that if the tranny *DOES* have that problem, you may well not find out until you have carved out the hole for it in your new chassis. But really, I would not worry about it. If the tranny "growls" when you load up the HV winding with the resistors you said you were planning, then that could be a bad sign.


Come to think of it, I have a tranny I recently salvaged that really makes an alarming and rather loud "glunk" sound when it's powered up and I have hesitated to use it for that reason. It works fine, though.

[trobbins]

A very convenient tool to add to your kit is an insulation resistance meter that tests at say 100V, 250V, 500V, 1kVDC.  That is the minimum 'safety' related testing I apply for any old or unknown transformers.  10-20yr old testers are very common on ebay, and double for testing coupling caps.  After winding resistance tests, then IR tests are the next action imho, after which loading tests will identify internal shorting problems.  Another convenient test tool recently available at very low cost is a new power meter  that give voltage, current, power, cosphi, VA etc with fair resolution and accuracy for quick testing - also very common on ebay and amazon etc.

[kagliostro]

Can you post a link for an example of this units ?


Thanks


Franco

[trobbins]

Franco, if you search for 'power meter' or power monitor in ebay you will see them as a piggy-back plug/socket (suited to your country) with an LCD screen and often with some function buttons.


They will typically display voltage, current, and power.  Limitations are often on the VAC range, current resolution, and PF range (not sure if that just means that accuracy of certain measurements degrade like real power), and failed units (they are not a quality item!).  The ENERGIE unit appears to be a bit more expensive, but also a bit better specification and quality than cheaper types.


There are also modules available that would need to be wired in to test equipment - they are also cheap, often include a wider range of measurements, but require fixed AC mains wiring, and very cheap modules may have suspect touch barriers, so best to put them behind perspex.  Some modules are din-rail mount style.  Some dinrail modules are meant for commercial/industrial use, so can be have quite good specs, but limited in display.


Yes there are more expensive modules and meters available, but you may find one is useful - but just be aware that specs are often very cursory, and so can hide a lack of performance that you may normally expect from good equipment.

[PRR]

Power monitors are good for reading the 120V/240V INput to the transformer. Unloaded (the way we usually start testing) this has little relation to the maximum safe OUTput.

Compare weight against catalog transformers of similar construction; that gives you an idea of the total V*A which it is probably good for.

Read the DCR and voltage of each secondary. Figure a load current that will give about 10% voltage drop at that DCR and voltage. That's a fair starting point.

[kagliostro]

Many thanks Tim now I've understand which kind of unit you mean (A friend show me one just this morning at a Ham Fest)


---


PRR I was suggesting to stay within a 5% of drop, is it too conservative ?


Thanks


Franco