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I have a small transformer that came from a small stereo, and probably isn't bigger than about 60-80VA. I was taking measurements of the various taps, and after having it plugged in for about 15 minutes I noticed it was getting warm. I taped my temp probe to it and let it sit for about an hour and a half, and the temp has risen to 130 degrees. I haven't measured current draw since I first started measuring things, but it drew 155mA at room temp (80 degrees)
Would you say that the temp and current draw is too high for an idle transformer?
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
poorly made cheap transformers are often made using a lower copper to current fraction, and use crap steel instead of mu metal. they often have higher idel current and parasitic losses, so they tend to run hot, even when idling.
to be sure, you would need to measure the primary current with no secondaries loaded, then re-run the temp and current test with it moderatly loaded. transformers (even poorly made ones) usually last for a long time, provided they arent run hard and kicked around earlier in there life.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
currentkills91 wrote ... ... the temp has risen to 130 degrees. I haven't measured current draw since I first started measuring things, but it drew 155mA at room temp (80 degrees) Would you say that the temp and current draw is too high for an idle transformer?
Sounds a little high, but not hugely out of line. Ever see how fast a MOT heats with no load?
1. Suppose the temerature rise would be 70 C (126 F) after an hour of operation at 100% load (unlikely in a stereo amplifier). That's not super hot, as transformers go. Especially those designed when green-ness was less important. A transformer that does not get pretty hot under full load is probably overdesigned -- needlessly heavy & expensive.
Now suppose that heat source was 60% copper loss and 40% core loss. Wiith no load, you still have all the core loss, so would expect a rise of 28 C (50 F).
2. The no-load input current is largely reactive (low power factor), so the loss wattage is less than V * I. And could have a distorted waveform, so without a true-RMS meter your current reading is questionable.
3. Might you be putting 120 V or higher on an old transformer with 115 or 110 V on the nameplate?
I haven't done this, but think one could estimate the true idle wattage by comparing the temperature rise to that from a known DC voltage and current in the primary and/or secondary windings. Not forgetting that the heat would now be developed in the copper instead of in the core.
Wish I knew where my meter was -- I'd measure a similar size transformer for comparison. Here's one pretty thorough tutorial about real transformers.
@klugesmith, I am using a true RMS meter, and the stereo the transformer came from says its rated for 120V input so I'm sure that's not the problem, under a small load the transformer doesn't really get any hotter.. I forgot to measure the current though.. I will just monitor the thing while its in use to make sure nothing goes too wrong
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Transformer Temperature
