Does your power transformer seem to you that it's running too hot?
Here's a method, from an NRI course book, for determining the temperature of a transformer winding. Basically, it's just measuring the resistance of a winding cold, and again while hot, and then using the following formula:

T(hot) - T(cold) = [[R(hot)/R(cold)]-1] x 235


Temperatures are in degrees centigrade, resistances in ohms.

Most digital ohm-meters will only read resistance to the nearest tenth of an ohm. This is not accurate enough for a winding of only a few ohms. But the high-voltage winding of a power transformer usually has a resistance of 3 digits, so you can measure it with sufficient accuracy. But if you have some lab grade equipment that will measure resistance to the hundredth or thousandth of an ohm, then you could theoretically use this method with any winding.

Here's a step-by-step easy method for a full-wave transformer as found in common AC tube type receivers.

[1] make sure your radio is stone cold and has been turned off for several hours.

[2] remove the rectifier tube and insert your ohm-meter probes into the plate pins in the empty socket. You can look up the tube at http://www.nj7p.org/Tube.php to find the correct pins. Remember to count the pins counter-clockwise because you're viewing from the top. Measure the resistance and write it down. This is the "cold" resistance of the high voltage winding of the transformer.

[3] put the rectifier tube back in the socket and turn on the radio. Let it play for an hour or so. Longer if you want.

[4] now turn off the radio and quickly remove the rectifier tube again. Careful! It will be very hot. Use a mitt or something to protect your hand. Quickly measure the resistance between the plate pins in the empty socket again, just like you did in step 2. Write down the value. This is the "hot" resistance of the high voltage winding of the transformer.

[5] now, according to the formula above, you divide the hot resistance by the cold resistance, subtract 1, and multiply the result by 235. This will give you the temperature difference between hot and cold. Simply add this difference to the room temperature (centigrade), which we assume is the same as the "cold" winding temperature, and you will have the "hot" temperature of the winding.

The UL code states that the maximum temperature rise of power transformers be less than 50 deg C. That's 122 deg F. So if your room temperature is 72 deg F, then the maximum allowable winding temperature would be 194 deg F.

I've tried this on several tranformers and seem to get reasonable results. The temperature obtained though, is always higher than what I measured the outside of the transfomer to be with a thermocouple. No doubt there is a temperature gradient between the winding and the outside of the transformer.

If its uncomfortable to rest your palm on it then its running too hot.

Carl

I don't know if I'm comfortable taking advice on putting my hand on a hot surface from someone who calls them self Burnt Fingers.

dit dit dit dit....dit dit. Hey Carl, he got you there! HI!
Curt

How about the new style infrared/laser guns.. ? I use a Fluke at work sometimes for equipment measurements..
I think the one we have was about $100 or so..
'F or 'C readouts

Actually, you can make your own contact temp meter for less than $20.

I use a digital thermometer with an attached probe. Looks a bit like the thermometers we used to check cargo temps. in reefer trailers. I just add on a bit to come close to internal temperature. Most times i rely on "Burnt Fingers" method.
best regards,
Sandy

Nope, I said PALM, and its a very quick love tap at first

Really though, never use fingers or palm in full contact at first pass. You might leave some skin behind as with a metal 6F6 or 6L6

Carl

Carl, you sound like you are speaking from past experience! Yeah, I've been burned/shocked a couple of big-times!

An infrared heat gun would do the job nicely. I recently purchased one. One of the things I wanted to check with it is the internal temperatures inside working radios. Haven't done that yet, but I figured if I read the temp. on several transformers then I'd know what was normal and which ones were having trouble.

It's great! Just point and read the temperature.

This is a very good method for measuring the average temperature of any electrical -copper- based winding. Accuracy is vital as a sensitivity analysis of the formulæ will show. Perhaps bridge methods would be most suitable.

We used this method to measure the rotor temperature of our 60MW and bigger turbo-alternators on-line through the brushgear. The rotor going round at 1500 or 3000 rpm. The method has been around for ¾ of a century or more

The limiting temperature for insulation will be the hot spot, for this embedded thermocouples are about the best way.

Mike

I use this:
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=93983

Curt, Curt, Curt!!! ..... Nearly 30K posts and you still haven't learned??? Nobody ever "gets" the comeback king..... When he's finally lying on that cold gray slab - he's gonna argue the undertaker that he ain't really dead.....