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 Post subject: Safe operating temperature for power transformers
PostPosted: Jun Sun 09, 2019 2:13 pm 
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I haven't seen this topic mentioned anywhere. How hot should a power transformer get in normal operation? I have encountered a few sets where the transformer seems to be running a little hotter than I would like, although the radios seem to be operating normally. No leaky capacitors, or other problems.

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 Post subject: Re: Safe operating temperature for power transformers
PostPosted: Jun Sun 09, 2019 2:22 pm 
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My very unscientific method.....if it is too hot to touch, things need to be checked.....starting with a no- load test.

If it gets more than barely warm with no load, then it is suspect.

If it does no get warm with no load, and the operating currents are right, then just get a fan....;) (and don't leave it running when you go out to dinner.....;) )

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 Post subject: Re: Safe operating temperature for power transformers
PostPosted: Jun Sun 09, 2019 6:03 pm 
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Location: Detroit, MI USA
There are specs available on modern transformers, which are allowed to run at a specific temperature rise over the ambient room temperature. They can safely run really hot, far beyond what you could touch comfortably.

This new standard does not apply to vintage transformers which will have a different quality of varnish and insulating paper inside them.

The "can you touch it comfortably for more than a couple of seconds" test seems to work for the older ones.

This topic has been discussed many times previously on ARF, and there's no 100% correct answer. Just to throw out numbers, if your transformer is running at 140 degrees F, it may still be OK but that also may be pushing the limits if you expect it to last indefinitely. If wax or tar is dripping out of it, or has in the past, then that could be a sign it's too hot although you can see that on many sets that still work just fine.

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 Post subject: Re: Safe operating temperature for power transformers
PostPosted: Jun Sun 09, 2019 8:54 pm 
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Location: Seattle WA US
If the transformer is too hot to touch comfortably, even if it is spec'ed to operate satisfactorily at that temperature, you have to ask:
Do I want that source of heat inside my radio ? What effect will that much heat have on all the other nearby components ? Perhaps such a transformer is OK if it is mounted on top of a chassis, with no cover to trap heat, and no other components near enough to be noticeably heated by it and some kind of screen to prevent human contact.

I prefer to avoid such situations.
-Chuck K7MCG


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 Post subject: Re: Safe operating temperature for power transformers
PostPosted: Jun Sun 09, 2019 8:58 pm 
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After several hours, the transformer in question stabilizes at 109 degrees, as measured by a digital kitchen thermometer. That's a little more than I would like, but no-load current draw is low. The unit is actually the radio/amplifier chassis that I am making into a modulation monitor for my Part 15 station.

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 Post subject: Re: Safe operating temperature for power transformers
PostPosted: Jun Sun 09, 2019 9:04 pm 
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Well, it's going to be a little bit hotter than that on the inside of the windings, but that number of 109 degrees F seems very reasonable. I have seen many that run 120-130 with no problems at all.

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 Post subject: Re: Safe operating temperature for power transformers
PostPosted: Jun Sun 09, 2019 10:59 pm 
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The two main sources of heat in power transformers are I^2R losses in the windings and magnetic loss (hysteresis) in the steel. Since the question is not so much how hot can it get, but how well can it get rid of the heat that is produced, it is standard practice to rate in terms of temperature rise above ambient. The subtlety is, if a transformer is in an enclosed space with poor ventilation, it may reach its maximum safe temperature rise at less than full load current.

So if you measure the room temperature (ambient) and then the temperature of the transformer in its enclosure at full load, after its temperature reaches equilibrium with its surroundings, the difference should be no more than 40 degrees C for most vintage transformers going back to the 1930s. Modern transformers are allowed much higher temperature rises.

“Warm to the touch” is hopelessly imprecise and unscientific, but if a transformer is running that cool it is probably not going to fail due to overheating.

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 Post subject: Re: Safe operating temperature for power transformers
PostPosted: Jun Sun 09, 2019 11:27 pm 
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I have a Crosley 7H3 which I've noticed the power transformer runs amazingly hot! But it seems to be fine.

I have talked to other people who have experienced the same thing with similar vintage Crosleys.

-Steve

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 Post subject: Re: Safe operating temperature for power transformers
PostPosted: Jun Mon 10, 2019 1:20 am 
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Location: Powell River BC Canada
If you were to measure the DC resistance of the primary of the transformer
right at the plug of a radio sitting at room temperature, you could measure it
again after it has operated for an hour or so.

If the house line voltage was the same, you have enough data to use an on-line
rise of resistance temperature calculator. Then you would know at least
how the primary winding got.

Use your best ohm meter , or better yet break out that 3 dial Wheatstone
stashed somewhere.

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 Post subject: Re: Safe operating temperature for power transformers
PostPosted: Jun Mon 10, 2019 4:11 am 
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I don't think I've ever had a radio where the transformer didn't run hot after 30 minutes or so. It's fine for shorter listening sessions, but I sure wouldn't play it all day like that.

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 Post subject: Re: Safe operating temperature for power transformers
PostPosted: Jun Mon 10, 2019 12:24 pm 
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In some of the early-30s "brick schoolhouse" sets, the PT will get only barely warm.

Brilliant idea just now about measuring the DC resistance and using that to find the core temperature...I would do it after the AC power switch--since the older switches can have some variation in ON resistance.

I think the "long tent pole" is the wire insulation. Today, the typical choices are 155C and 200C. In the 30s, probably a lot less

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


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 Post subject: Re: Safe operating temperature for power transformers
PostPosted: Jun Mon 10, 2019 12:40 pm 
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I’ll just add that “heat kills” when it comes to electronics. And those about to flame me about mercury vapor tubes, yes there are always exceptions.

In general keep your cool and electronics will last longer. Within design limits is a good start, and then de-rate for age. Not yours, the radio or instruments:-D

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 Post subject: Re: Safe operating temperature for power transformers
PostPosted: Jun Mon 10, 2019 11:07 pm 
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Location: Saskatoon
For a 90°C operating temperature which should be acceptable, even for old transformers, the hot resistance will be no more than 1.27 times the cold resistance. So, if you measure the primary resistance at room temperature, then run the radio for a couple of hours, and then measure it again, if it's no more than 1.27 times higher, then it should be fine.


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 Post subject: Re: Safe operating temperature for power transformers
PostPosted: Jun Tue 11, 2019 3:50 am 
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Dumb question:

Wouldn't it be OK to measure the HV secondary resistance? This should be higher than the primary to begin with and therefore the delta temp should be greater, allowing for more accurate measurement. Or are we concerned with winding proximity to the core?

If the latter, it brings up an interesting theoretical question: What is the thermal gradient expected across the winding bobbin, or from the geometric center of the transformer to an outside winding?

Regards,

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 Post subject: Re: Safe operating temperature for power transformers
PostPosted: Jun Tue 11, 2019 7:19 am 
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Location: Powell River BC Canada
The current density is (amps) per circular mil but the power in the wire is that squared multiplied
by resistance per turn. Turns closer to core have less resistance per turn than those outer ones.

Probably wrong, but perhaps a start ?

My most interesting instructor said, about windings, if there is doubt about what is
happening inside them, drive a chisel through and look.

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 Post subject: Re: Safe operating temperature for power transformers
PostPosted: Jun Tue 11, 2019 7:47 am 
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My most interesting instructor said, about windings, if there is doubt about what is
happening inside them, drive a chisel through and look.

:shock:

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 Post subject: Re: Safe operating temperature for power transformers
PostPosted: Jun Tue 11, 2019 8:58 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Saskatoon
Jthorusen wrote:
Wouldn't it be OK to measure the HV secondary resistance?

Yes, you could do that too, but as you also mentioned, the primary is closer to the core, and will be the hottest winding. So, it would be the most critical. The primary won't have as high resistance as the HV secondary, but it will still be high enough that it's easy to measure with an ordinary ohmmeter.


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 Post subject: Re: Safe operating temperature for power transformers
PostPosted: Jun Tue 11, 2019 12:15 pm 
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....wouldn't a high speed steel drill be easier than a hammer and chisel? :shock:

Slide rules and calculators aside, I still say cooler is better :)

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 Post subject: Re: Safe operating temperature for power transformers
PostPosted: Jun Wed 12, 2019 4:03 am 
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Joined: Sep Thu 23, 2010 6:37 am
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Location: Powell River BC Canada
tubes4life wrote:
My most interesting instructor said, about windings, if there is doubt about what is
happening inside them, drive a chisel through and look.

:shock:


The instructor was late the Dan Tuley, retired utility engineer, in 1972,
in a motor winding class.

I think we were arguing about nifty ways to know if rewinding was needed.

Another of his: If you don't know, ask an apprentice. He will think he should know it,
and will scour Hell to get the answer.

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VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
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