Greetings all
The last project I built uses a Hammond 370FX power transformer, and I have been uncomfortable with the kinds of temperatures it reaches after about an hour's operation.
To the point that I ordered a larger unit to replace it with, since I am concerned about reliability.
I understand from some other threads and discussions elsewhere that this is common behaviour for these.
I did an experiment and measured the temperature rise over 3 hours operation and wrote it up here
I'd be interested if this matches others' experiences, and more importantly, in my tests I made an uninformed assumption that the windings could be expected to be 20°C hotter than the laminations on top.
Does anyone have any deeper insight or better knowledge on this?
Please be gentle, I'm still learning this stuff.
The last project I built uses a Hammond 370FX power transformer, and I have been uncomfortable with the kinds of temperatures it reaches after about an hour's operation.
To the point that I ordered a larger unit to replace it with, since I am concerned about reliability.
I understand from some other threads and discussions elsewhere that this is common behaviour for these.
I did an experiment and measured the temperature rise over 3 hours operation and wrote it up here
I'd be interested if this matches others' experiences, and more importantly, in my tests I made an uninformed assumption that the windings could be expected to be 20°C hotter than the laminations on top.
Does anyone have any deeper insight or better knowledge on this?
Please be gentle, I'm still learning this stuff.
What you must find out is if it runs hot with no load. If it does there is either a shorted turn or two or the laminations have shorted out, (usually due to no insulation alongside the mounting bolts that clamp the laminations together or no fibre washer under at least one end of those bolts).
If it runs cold with no load check the load you are subjecting it to. In an ideal world, the transformer should run cool and be heated only from the radiation from the output valves. I usually suspect poor design/quality if they run hot.
If it runs cold with no load check the load you are subjecting it to. In an ideal world, the transformer should run cool and be heated only from the radiation from the output valves. I usually suspect poor design/quality if they run hot.
OK now I am puzzled. Do the individual E-I pieces need to be insulated from each other?
If so, then I have learned something
Please read the maker's short-paper on transformer temperature.
What is meant by “Class” in insulation? | HPS | Hammond Power Solutions
Since the 370FX has no "class", assume it is temperature class "A", although perhaps built with some better-than-A stuff.
Your 40 deg C rise appears to be entirely safe.
Your 62 deg C asymptote, even allowing for heat path, is far below the 105 deg C rating of class A stuff.
It is entirely normal for (>20VA) transformers to run very warm. Heat rise is part of the trade-off against cost and size.
What is meant by “Class” in insulation? | HPS | Hammond Power Solutions
Since the 370FX has no "class", assume it is temperature class "A", although perhaps built with some better-than-A stuff.
Your 40 deg C rise appears to be entirely safe.
Your 62 deg C asymptote, even allowing for heat path, is far below the 105 deg C rating of class A stuff.
It is entirely normal for (>20VA) transformers to run very warm. Heat rise is part of the trade-off against cost and size.
Some of my hammonds run hot, some not.
A 5V rectifier can be converted to diodes, if not mandatory for your amp and you can have a soft start module, this will lower the temperature.
A support transformer can heat up the tubes elements which can too reduce your Hammond temperature.
On a humoristique note, in the factories I worked 2 of the 50Kv to 600V were running in the 100+ degree celcius (in the red on the indicator :, danger zone : ) without any buzz or problems, cost saving on air-conditioning etc.
So unless the transformer vibrates or emit a noise, up to 80-90 C on the surface should not break the transformer insulation prematurely.
However 50C should be your maximum target especially if people can touch it or in hot weathers.
A 5V rectifier can be converted to diodes, if not mandatory for your amp and you can have a soft start module, this will lower the temperature.
A support transformer can heat up the tubes elements which can too reduce your Hammond temperature.
On a humoristique note, in the factories I worked 2 of the 50Kv to 600V were running in the 100+ degree celcius (in the red on the indicator :, danger zone : ) without any buzz or problems, cost saving on air-conditioning etc.
So unless the transformer vibrates or emit a noise, up to 80-90 C on the surface should not break the transformer insulation prematurely.
However 50C should be your maximum target especially if people can touch it or in hot weathers.
Yeah well, BIG iron economics are different from soup-can iron.
10VA doorbell transformers really won't overheat because the surface/volume ratio is large. All the heat that fits in the winding, the surface area will shed without huge temp rise. OTOH a big tranny can fit a lot of power/heat in its volume, but the surface area does not increase as much, and it tends to run HOT.
You see in that Hammond puff-sheet that insulation ratings run from "A" (formerly paper/cloth/varnish) up to Really Hot. The tough stuff costs more, of course. But even a 25KVA pole-pig is likely wound of high-temp stuff.
Agree that 50-ish rise is probably better for our stuff, leave the 100 deg C iron for guys on salary.
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