Normal/expected temp rise of a 600 watt mains transformer

he

I would expect most transformers to temperature rise on the order of

But, you've said you've closed off the air space the torroid is in? Not good. Heat then must transfer from torroid to air, air to box, box to outside air. When you add up all the thermal resistances, that torroid will run VERY hot.

Rule of thumb: 1 degree rise for every 1 watt with an area of 100 sq.in. located in free air.

Assume 95% efficient transformer 30W!

Two layers, we're talking over 60 degree rise from outside air to inside torroid. Just an estimate. Try using some finite element analysis software [free femm 4.2] to analyze the static thermal rise. I've found it was fairly accurate in estimating rise of temperature of fat/flesh for a SAR analysis.

the

I would expect most transformers to temperature rise on the order of

But, you've said you've closed off the air space the torroid is in? Not good. Heat then must transfer from torroid to air, air to box, box to outside air. When you add up all the thermal resistances, that torroid will run VERY hot.

Rule of thumb: 1 degree rise for every 1 watt with an area of 100 sq.in. located in free air.

Assume 95% efficient transformer 30W!

Two layers, we're talking over 60 degree rise from outside air to inside torroid. Just an estimate. Try using some finite element analysis software [free femm 4.2] to analyze the static thermal rise. I've found it was fairly accurate in estimating rise of temperature of fat/flesh for a SAR analysis.

That was worse case scenario. It is an existing amp and wanted to add a thermal fuse external to the transformer as I did not like this almost thermally enclosed siting. I added a 110 degree C , 10 amp thermal fuse to the outer surface of the tx. Should never be run at 600 watt and there is some forced air ventillation as the amp heatsink is not totally airtight and slight holes around the casing, but these transformers were made without internal thermal fuse.

es

and

Is that a power transformer, or the audio transformer?

If power, tends to run hotter, because no one cares much

If audio, it must operate very well over a broad band of spectrum, tends to run COOL, because it can't run hot unless the core is operating around its full range BH curve, which means distortion! So, I think most designers, back way down the operating point and that transformer should be running fairly cool. Did you ever measure its temperature?

A way to measure INTERNAL temperatures in a transformer without having to dig into them: Run the thing until temp is stable. Shut off and immediately start measuring winding resistance as a function of time. Plot the curve and recreate the missing section of plot between t0 to t1, where t0 is when you shut off and t1 is your first measurement. The resistance at t0 tells you what the winding temperature was. Yes, you need to know what the winding resistance was at room temperature to get a calibration point.

secondaries

and

Is that a power transformer, or the audio transformer?

If power, tends to run hotter, because no one cares much

If audio, it must operate very well over a broad band of spectrum, tends to run COOL, because it can't run hot unless the core is operating around its full range BH curve, which means distortion! So, I think most designers, back way down the operating point and that transformer should be running fairly cool. Did you ever measure its temperature?

A way to measure INTERNAL temperatures in a transformer without having to dig into them: Run the thing until temp is stable. Shut off and immediately start measuring winding resistance as a function of time. Plot the curve and recreate the missing section of plot between t0 to t1, where t0 is when you shut off and t1 is your first measurement. The resistance at t0 tells you what the winding temperature was. Yes, you need to know what the winding resistance was at room temperature to get a calibration point.

I hadn't thought about the wiring being hotter than the platteau temperature , but of course there must be a temperature gradient between the heat source and the periphery.

ure

rce

In the old lamination type power transformers I used to find that half the power went into the core and half went into the copper, with half [one fourth of total power] of that into primary and the other half into the secondary.