At 09:27 AM 1/9/2005 -0800, you wrote:
>For 60 Hz transformers, I've always used 25 watts per square inch as a
>conservative rule of thumb. That's cross-sectional area of the center part
>of the core. Since these transformers you speak of are complete and you
>can't get to the center to measure, double the area of one outer length.
>
>Cheerful regards,
>
>Bob

Another tack is to apply a load, allow some hours for the transformer
to heat up thoroughly, then disconnect it (!) and measure the winding
resistance. Compare with the winding resistance cold. The resistance
will increase by about 3850 ppm/K. Solve for average temperature of
the winding.

You do have to assume the insulation class of the transformer, but if
you make a conservative assumption on that, you should be safe.

This method is very useful for checking the actual temperature rise
in transformers you do have data on- especially if you can't get a
thermocouple into the middle of the winding.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com




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