Toroid transformer temperature

> From: Chris L <datac@...>
>Subject: Re: Re: Torroid Question
>
>
>Thanks Bob,
>You make sense here.... Sure couldn't determine much without a long term
>test. Obviously, If I ran it for a full day at moderate temps, that
>would be pretty good.
>I assume that we don't want to rely on "Fan Cooling" for a little more
>ooomph.......
>Do you have an opinion on what is a good target temp ?? Caudlet
>mentioned 80C I believe....

If you really want to know how hot the windings are getting, there is a
standard test used with motors and transformers based on the change of
resistance with temperature. You measure the winding resistance cold, then
run the motor or transformer under some specified load (probably full load
in your case) for many hours to get the temperature to stabilize, then
measure the resistance again. Since the temperature coefficient of copper
is known, you can calculate the average winding temperature.

The Leeson Motors page at
<http://www.iprocessmart.com/leeson/leeson_motor_temperature_article.htm>
contains the formula and a bunch of other useful info. Although it's
oriented to motors, I believe it applies to transformers too. Transformers
are also built with different insulation classes and thus different ability
to withstand heat. A transformer may be operating normally when it is too
hot to touch.

Dave