Re: Question about electromagnet heat
- From: Jon Elson <jmelson@xxxxxxxxxxxxxxxx>
- Date: Wed, 29 Nov 2006 15:26:26 -0600
Bob Engelhardt wrote:
I have a nice electromagnet that I got on eBay ( http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=200045827548 ). I'd like to determine it's maximum current for 100% duty cycle. My plan is to apply a small voltage (say 1 volt), let it reach steady state temperature (minimum current) and calculate the temperature using R=Rs[1+a(T-Ts)]. Increase the voltage and repeat until a maximum temperature is reached. The magnet would be sitting on a piece of steel approx. 12" sq x 1/4" thick, to simulate its intended operating environment (power tool table).
1. Are there any flaws in this approach? Or should I say, what are the flaws in this approach - there must be some.
2. What maximum temperature that I should use? I assume that the magnet wire varnish would be the weak link, but that still doesn't help, much. The magnet is potted in "polymer", would that limit the temperature more than the varnish?
Your problem is you need to know the temperature INSIDE the
magnet winding. Also, you need to know the insulation class
of that magnet wire. You can make educated guesses, and throw
in a best guess safety factor. You can't accurately calculate the
highest temperature within the winding with a resistance
measurement, because the outer part of the winding can be
presumed to be cooler, and has the most length of wire. The
shorter inner turns will most likely be the hottest.
Jon
.
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