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Question about electromagnet heat

I have a nice electromagnet that I got on eBay (

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). 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.

  1. 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?

Thanks, Bob

PS - if there's a better newsgroup to ask this question in, I'd appreciate being pointed there, thanks.

Reply to
Bob Engelhardt
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As a wild guess; do not exceed 150 C.

Reply to
Charles Schuler

with or without coolant flowing over the surface?

it'll take a long time.

it ignores other environmental heating or cooling sources that it will see in use.

Unlikely, the potting compound will be as good as the varnish,

surface temperature will be less than core temperature.

the best people to ask are the makers.

Bye. Jasen

Reply to
jasen

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

Reply to
Jon Elson

Without. Still air only.

I hadn't thought about it, but now that you mention it, I wouldn't have to wait until temperature stabilized. I'd just have to wait as long as the longest time that it would be energized in use

I'll assume that the test plate is a reasonable approximation of the working environment. Maybe verify that at the end by making a couple of measurements in use.

Good

Right, but I'll be using the coil's resistance as the thermometer & so measuring it directly.

If I only knew who they were 8-(

Thanks for your reply, Bob

Reply to
Bob Engelhardt

Right, do you know what kind of range of insulation might be used on a magnet like this (132 turns of 24 ga wire, if it matters)? Do you happen to know the operating temperature of any magnet wire insulation? Motor winding insulation is classified as A,B,F, or H. Is it likely that one of those classes would be used here?

I hadn't thought of that, but I think that the geometry of the core and winding would result in pretty even heating:

- the core would be the main heat conductor

- the core is E-shaped & so encloses the winding, mostly

- on the ends, the winding has polymer on the outside and core on the inside - I think it might be hotter on the outside

- the winding is not very thick (see cross section in listing), so there wouldn't be much of a thermal gradient. Also, the high thermal conductivity of the copper wire would lessen the gradient.

Lots of considerations, but I don't need to run on the hairy edge.

Thanks for your thoughts, Bob

Reply to
Bob Engelhardt

You could measure the resistance of the coil at ambient temperature and then at working temperature. That would give a close approximation to the average temperature.

Reply to
Homer J Simpson

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