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Question about electromagnet heat

B

Bob Engelhardt

Jan 1, 1970
0
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?

Thanks,
Bob

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

Charles Schuler

Jan 1, 1970
0
Bob Engelhardt said:
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?

As a wild guess; do not exceed 150 C.
 
J

jasen

Jan 1, 1970
0
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).

with or without coolant flowing over the surface?
1. Are there any flaws in this approach? Or should I say, what are the
flaws in this approach - there must be some.

it'll take a long time.

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

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
 
J

Jon Elson

Jan 1, 1970
0
Bob said:
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
 
B

Bob Engelhardt

Jan 1, 1970
0
jasen said:
with or without coolant flowing over the surface?
Without. Still air only.
it'll take a long time.
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
it ignores other environmental heating or cooling sources that it will see
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.
Unlikely, the potting compound will be as good as the varnish, Good

surface temperature will be less than core temperature.
Right, but I'll be using the coil's resistance as the thermometer & so
measuring it directly.
the best people to ask are the makers.
If I only knew who they were 8-(

Thanks for your reply,
Bob
 
B

Bob Engelhardt

Jan 1, 1970
0
Jon said:
... Also, you need to know the insulation class
of that magnet wire. ...
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?
... the outer part of the winding can be
presumed to be cooler ...

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
 
H

Homer J Simpson

Jan 1, 1970
0
I hadn't thought of that, but I think that the geometry of the core and
winding would result in pretty even heating:

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.
 
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