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Home > Archive > Electrical Engineering > October 2007 > electromagnet stuff
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electromagnet stuff
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| dear benj,
> thanks for your insight into my complex magnet problem. let me give you
> some more info on what I'm doing. currently i have an electromagnet that
> has a 1" core diamter x 3" long. the outside maximum diameter of the wraps
> not to exceed 2.75". My purpose is a strong field at the end of the core
> for pushing a N50 1" dia. x .5"l permanent magnet. tried numerous sizes of
> wire, different power supplys etc. what power supply and wire size would
> you recomend to acheive the best possible scenario for a magnet of this
> type. i expect to operate this magnet nearly continuous. i have machine
> shop capabilities and any design that we could replicate that would
> provide a good pushing would be much appreciated
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jp wrote:[color=darkred]
> dear benj,
Since you mentioned 12 volts before, if you wind that coil with 20
gauge film insulated magnet wire( about 1000 feet; coil will be
approximately 26 layers at 88 turns per layer), you'll have about 12
watts into the coil (will draw about 1 amp and if you care, the
inductance will be about 84 milliHenry) and get about 400 gauss at the
center of the coil with no iron in it. Iron will increase the field.
This should be in a safe continuous duty temperature range. To push
things you can drop the wire to 18 gauge which increases the center
field to about 800 gauss, but will put 45 watts into the coil. That
baby will likely get HOT! If this is too weak then you've got to go
with winding the coil with tiny copper tubing use low voltage and
heavy current and run water through it to take the heat away. Any way
to remove heat will help such as fans or heat sinks. But the problem
is the heat is going into the center of the coil where air currents or
heat sinks can't carry it away. Why not use another permanent magnet
as a "pusher"? They'll be plenty strong.
Good luck!
Benj
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| Don Kelly 2007-10-14, 8:25 pm |
| ----------------------------
"Benj" <bjacoby@iwaynet.net> wrote in message
news:1192363952.397109.45400@q5g2000prf.googlegroups.com...
>
> jp wrote:
>
> Since you mentioned 12 volts before, if you wind that coil with 20
> gauge film insulated magnet wire( about 1000 feet; coil will be
> approximately 26 layers at 88 turns per layer), you'll have about 12
> watts into the coil (will draw about 1 amp and if you care, the
> inductance will be about 84 milliHenry) and get about 400 gauss at the
> center of the coil with no iron in it. Iron will increase the field.
> This should be in a safe continuous duty temperature range. To push
> things you can drop the wire to 18 gauge which increases the center
> field to about 800 gauss, but will put 45 watts into the coil. That
> baby will likely get HOT! If this is too weak then you've got to go
> with winding the coil with tiny copper tubing use low voltage and
> heavy current and run water through it to take the heat away. Any way
> to remove heat will help such as fans or heat sinks. But the problem
> is the heat is going into the center of the coil where air currents or
> heat sinks can't carry it away. Why not use another permanent magnet
> as a "pusher"? They'll be plenty strong.
>
> Good luck!
>
> Benj
>
The problem is that the flux at the center of the solenoid is not of
importance. The flux at the end of the solenoid is more important for a
relatively short device. This appears to be of the order of 0.00013 T or 130
Gauss, at maximum (distance 0 from near end of solenoid) This is
considerably less than the 400 gauss correctly found for a long solenoid.
(http://www.netdenizen.com/emagnet/s...enoidonaxis.htm ).
Adding iron will modify this all to hell as then one doesn't have the same
partial leakages and an estimate would be roughly dependent on the external
air gap, ignoring the negligable iron mmf partial leakage pathsso an
estimate based on a 5 inch average air path length would be about 0.23
Gauss.This may be sufficient for the purpose but it is considerably less
than the 400 Gauss based on along coil no iron case.
It appears that, for this application, a "bar" must be used rather than some
other shape which minimizes the external air gap. That is too bad as it
certainly limits what can be done with the given size and thermal
constraints.
Don Kelly dhky@shawcross.ca
remove the X to answer
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