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pauleddy

Sun Aug 01 2010, 09:22PM

Registered Member #2909 Joined: Wed Jun 09 2010, 12:31AM
Location: fort belvoir, Va USA ( south of DC)
Posts: 145

Does any know anything about super cooled coil guns? I remember from some where that if you cool a conductor down it lowers the resistance of the conductor, and a lower resent wire can produce a better field strength right? And that should improve efficiency of a magnetic coil. So my question is can you build a better coil gun if you cool the coil down to a relatively low temp with dry ice with alcohol, liquid nitrogen or better liquid helium.
Any ideas on this would be great.

Turkey9

Sun Aug 01 2010, 10:30PM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

With a super-conducting coil, you could put much more current through it. This would give a much larger magnetic field, but you would need a larger energy source ie. capacitor. I think that the only benefit would be when building very high energy guns or very physically small guns.

If you have the resources, why not try it out? There is A LOT out there to learn about superconductors before trying it out, though. Cooling your coil with dry ice might help with copper but you need much lower temps and much more exotic materials before you can really take advantage of reduced resistance.

pauleddy

Sun Aug 01 2010, 11:42PM

Registered Member #2909 Joined: Wed Jun 09 2010, 12:31AM
Location: fort belvoir, Va USA ( south of DC)
Posts: 145

i have recently build a 6.5kj bank running at 900v, i did make a small coil gun with this bank it was 20-30 turns of 14 gauge (I think) wire, two layer around a aluminum tube about 1/4" in diameter. But it keeps shoring out and getting hot. I was thinkinking of ways to improve efficiency with this bank.

ramses

Mon Aug 02 2010, 12:34AM

Registered Member #1208 Joined: Thu Jan 03 2008, 05:30PM
Location: Chesterland, OH
Posts: 154

At higher energies, the main limiting factor is magnetic saturation of the projectile, so it would make sense to address that first.

The aluminum tube may have contributed to the heating and shorting out through eddy current losses, but the joule heating in the windings cannot be ignored. Active cooling even without superconducting would be beneficial, beyond keeping the insulation from burning.

BridgetRitter

[Edit: 400 pixels]

pauleddy

Mon Aug 02 2010, 01:21AM

Registered Member #2909 Joined: Wed Jun 09 2010, 12:31AM
Location: fort belvoir, Va USA ( south of DC)
Posts: 145

I guess with small projectiles saturation would be a problem. But if thatâ€™s chart is right, seeing that dry ice is about 200K thatâ€™s nearly a drop in half of resistance, nice. That may help with improve efficiency of coil guns. But as you said saturation would be an issue, that would mean a larger projectile (more mass, more energy needed) or better materials

HVgeek

Mon Aug 02 2010, 10:30AM

Registered Member #2998 Joined: Tue Jul 13 2010, 08:34PM
Location: Swedish forests.
Posts: 26

I think having a conducting tube might have contributed to that heating. And the only way to keep a coil from shorting at higher voltages is more insulation. Even if the resistivity of the coil is low, layer- to- layer voltage will be high due to the inductance of the coil. Add a couple of layers of tape in between coiling layers to solve the shorting out problem.

Cooling might just be worth it, seeing these charts... Maybe I'll salvage an old freezer to tweak out and ty this myself. If you have surplus dry ice, that would obviously be better though. Just my two cents.

klugesmith

Mon Aug 02 2010, 05:56PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

ramses wrote ...
... but the joule heating in the windings cannot be ignored. Active cooling even without superconducting would be beneficial, beyond keeping the insulation from burning.

I think Paul wants to get the most out of a specific capacitor bank energy, e.g. 6500 J. !!!
Chilling the coil, to reduce its resistance, will mean much less coil power per ampere.
But it won't reduce the coil heating per joule of bank energy, which is always almost 1:1.
Example: 6500 J discharge into 1/4 pound of copper: temperature rise is about 150 degrees C.
Reducing the initial temperature increases the tolerable temperature rise per shot.

As for getting a stronger magnetic field by reducing resistance to increase the peak current:
Not a bad idea, but don't expect too much.
In the absolute limit (superconducting coil), the magnetic field at it peak holds 100% of the original bank energy,
and the RLC circuit is undamped. That's bad for capacitor voltage reversal and for the magnetic pulse not dying out quickly.
That maximum field strength is e times the field strength in critically damped RLC.
If coil circuit is strongly overdamped at room temperature, then pre-chilling the coil is a win-win proposition.

pauleddy

Tue Aug 03 2010, 03:53AM

Registered Member #2909 Joined: Wed Jun 09 2010, 12:31AM
Location: fort belvoir, Va USA ( south of DC)
Posts: 145

Well I think my main problem with shorting is the age of the magnetic wire I am using, I found in a back of a desk from my physics class, my teacher didnâ€™t know it was there and he hade been working there for years. The insulation was flakey I did try to use masking tape to insolate the layers but after the first fire the tape got squeezed out some how. So I think I need to make a new coil and find a non conductive barrel.
Now would the inductance change with temp as well?
And what do you mean my damped?

HVgeek

Tue Aug 03 2010, 07:22AM

Registered Member #2998 Joined: Tue Jul 13 2010, 08:34PM
Location: Swedish forests.
Posts: 26

The inductance would stay the same, only you could push more current through the same-size coil wich means higher magnetic field density. And of course cooling would lower the threshold of wour insulation melting.

About that insulation: Get better magnet wire. If the insulation is flaking, you're screwed. Either buy it or find some discarded appliance that isn't decades old. I saw open MOT-cores for my magnet wire, not from being cheap, but because there simply isn't any magnet wire for sale in the swedish inland.

By dampened he means that the circuit has resistance to it, wich would kill off resonance between the capacitors and the inductance. If there is too much resistance your performance would suffer since it'd block maximum current (this is almost always the case). If it's too low, then you would have a L-C resonance circuit that would mess up your capacitors and produce a vibrating magnetic field that doesen't "turn off" fast enough and would snag your projectile.

GhostNull

Tue Aug 03 2010, 12:03PM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

I think there was some one that cooled their coil in liquid Nitrogen a while back... if I can find the thread...

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