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Plastic Insulated Wire Usable For Coil In A Coil Gun?

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evilgecko

Thu Apr 13 2006, 06:07AM

Registered Member #288 Joined: Mon Mar 06 2006, 07:52AM
Location: Palmerston North
Posts: 32

I building a relativly large coil gun (3.5kJ stored energy) with currents travelling through the coil at (3000 to 6000A). The largest enamled wire I can source is 18SWG (1.0mm). I need about 10SWG to handle a 3000A current. I was wondering if I could use plastic insulated copper wire instead of the traditional without sacrificing to much efficiency. I know the primary reason why enamled wire is used is because you can fit more turns in the same space. But if i found thin insulated wire it wouldn't effect the coil diameter and length that much would it?

Bjørn

Thu Apr 13 2006, 06:18AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

If the insulation is very thin compared to the diameter of the wire it will not make a large difference. You can also use several strands of thinner wire in parallel.

evilgecko

Thu Apr 13 2006, 08:29AM

Registered Member #288 Joined: Mon Mar 06 2006, 07:52AM
Location: Palmerston North
Posts: 32

So if I ran two strands of 18SWG side by side on the coil it would be equal to the diameter doubled, no thinking about it wouldn't it be the cross section doubled? But since they are lineraly related it doesn't matter what you use.

So using 2 strands of 18AWG (not SWG) would equal about one strand of 12AWG.

Is there any difference between the resistances? I calculated it and it seemed that the resistance of two strands of 18AWG is about twice that of a 12AWG strand of equal length!

Bjørn

Thu Apr 13 2006, 08:57AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

If you double the number of wires you will double the area and halve the resistance for DC current. Only the total area is interesting.

Because of the skin effect (http://wiki.4hv.org/index.php/Skin_effect) using several insulated strands can in some cases be more efficient than one thick wire.

Simon

Fri Apr 14 2006, 06:27AM

Registered Member #32 Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549

evilgecko wrote ...

So if I ran two strands of 18SWG side by side on the coil it would be equal to the diameter doubled, no thinking about it wouldn't it be the cross section doubled? But since they are lineraly related it doesn't matter what you use.

It does matter but you got it right with the area. Remember, area is proportional to the square on diameter.

Using insulated wire will have a significant impact on current density. If you think about it, for just a typical bit of insulated wire the density is probably something like half. If you're an amateur, however, sometimes you just have to work with what you have.

evilgecko

Fri Apr 14 2006, 12:38PM

Registered Member #288 Joined: Mon Mar 06 2006, 07:52AM
Location: Palmerston North
Posts: 32

Hmmm ok plastic insulated wire isn't seeming such a good coice then. I'm looking to make a powerful gun with 3% efficiency (I'm hoping for a 15g bullet to exit at 140ms-1). Is it posible to insulate your own wire? Maybe a nitrocellulose kind of laquer which is flexible.

rupidust

Fri Apr 14 2006, 02:31PM

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Registered Member #110 Joined: Fri Feb 10 2006, 12:23AM
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evilgecko wrote ...

Hmmm ok plastic insulated wire isn't seeming such a good coice then. I'm looking to make a powerful gun with 3% efficiency (I'm hoping for a 15g bullet to exit at 140ms-1). Is it posible to insulate your own wire? Maybe a nitrocellulose kind of laquer which is flexible.

Dont dispmiss without actually trying. 3% is small and I see no reason as to why plastic insulated wire could not get 3%. Also see no reason not to wind the coil, test, measure, then wind enamel coil whenever such wire becomes available. You migh end up winding 4+ coils regardless of insulation. Now as for this power level you want, 15 grams at 140 meters/second = 147 Joules of kinetic energy. This will require 4,410 Joules of potential energy for 3% conversion. If this will be in a single stage, then you will need a fast pulse and very high voltage. Single stage or multiple stage, you are going to need skills above and beyond posting about which wire insulation to use to get a 15 gram slug to that energy level.

evilgecko

Fri Apr 14 2006, 11:13PM

Registered Member #288 Joined: Mon Mar 06 2006, 07:52AM
Location: Palmerston North
Posts: 32

Sorry that speed was calulated for a 10g projectile. But I deicided to make both the coil and projectile 30mm long and the projectile turned out to weigh 15g.

Total stored energy = 3645J
3% efficiency = 109.35J and a speed of 121ms-1

The problem I'm facing is that yes, I need short pulses but no the maximum voltage my SCR's can handle is 600V. I have got only 2 SCR's which can handle a pulse of 3300A. If I make it two stage the projectile will be accelerated to 85.4ms-1 in the first coil (assuming each coils efficiency is 3% and cap bank is halved).

If I make the coil 30mm long then it will take 0.35ms (assuming constant acceleration t=2d/v) to travel from outside the coil to the center. This should be the maximum pulse width!

Using the MGC calculator the pulse width is 1.83ms and peak at 1.07ms. It is quite over dampened.

How can I prevent this. One way would be to use a bigger projectile, but that would decrease velocity. Is 120ms-1 unrealistic? I guess a larger projectile will have some momentum. I aiming for the gun to give the most punching power (ie can piercing).

Or decrease the capacitance but that seems self sacrificial.

Any ideas?

The Zander

Sat Apr 15 2006, 01:26AM

Registered Member #295 Joined: Tue Mar 07 2006, 06:59PM
Location:
Posts: 23

Just out of curiosity what are your capacitor banks apecs? 6000A does seem a little optimistic, and even if it's not 18AWG wire should be able to handle that for the length of a pulse (probably < 1mS with a projectile that light). we can use high school chem to prove this:

lets assume your coil has an inner diameter of 1/4" is 1" long and is comprised of 5 layers of 18AWG wire (these numbers are completely arbitrary, with no real bearing on your project, once we have specs for your coil we can derive more conclusive results).

the specific heat capacity of pure copper is ~380J/KG-K, using barrys inductor simulator (thanx barry, you apps are awsome) you see that you have 20g of copper in your coil. lets calculate the power dissipated using I^2R, where I will be 4000A (to account for the fact that the current ramps up then down, and i really don't feeling like doing any integration), and R (again from barry) is 0.062A.

[(380)(0.02 Kg) / ((4000^2 A)(0.062 ohms)(0.001 seconds))]^-1 = Delta T (degrees)

After everything is said and done, after one shot using 18AWG wire and the coil described above you would see a temperature increase of ~130 degrees C, and this also assumes the coil is in a vaccum (aka: no heat is dissipated to the environment). i think if you space your shots far enough apar your coil will easily survive.

Just my $0.02CDN

-TheZander

[edit]
in regards to achieving the most punching power, i would tend towards a heavier projectile (1/2" diamater maybe and inch long), from what we seen, because of the inherently long pulse times big caps like your yeild heavier = more efficient

evilgecko

Sat Apr 15 2006, 01:54AM

Registered Member #288 Joined: Mon Mar 06 2006, 07:52AM
Location: Palmerston North
Posts: 32

I've always wondered what 20g of vaporised copper would look and sound like. 130 degrees C! Almost seems impossible for that amount of copper tor reach that in 1 millisecond. My cap bank consists of 20 450V 1800uF caps which I picked up for $4 each. I could only reach 6000A if i paralleled the two SCR's and I'm tending towards the multi coil technique. My projectile is 10mm in diameter (nearly 1/2 inch like you said) and 30mm long from point to base.

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