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Wire gauge for coilgun?

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El_Roberto

Fri Oct 31 2008, 09:42AM

Registered Member #1774 Joined: Wed Oct 22 2008, 02:51AM
Location:
Posts: 135

As I have said in another post I am Beggining to build a 20 stage Coilgun and I am just looking up prices for copper wire and I want to know what size you guys would reccomend. I would like to use 25swg as I could get two coils out of a roll. Realistically the biggest I can go is 19swg because anything bigger comes in too short rolls and I can't get a deffern't supplier. Each coil will have about 300amps at 225v for 5 micro seconds. (using 225v 2200uf capcitors).

Barry

Fri Oct 31 2008, 06:33PM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

Get the thickest wire you can, and you can always use parallel strands to make it behave as if it were thicker wire.

With a 20-stage coilgun, the ending stages will require lower inductance coils to get the shorter discharge pulse. This generally means fewer turns of thicker wire -- but parallel strands of thinner wire will behave similarly.

Indeed, the losses due to skin effect are decreased by using parallel strands of thin wire. This will become more important at higher frequencies such as 5 micro second pulses.

Cheers, Barry
It's been so long since I've had sex that I've forgotten who gets tied up.

El_Roberto

Fri Oct 31 2008, 08:47PM

Registered Member #1774 Joined: Wed Oct 22 2008, 02:51AM
Location:
Posts: 135

Ok thanks Barry, Ill get the 19AWG wire this means itll cost more but I wont have to worry about frying my coils. I know 19awg can definately hadle it as my current coilgun setup uses it and using your calculator it has aabout 1200amps and 180v going throught it for about 5 microseconds

Killa-X

Sun Nov 02 2008, 01:42AM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
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Posts: 1039

My question is, This guy got a 400V 470uf single capacitor to fire a bolt through a coffee can using a 22 AWG, 30mm wide, 12 layers, and 8.9mm inner core. So I was going to go buy some wire. But heres the deal.

22AWG at 250ft
20AWG at 150ft
18AWG at 100ft
16AWG at 60ft
14AWG at 40ft

Whats the best guage to get? Of corse I cant get the 16 or 14, because 60ft isn't a lot. I know the 22AWG works good, but I wanted to know what about the others there? Or would they require so many layers that the amount of feet selling with it, wouldn't work good?

Mattski

Mon Nov 03 2008, 12:04AM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

Because of the nature of the tradeoffs and variety of potential designs, there is no single correct answer for what size to buy. To select the correct size requires a set of design requirements and then an analysis based of trading off the remaining design factors. The basic idea though is that for a given coil size, with thinner wire you will have more turns (thus more inductance/magnetic force) but higher resistance from the longer length and smaller wire, which could limit your peak current. Barry's coilgun site has a RLC simulator for viewing the response of a coilgun system which you can here. I think if you come up with several designs on paper by changing various parameters you will understand better how to choose the wire size.

I only built a single coilgun a long time ago, so maybe others will have different recommendations, but I started out with 22AWG, but got better results with my final coil built from 16AWG, because of the higher current. So as a general recommendation, getting something like the 18AWG on your list would probably be a good compromise: you get a fair amount of length but it's still reasonably thick. Depending on how many coils of what size you want to make though, the 60ft of 16AWG might last longer than you think.

Edit: I reread your first post, and it seems like you've already got some of the design set (number of stages and capacitor size), so from that you just need to crunch numbers to see if you can get a good coilgun out of a particular wire size. The coils will need to be differently sized so that the pulse width at each coil matches the velocity of the projectile as it passes through each coil, which will be especially important for so many stages. You will also want to take into account what you will use to switch each coil to make sure you can handle the peak current of a particular setup.

Barry

Mon Nov 03 2008, 12:04AM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

Most places are basically selling magnet wire by the pound. Or rather, all wire comes on spools that are the same size which can be lined up neatly on a shelf. So they all weigh about the same when the spool is full.

This is okay ... it's usually the same way that you use it. You will generally wind the same physical outer dimensions for a given coil on a coilgun no matter the AWG size. Why? Because inductance is proportional to both turns-squared and o.d.-squared, and therefore larger coils quickly have too much inductance.

What are your best coil dimensions? Jeez that's a tough one! All the factors depend on each other ... coil size, wire size, coil resistance and inductance, discharge time, capacitance, voltage, projectile size and mass.

If you've already built the coilgun, run a voltage sweep test and draw a graph to find the best performance. Then remove about 10% of the turns and run another voltage sweep for comparison. If it's better, remove another 10% of the turns. If it's worse, wind a new coil using the next size thicker wire and repeat.

Very tedious, yes. Dunno how else to tease apart the factors.

In my experience the end result will be fewer turns of heavier wire, with 2 or 3 layers or 4 at the most. The math I've done found that coilgun performance is sensitive to resistance, and even a few milliohms can make a difference.

Barry
Today is a day for firm decisions! Or is it?

rp181

Mon Nov 03 2008, 01:27AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

50 ft goes along way. 50 ft. of 12 AWG got me 3 coils for a multistage coilgun, and a coil for a single stage. There was two layers, and 1.5in long and 3 in long respectivle. The coilform was .5in PVC.

Killa-X

Mon Nov 03 2008, 01:45AM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039

Ah, So you took a 0.5 inch PVC pipe, and used 12 guage wire to do coils only 2 layers at 1.5 and 3 inches.. mmmm... I dont know.. Do you guys think i should go with the instructions, 22awg, or go my own, ways, and go 14 AWG at 40feet and just mess around with the size until it works greate? The 14AWG is like 2 dollars cheaper than the 250ft of 22awg. I know the tutorial I was going to follow, used 22 awg, same capacitor, and it can fire into a coffee can. I want the same results, and Maybe thick wire would be better..?

big5824

Mon Nov 03 2008, 06:36PM

Registered Member #1687 Joined: Tue Sept 09 2008, 08:47PM
Location: UK, Darlington
Posts: 240

can you link the tut? id like to take a look

guitardude012

Tue Nov 04 2008, 09:22PM

Registered Member #968 Joined: Fri Aug 24 2007, 04:54PM
Location: Norman, Oklahoma
Posts: 104

If we are thinking of the same tutorial, he wasn't using anywhere near the amount of Joules you will be switching. You will most likely want thicker wire. As Barry said, "Get the thickest wire you can" (within reason.... we are assuming you won't be wrapping a copper bar around the barrel and insulating it lol) The most wire I have used so far for a 14 awg coil was 13m total(4 layers).

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