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Coilgun: long coils vs short ones.

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LAN_403

Oatmeal

Sat Apr 28 2012, 05:54AM

Registered Member #4540 Joined: Sun Apr 22 2012, 11:26AM
Location: Wollongong, Australia.
Posts: 2

Hi 4HV!

I've been lurking here for a few months and taken a crack at a few projects you guys have posted about. Recently I've started working on a coilgun and had some okay results and a lot of fun. Barry's coilgun site provided a wealth of info to get started - great stuff!

One question I've not been able to find too much info about: for a given length of wire, what advantages does a long coil with few layers have over a shorter coil with more layers? If I'm not mistaken, the resistance of the wire will stay constant, though the inductance will be larger for a shorter, thicker coil. I figure the length of the projectile (armature?) has some bearing on determining a good coil length as well as the time required to discharge the cap bank, but I was wondering if there were any other tradeoffs?

I've been reading through my physics textbook and I'm finding electromag to be pretty full on.

Just by the way, my current setup is:

Coil, 1.25mm magnet wire (~16.5 AWG?), 2 layers, 100mm long, 12mm int diameter, approx 50uH, 130mohm)
Cap bank rated for 80V, 10mF - When I make a proper charging circuit, I'll get higher V rated caps - just direct charging from my power supply at this point (~64V).
Projectile 10mm diameter, 70mm long, steel rod (standard hardware store stock).
Using a IRFP260N MOSFET as a trigger, microcontroller + gate driver for dictating pulse width.

Schematic of my setup: http://img836.imageshack.us/img836/3184/coilgunmkischematic.png
Actual Setup: http://img801.imageshack.us/img801/8827/20120428001.jpg

Pinkamena

Sat Apr 28 2012, 10:37AM

Registered Member #4237 Joined: Tue Nov 29 2011, 02:49PM
Location:
Posts: 117

There is no need for a coil longer than your projectile, as it won't be experiencing any forces when it's fully inside your coil. I've found through simulations that it's best to have a coil that's exactly as long as your projectile.

Oatmeal

Sun Apr 29 2012, 08:06AM

Registered Member #4540 Joined: Sun Apr 22 2012, 11:26AM
Location: Wollongong, Australia.
Posts: 2

Fair enough, cheers Pinkamena.

What about projectile length in general? For low power coilguns, I'm assuming a shorter, smaller diameter projectile in a tighter coil would be better.

I guess my next question is: would having a coil with 100 turns over two layers and a projectile as long as that coil fare better than a coil also with 100 turns, but only with a single layer and a projectile twice as long as the previous one?

Pinkamena

Mon May 21 2012, 07:10PM

Registered Member #4237 Joined: Tue Nov 29 2011, 02:49PM
Location:
Posts: 117

I am not sure about that. I assume you'd get better efficiency using the short coil. Since there's the same amount of turns (and thus also the same amount of current), you'll get the same force in both the coils. It's hard to say exactly though, as the geometry of the coil will change many of its properties.

Barry

Wed Jun 13 2012, 08:29PM

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

Pinkamena wrote ...

There is no need for a coil longer than your projectile, as it won't be experiencing any forces when it's fully inside your coil. I've found through simulations that it's best to have a coil that's exactly as long as your projectile.

Ditto. This is also my experience from running experiments. My thinking is:

A coil longer than the projectile will have a portion of wire that dissipates energy (resistance) without transferring energy to the armature.

A coil shorter than the projectile will leave you with extra projectile mass that doesn't absorb energy.

But the effect is pretty broad. As long as the coil length is within +/- 20% of the projectile length, you won't notice the difference.

Cheers, Barry
PS - Glad to hear you liked my website.

Josh Campbell

Wed Jun 13 2012, 11:19PM

Registered Member #5258 Joined: Sun Jun 10 2012, 10:15PM
Location: Missouri - USA
Posts: 119

Another vote for shot coils.
In multi-stage setups it beneficial to have very short coils so you can precisely place them and know exactly where the strongest magnetic field will develop. Rather than having long coils with say, stronger magnetic fields at the ends than in the middle and skewing your guess as to where it should be placed after the sensor.

Yandersen

Thu Oct 04 2012, 04:07AM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

Topic is dead, but I still must contribute. Vote for the short coils with both hands, efficiency wise. But design starts from the projectile. Consider this: the shorter projectile lets bigger efficiency (my practical exp + thinking gives the same answer) than longer one, but the last gives better penetrating results as a bullet. Trade off. I choose efficiency. The highest I have achieved with projectile of just 1.5 times longer in length than it's diameter. And coil must be really thick in shape: the number of layers must ideally be the same as number of turns in each layer. Well, a little less layers may be little better, howether. That means a finger-thick wire too, unless you dare enough to get Really High Voltage and small capacitance..

The long coil is inefficient because all that it's central part does is dessipates energy. Wire a single-layer coil on a pipe and let DC current to flow through it. Then probe an iron stick into, checking the pulling force. After that rewire all that wire on one spot of a pipe creating a wire bubble. Check the difference. Why so? Because projectile pulled not simply by the strong mag field, but by it's gradient over the projectile. In fact, iron is pulled closer to the wire, not to the geometry center of a coil. If coil is in front of projectile, projectile is pulled forward. But being inside the coil projectile is pulled to the walls of a barrel - not a desired result, agree? In other words, only the edges of a coil contribute - so why to waste wire on a central part?

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