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Something similar to railgun and coligun.

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LAN_403

Ivan Kalafatic

Sun May 25 2008, 11:37AM

Registered Member #1502 Joined: Sun May 25 2008, 11:19AM
Location: Croatia, Pozega
Posts: 12

Hello folks.. newguy on forum.. My brother and I have created coilgun. Input voltage 15V and capacitor banks total 40 000 uF (various capacitors from TV-s, VCR and so on..). That is 4.5J of capacitor energy.. But we are unhappy with efficiency of that coilgun. Something like 0.05%. I have winded the coil like 6-7 times. Current coil is the best for our design. Projectile flyes maybe 0.5 meter. We've tryed many types of projectiles and efficiency is still VERY low.. So.. We're looking for device (gun) similar to railgun and coilgun but with better efficiency.. Thanks!

Barry

Sun May 25 2008, 12:26PM

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

For better efficiency at low power, have a look at an induction launcher, also known as a Thompson Jumping Ring. That is, a flat pancake coil with an aluminum disk on top. This geometry puts the most amount of wire close to the metal, and so gives the best coupling between coil and projectile. The pancake coil inherently works better than colinear coils.

Since you have large capacitance, you'll want to match it with small inductance. So be thinking of a coil with heavy wire and a fairly small number of turns. Minimize resistance as much as possible by keep connections short, using very heavy hookup wire, and solder the connections.

Let us know how the design comes along! Send pictures, lol.
Bestest regardses, Barry

Ivan Kalafatic

Sun May 25 2008, 01:04PM

Registered Member #1502 Joined: Sun May 25 2008, 11:19AM
Location: Croatia, Pozega
Posts: 12

Hello Barry. First of all, thanks for the reply. About the coil, will 0.13mH do the trick? (19AWG, inner diameter 15 mm, outer 31, lenght app. 11mm. That is about 88 turns. I will send pictures, after i wind the coil. Greets

Quantum Singularity

Mon May 26 2008, 02:22AM

Registered Member #158 Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282

Ivan Kalafatic wrote ...

Hello folks.. newguy on forum.. My brother and I have created coilgun. Input voltage 15V and capacitor banks total 40 000 uF (various capacitors from TV-s, VCR and so on..). That is 4.5J of capacitor energy.. But we are unhappy with efficiency of that coilgun. Something like 0.05%. I have winded the coil like 6-7 times. Current coil is the best for our design. Projectile flyes maybe 0.5 meter. We've tryed many types of projectiles and efficiency is still VERY low.. So.. We're looking for device (gun) similar to railgun and coilgun but with better efficiency.. Thanks!

I think your efficiency will suffer at those low voltages. Most railguns and coilguns here you will see are in the few hundred volt to kilovolt range. And starting out with only 4.5J of stored energy is incredibly small. You just need to up the power and voltage a lot IMO. Go find yourself some 450V lytics or similar, or something at least like a bunch of photoflash caps. And just BTW, there is no device "similar to a railgun and coilgun", they are two separate things utilizing completely different principles. And if there was something significantly more efficient than either, then I'm sure we would all be doing it. But both railguns and coilguns can get a lot higher efficiency than .05%, a well built one you can probably exceed 10% and a rare few much higher than that. And my last one which was thrown together with no specific parameters or design was almost 10%. And it was closely related to voltage... changing nothing other than charge voltage on one particular run showed efficiency doubling between 400V and 650V, wish I'd have been able to go to 1,000V and seen what would have happened, if the trend was correct I could have hit 15%. But overall, yes, amateur coilguns and railguns are pretty lousy in terms of efficiency.

Barry

Mon May 26 2008, 05:28PM

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

Ivan Kalafatic wrote ...

Hello Barry. First of all, thanks for the reply. About the coil, will 0.13mH do the trick? (19AWG, inner diameter 15 mm, outer 31, lenght app. 11mm. That is about 88 turns. I will send pictures, after i wind the coil. Greets

Hi again! Your new coil sounds a lot better. I checked it with the RLC Simulator using your values (15v - 40,000 uF - 130 uH) it estimates that you'll get a 7 msec pulse.

A 7 msec pulse seems about right for a small low-energy coilgun. But keep in mind, to get faster speeds you would necessarily need a shorter pulse. The pulse width puts a limit on the exit speed. So if anything, you could use 5-10% few turns on the coil, but it's hard to say in advance.

Think of it this way ... During those 7 msec, ideally your projectile will move just outside the coil into the center of the coil. For maximum efficiency you want to use the whole coil length like this for acceleration.

Don't worry about losing field strength with fewer turns -- the lower inductance also results in higher current which will make up for it.

Also for maximum efficiency you want the projectile length approximately equal to the coil length. This is what my measurements found when testing a coilgun of about the same energy and voltage as yours. It was not sensitive to the exact length, as long as it was close to the coil's length.

To make this happen, there is a balancing act between projectile length and mass. I recommend testing projectiles of different thickness but the same length -- about the same length as the coil itself. For example, you might cut different nails to the same length for your testing.

Let us know how it works out! Good luck!
Barry

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