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9KJ coilgun construction

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LAN_403

rp181

Sun Jun 07 2009, 11:24PM

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

Very nice, neat.

Nice SCR clamp, make sure on the tension.

What are the coil supports made of (What guage wire do you plan?)? You should try iron powder matrix shells, It may help efficiency.

Hard to tell, how long is the barrel?
BTW, I have some power resistors (1kohm 300w) if you want, two in parallel make great discharge resistors for 400v, though light bulbs are more economical.

Good luck.

big5824

Mon Jun 08 2009, 09:14AM

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

Im planning on using 16awg for the first coils, 14awg for the later ones. The coil supports are just Acrylic cut on a laser cutter, same as the break beam sensors and the SCR clamp spacers. Im going to set them in place with epoxy once iv got this barrel slotted.

As for the iron powder shells, im considering it, but from looking other research a while ago (cant remember the source) it seems they make little difference at high powers, but I will try anyway. Im guessing mixing a lot of iron filings with some PVC glue or something then leaving it to dry in a mould would work best? Could even push transparencies into the mixture to create laminations.

And thanks for the offer, but the light bulbs do the job and are already integrated into the cap bank, so il stick with em for now :)

rp181

Mon Jun 08 2009, 06:10PM

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

I think a epoxy mixture will work better, PVC glue is hard to dry in large quantities, and is noxious.
You should go thicker wire, repeated shots (2-3) with 1.2kJ in a 12 AWG coil made it warm. I say 12, or maybe 10.

I am also wondering about the iron powder shells at high powers, hope that turns out good. What is your efeciany

big5824

Mon Jun 08 2009, 07:51PM

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

I need to use thinner wire to keep the pulse as long as possible, as the projectile will definitely be fully saturated, so having a short pulse and thus huge field would waste a lot more energy in heating the projectile. However like you said using the thin wire and thus a longer pulse will lose a lot of energy to heat (around 40% on the first stage :S), im just hoping this is a smaller loss than the effect of saturation using a short pulse. Luckily the ohmic losses reduce on later stages, but the losses to saturation will increase.

Once I get the gun finished and tested I will try to track down a high saturation alloy, and then swap the coils for thick wire and use much shorter pulses, but for now im sticking with steel.

oh and also, wouldnt it be a bit expensive using epoxy for that task?

rp181

Mon Jun 08 2009, 09:53PM

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

Epoxy is not all that expensive in the US, also keep in mind most of it will be metal powder.

You could use thick wire, and do a longer thicker coil. When the projectile saturates, energy is not wasted, it still accelerates the projectile, but yes, it is worse.

Would a long coil with a current limiting inductor work?

big5824

Mon Jun 08 2009, 10:06PM

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

Yea a longer coil would help, but sadly the barrel is only 50cm long, so my coils are going to be limited to around 6 or 7cm max.

Something else I just though though, would the iron filings saturate also? and would that then waste energy as heat? Or would the large cross sectional area reduce the flux density and thus stop it saturating? (I dont know too much about magnetics)

rp181

Mon Jun 08 2009, 11:08PM

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

Im not sure, but I think the iron shell "confines" the magnetic field, increasing the strength in the middle of the coil. I think the shell will saturate, but it will not matter. The shell basically "stores" energy, and lengthens the pulse a little. There will be a little waste in magnetizing the the iron, but that will reduce after the first shot.

Do you have a decent LCR meter and a fast current sensor (ideally a rogowski coil) available? You could plug in the numbers in barry's simulator, and see how the waveform is different with the rogowski coil, though you would have to still do it before the shell, as the projectile changes the inductance. You may be able to measure the inductance with the shell.
Best thing to do is just try it.

klugesmith

Tue Jun 09 2009, 12:13AM

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

Regarding "iron shell" ferromagnetic cores for coil guns.
Has anyone tried a long strip of thin sheet steel (e.g. shim stock or roof flashing) coiled to form a laminated cylinder? Would need insulation between layers to avoid the shorted-turn problem (circumferential "eddy current"). Still has the problem of eddy currents within the thickness of a lamination, but if 0.020" is thin enough for real transformers at mains frequency, it might be thin enough not to cancel the benefits of having a core. The sheet metal would saturate at a much higher flux density than a homebrew iron-powder-filled-resin concoction, IMHO.

Rich

rp181

Tue Jun 09 2009, 12:22AM

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

Eddy currents will always be present, but by disturbing the path, it becomes less and less. That means the smaller the particles, the less eddy currents there are.

big5824

Tue Jun 09 2009, 08:29PM

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

Just wound the first layer on the first coil, i forgot how much of a pain copper wire and epoxy can be....

Once this coil has had its second and third layer added, I might try hooking just one or two caps up to it and using it as a single stage gun to see how the barrel etc holds up, plus an excuse to shoot stuff :)

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