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optimal coil length

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big5824

Tue Feb 03 2009, 08:09PM

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

Im busy planning my 10kj coilgun, but im a little stuck on how long my coils should be (projectile is already made at 25mm 6.8mm diameter). Iv heard from many sources that the optimal coil length is 4/3 of the projectile, but where has this fact come from? Does anybody know of anyone who has explored this and found evidence supporting or disproving this?

Marauder709

Wed Feb 04 2009, 04:42AM

Registered Member #1895 Joined: Thu Jan 01 2009, 03:12AM
Location:
Posts: 22

I think it is more prudent to find the optimal coil size for a given voltage source and barrel diameter and then fabricate the projectile. I think you will agree that the largest dissipation of energy will be from electrical inefficiencies (resistance, inductive reactance, etc.). You should therefore make your coil as electrically efficient as possible before you think about magnetic efficiency (eddy currents, reluctance, etc.). It is kind of an involved process, but it may pay large dividends given the size of you capacitor bank.

However, if you have your heart set on using your projectile, I think a coil as long or slightly longer than the projectile is fine.

Barry

Wed Feb 04 2009, 02:22PM

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

The recommendation may have come from me. I ran both physical experiments and FEM simulations to reach the conclusion.

For my small coilgun, I found a broad peak in kinetic energy when the projectile length is within -25% to +25% of the coil length (40mm). There seems to be a constant energy transfer for projectiles nearly the same length as the coil. Or at least the peak is within 10% of being flat. Experimental details are here

Hope this helps! Barry
PS - 10kj will be exciting!

KLH

Wed Feb 04 2009, 03:13PM

Registered Member #1819 Joined: Thu Nov 20 2008, 04:05PM
Location:
Posts: 137

Marauder709 wrote ...

I think it is more prudent to find the optimal coil size for a given voltage source and barrel diameter and then fabricate the projectile. I think you will agree that the largest dissipation of energy will be from electrical inefficiencies (resistance, inductive reactance, etc.). You should therefore make your coil as electrically efficient as possible before you think about magnetic efficiency (eddy currents, reluctance, etc.). It is kind of an involved process, but it may pay large dividends given the size of you capacitor bank.

However, if you have your heart set on using your projectile, I think a coil as long or slightly longer than the projectile is fine.

I agree with going for low resistance; resistances as low as 0.01 ohm can cause serious problems in power dissipation (10AWG in 100uH 0.04 ohm coil with 1903.5 J capacitor bank = <50% efficiency!!!). You might have to use very thick wire, or even really thick litz wire (if your required peak current is very high).

But doesn't the inductance only affect peak current, and not power dissipation? This isn't an AC circuit, if you discount the ringing caused by residual energy from the inductor.

Berni

Wed Feb 04 2009, 04:38PM

Registered Member #1132 Joined: Mon Nov 19 2007, 06:15PM
Location: Slovenia
Posts: 84

As sead you are going to need really fat wire for 10kJ. It might be a better idea to spread that energy across 3 or 4 stages, that will mean much smaller wire is needed and the efficiency will be a lot higher so that means the projectile will go much faster.

big5824

Wed Feb 04 2009, 07:31PM

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

yup i was planning on spreading it over 5 stages using one bank to power all 5, but now iv done some more number crunching it seems like the coils will have to have stupidly low inductance and resistance to dump 2kj into each stage before the projectile has passed (turning on when the front of the projectile is about 10mm from coil)...pulse times would be around 0.7ms for the first stage and 0.3 for the second, so i think im either going to switch to a larger projectile and just aim for high efficiency, or only use 500-1kj per stage and stick with my current high velocity goal.

oh and also, i wasnt intending for the gun to be 10kj, i thought i was only bidding on 2kj of caps :P i get the feeling im in way over my head now :)

Electroholic

Thu Feb 05 2009, 05:26AM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

you pretty much said it yourself, stupidly low inductance coils and fast pulses. First few stages are always easy, but when the projectile is already moving so fast, Di/Dt required would be quite hard to generate. low ESL ESR caps, the Switch...
Good luck.

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