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Registered Member #2848
Joined: Tue May 04 2010, 05:19AM
Location:
Posts: 44
I have a coil gun using Capacitors charged to 360 volts. The total capacitance is 3430 uf.
I need help designing a coil to work with this gun. How long should it be and how thick? I was using a coil that was about 6in long and 10 turns deep (18 gauge speaker wire) and when I fired the gun, the suckback was so bad, it actually fired the projectile backwards.
I'm not trying to make the most efficient coil. I want something that works.
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
Your capacitors are too slow to discharge so you need to reduce the inductance of your coil. Have a play with Barry's RLC simulator to get a feel for discharge times and unwanted oscillations:
Then look here: and here:
Everything else is in the archives on the main menu.
When you have decided of a suitable inductance for the pulse length you want, you look up the formula for calculating the inductance of a coil. If you just want it to fire the correct way then make a coil with a lot less turns while keeping in mind that some coil configuration will put a lot of wear on your capacitors.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
Welcome to 4hv! Be sure to come back and tell us about your next design.
Don't overlook Barry's inductor simulator -- as you move sliders for coil dimensions and wire gauge, it not only estimates Inductance (applying Wheeler's formula), but also figures the total wire length, resistance, layer count, turns per layer, etc.
You can get quick answers about maximum current for a given inductance, based on energy in magnetic field . With zero resistance (undamped oscillation), 100% of energy from capacitor is transferred to inductor. 0.5 * L * I^2 = 0.5 * C * V^2, so Imax = Vmax * sqrt(C/L). In a coil with enough resistance for critical damping, about 14% of energy from capacitor is transferred to inductor. Imax = Vmax * sqrt(C/L) / e. Here's a derivation:
With a fixed damping ratio and coil size, lowering the inductance by a factor of four will * halve the pulse duration * double the peak current * not change the peak magnetic field
A convenient substitute for thicker wire can be several strands of the same old wire, wound as one. For example if you wound a coil with 100 turns of zip cord and externally wired the two conductors in series, it would be a 200 turn coil (or zero net turns, depending on connection polarity). Externally connected in parallel it would be a 100 turn coil with 1/4 of the resistance and 1/4 of the inductance of the series coil. You could easily switch between the two configurations.
Registered Member #2848
Joined: Tue May 04 2010, 05:19AM
Location:
Posts: 44
Thanks for the help so far. I've learned a lot after reading through those links. This might sound stupid, but on Barry's RLC simulator, what is the L? Like what does "L" stand for. I'm sorry lol. Is that the length of the coil? I've already started designing a better coil and hope to test it soon.
Registered Member #853
Joined: Thu Jun 21 2007, 03:08PM
Location:
Posts: 14
Or you could make one of different designs of speedtraps, then just make a huge coil with two connection points, see which connection that gives you the highest velocity, then add wire or strip wire. It is very manual, but you can't go wrong! (And you might need this method for a multistage later)
Registered Member #1062
Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529
To start with, try a coil around 1.5" long, and around 3-4 layers. Use a projectile that is a cylinder from 1-1.25" long. Diameter should stay under 1/4". From there, you can optimize.
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Coil Gun coil design.
