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Hi, I have recently had the idea for a paper-clip coilgun that I am adamant on building. As the paper-clip is oval at the cross-section, it seems to make sense to use an oval shaped coil. I have three questions that I would be very grateful if you were kind enough to consider. 1. Would the coil work (would the electrons still move in such a way that the projectile is attracted into the coil)? 2. Is it possible to predict the inductance with Barry's inductor sim (possibly by entering [circumference/3.14] as the diameter)? 3. Would there be any unexpected efficiency losses? Cheers, Jack Allison
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
I would expect; 1. Yes, the coil will still work 2. No, the inductance will depend upon the actual geometry i.e. a very 'flat' coil will have less inductance than a circular one so calculate as if it's a round coil (dia = circumference/pi) then expect maybe 1/2 that value. 3. The main 'efficiency' issue would be that the coil would be filled with more air than iron, but with typical coilgun efficiencies don't let that worry you.
Just build the coilgun then add/remove turns for best performance.
Registered Member #90
Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301
Yes, it will work fine. I believe you gain efficiency by building a coil that closely conforms to the projectile shape. This is true for all coilguns - by placing the windings closer to the iron, the coupling between them is improved. In general, empty air space should be designed out if possible, as it lowers the magnetic energy density inside the enclosed space.
To estimate inductance, you might try using Wheeler's Formula which is based on the "average coil radius". Come to think of it, my simulator is simply calculating Wheeler's Formula anyway, so dial in the numbers and mentally convert somehow from its round-average-coil-radius to your oval shape's average radius.
It's only an estimate anyway -- although the math in Wheeler's Formula is good for 1% accuracy, the building technique (exact size, packing factor, insulation thickness, etc) cause the actual inductance to vary by perhaps 10% or more.
Spend minimal time analyzing this in advance. Just build it and see, then adjust as needed.
Cheers, Barry If you keep sending me farmville requests, then I will sacrifice your goats, stampede your cattle and burn down your barn!
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Oval shaped coil?
