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Registered Member #1563
Joined: Wed Jun 25 2008, 03:55AM
Location: Wimer Oregon, Wewt for sticks!
Posts: 30
Well, been working on my coilgun some more and finally got it into decent working order. There's an issue however... efficiency seems to be an issue in the way opposite to that of which would be expected. I was working though my projectile/voltage/distance from center of coil combination and things seemed to be working out well. 1.5cm 3.33g projectile getting .01% efficiency at 250v (coil is 8cm long by the way), same projectile reaching is efficiency apex at .16% efficiency. 4cm, 8.79g rod at 200v getting .6% eff.... oh not bad, and once I find its sweet spot... 2.6%! repeatable? yes! .... wtf? I keep testing on, who knows, maybe its a good combo, as voltage went up, eff went down. Slap in a monstrous 8cm 17.4g rod (maches coil length) and guess at a good position... WHAP! 5.6%... crap, something has to be wrong with my optical sensors. Redesigned and retested with other shot specs... turns out the same... Is it my math? Here's a rundown: 1100uf caps charged to 200v: 1/2(.0011F)*200v^2 = 22J velocity (time to travel between two sensors 5cm apart): (5cm*.01)/.0042s = 11.9 m/s K energy: 1/2(.0174kg)*11.9m/s^2 = 1.233j Efficiency: (1.233J/22J)*100 = 5.6% ------------------------------------------
Now, is my math wrong, or should I know something about optical sensors and using my laptops sound card to measure velocity. I know phototransistors have off and on times and that the soundcard is indirectly coupled to the incoming signal. So... even though I'm using a HUGE coil, thats completely untuned (200ft or so of 14AWG wire) are these numbers right?
You are using a very large projectile for your capacitor size, so yes 5% efficiency is very probable. Using even larger projectiles its possible to push the efficiency up to the 20% area, but then the velocity starts to drop below usable limits.
Still, 5% is good for an amature gun, well done :)
Registered Member #2064
Joined: Sat Apr 04 2009, 09:26PM
Location: NJ, USA
Posts: 16
how can a 5% eff. coilgun shoot faster than the same hardware at 20%? julage for a projectile is .5(9kg)(m/s)^2, so a projectile moving faster delivers more energy. In order to move faster, it must have absorbed more energy from the coil and caps...and thus be more efficient. help?
Registered Member #14
Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383
Actually 5% can be possible, I recoded a 10% efficiency for latter stages using halfbridge topologies, and the first stage was 6,4% (used a small sacrifical pre-stage to impart an initial velocity.
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My math must be wrong, 5% efficiency?
