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Registered Member #2767
Joined: Sun Mar 28 2010, 10:31PM
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So update on the coils...Klugesmith PMd me (which was really nice since I'm rushing now) and suggested I try a bifilar coil...the reason makes a lot of sense to me...so I've given it a quick CAD to confirm if this is indeed what was suggested and that it will be valid.
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Joined: Wed Apr 29 2009, 12:22AM
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Flochi wrote ... Alright! So I spoke with the higher up, who has allowed me to go so far as 150V, although he would rather I stay less than 100V.
That's great. You will see that series connections allow shorter pulses with the same energy, same peak current, and same coil.
Flochi wrote ... Regarding the coiling, if you're suggesting having more than one single layer coil, one inside the other ...
Nothing that complicated. Looks like you got it. (Great CAD pictures!) It only makes sense if you want fewer turns than a simple winding that fills the volume.
Flochi wrote ... Could you advise me on the inductance and current expected for a 2 series 15 parallel config with two layers of AWG12 at 61mm long (? length still 61 or longer/shorter?)
Are you having any trouble using Barry's inductor simulator and RLC simulator? Do you know how to compute the capacitance and stored energy of a series-parallel bank? Do you know how to compute the stored energy in a coil, given L and I?
Then here's a two step design process where you don't need to do any more math. First: use interactive RLC simulator to -quickly- explore the design space with 3500 ampere limit and no attention to physical coil design. You will get an idea of target L and R values, and how close is good enough. Second: use the inductor simulator to find how to wind your 61mm bobbins to get those L and R values -- you may be pleasantly surprised.
Your tour guide suggests that you compare three different 30-capacitor bank configurations: 30P at 50V, 2S15P at 100V, and 3S10P at 150V. (S,P nomenclature borrowed from model airplane battery-pack jargon). In the RLC simulator, set resistance to zero (undamped oscillation) and see what inductance value will limit the current to 3500 A. Check: should be around 110 uH for any of the three C,V combinations (it's the L value that holds 100% of capacitor bank energy at 3500 A). But you see the frequencies are different in 1:2:3 ratio.
Of course resistance is real, in more ways than one. You want enough to make the current die out before your projectile gets too far past middle of coil. And you want enough to prevent the capacitor voltage from reversing. The simulator has a check box to show voltage on the graph. People with actual coilgun experience may have advice about how much overshoot is OK. Dial in some R, then back off on L to bring the peak current back to 3500 A. Find the R,L point where the current decays from 3500 A and barely does not overshoot. This is "critically damped" and gives the fastest stoppage.
With the dual targets of 3500 A and critical damping, all three capacitor configs will want the same inductance. It's the value which, at 3500 A, holds a secret fraction of initial capacitor energy. But as you progress from 1S to 2S to 3S banks, the critical resistance goes up and the pulse width goes down (by ratio 1 : 1/2 : 1/3).
At this point, make some choices about C, V, L, and R. Now go to the inductor simulator and work out the winding details. Please tell us about it. Good luck!
I have to tune out for a couple days. Hope there are other readers who can step in for next round.
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3 Stage Coilgun...advice on current and design?
But as you progress from 1S to 2S to 3S banks, the critical resistance goes up and the pulse width goes down (by ratio 1 : 1/2 : 1/3).