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New Coilgun Project

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Saz43

Sun Dec 09 2012, 06:52PM

Registered Member #1525 Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294

Yandersen wrote ...

Nope. After cap drops to 0V, current keeps running in two diode-thyristor loops. Maybe, extra third thyristor for V-switch in parallel to the coil will close everything up?..

Remember than in an RLC circuit, voltage and current are 90 degrees out of phase. So when capacitor voltage is at zero, current is at it's peak value. SCRs shut off when zero current runs through the anode, which in an RLC circuit coincides with peak negative capacitor voltage.

This weekend I had a bit of extra time to breadboard the injector control circuit, so far it seems to work as designed. It has semi and full-auto modes, with an NE555 causing a delay that sets the maximum ROF for full auto (currently set to 10.5 RPS).

BigBad

Wed Dec 12 2012, 03:41AM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

Steve Conner wrote ...

I could be imagining it, but didn't we prove a long time ago that the optimal coil is the same length as the projectile?

Yes, but that's not the precise issue. The issue is the optimum diameter/length ratio of the coil, not the relative length (which IMO should be about the same).

It turns out that both small diameter/length ratios and large diameter/length ratios are very suboptimal.

In other words there's a happy medium.

As a related thing, I've been doing simulations of simple permanent magnets, and it turns out there's an optimum shape for magnets to give the best total pull; two cubes, or two cylinders of roughly equal diameter/width both give really good pull strength. Flat discs face-face or bar magnets end-end give much lower total pull. I think this is the same effect.

When you energise your coil, you're making a magnet (and a second one from the iron), and you need the shapes to be geometrically optimal to get the best pull from the energy you've put in.

wrote ...

Also, to prevent suckback, the current has to have decayed away by the time the projectile reaches the centre of the coil.

Ideally wayyyy before that. Any energy in the coil after any part of the projectile has entered already has parts of the coil pulling on the projectile the wrong way! You are already getting partial suckback.

Yandersen

Wed Dec 12 2012, 04:07AM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

"Ideally wayyyy before that. Any energy in the coil after any part of the projectile has entered already has parts of the coil pulling on the projectile the wrong way! You are already getting partial suckback."

Seeing the pull force in a different entering points for the coil with constant current (FEMM simulation) I would say that pull force is at the peak when projectile is inside the coil from 1/3 to 2/3 of the coil length. If only we can start and stop the current infinitely fast, the highest efficiency will be achieved when coil consumes energy while projectile passes only that little distance.

BigBad

Wed Dec 12 2012, 10:40AM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

Right, and you can pretty much do that by capacitively cancelling out the inductance of the coil to give just a resistive load to your driving circuit. I haven't analysed your circuit but that's pretty much what you're doing isn't it?

And it goes without saying really, that what you ideally want to do is keep the projectile in that optimum field the whole time as it accelerates by phasing a bunch of coils correctly.

Turkey9

Thu Dec 13 2012, 01:07AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

What do you mean by capacitively cancelling the inductance? There is no way to make an inductor look like a purely resistive load to a circuit. Even if you apply a direct current (say from an ideal voltage source) the current through the coil will ramp up in an exponential way.

EDIT*** Of course it's linear, my apologies! Forgot I was talking about an ideal case.

Yandersen

Thu Dec 13 2012, 01:12AM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

Linearly, actually.

BigBad

Thu Dec 13 2012, 06:48PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

Turkey9 wrote ...

What do you mean by capacitively cancelling the inductance? There is no way to make an inductor look like a purely resistive load to a circuit. Even if you apply a direct current (say from an ideal voltage source) the current through the coil will ramp up in an exponential way.

EDIT*** Of course it's linear, my apologies! Forgot I was talking about an ideal case.

You certainly can make a capacitively loaded inductor look like a resistor at resonance, by driving it at the critical frequency; the capacitance builds up the high voltages for you and bullies the inductor into submission.

Theoretically one design of coilgun could consists of a bunch of capacitively loaded coils in parallel, each with their own resonant frequency and you just feed it with ever increasing pitch.

Not sure if that particular idea is practical, but it's not inconceivable that it could be made to work.

edit: might work pretty well for an induction motor, you really need multiple cycles, not sure about more traditional coil guns

Turkey9

Fri Dec 14 2012, 09:01AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

Ah I see. The only problem is that it requires a sinusoidal steady state to achieve this. Too bad that starting and stopping a sinusoid will create harmonics that will through off the tuning. I'm talking about the practical case here.

BigBad

Sun Dec 16 2012, 05:07PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

A few cycles, depends on the Q factor- which may not be very high.

But the other point is that in these kinds of circuits you don't really want very high current; because past saturation, high currents give increased i^2 R, but only linear force. So you want to pump current into the coil quickly up to that limit, and then suck it back out again, so you're not going so far up the linear ramp and so it all happens much more quickly anyway.

Yandersen

Sun Dec 16 2012, 05:53PM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

Coilgun without saturation, huh? Meters of barrel for 30m/s? But it may be energy efficient, of course.

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