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Help with induction CG design

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TodX

Thu Dec 11 2008, 10:28AM

Registered Member #1118 Joined: Wed Nov 14 2007, 09:42AM
Location:
Posts: 5

Hi guys, I've been planning on making an induction CG for a while, and it looks like I'll have a few weeks to work on it soon, but the design I'm thinking of has one major down fall: it would use the projectile as a moving switch to progressively "turn-on" the next stage of coils.

The setup I'm thinking of looks kind of like a mix between a conventional cg and a railgun, though it's still functioning on induction cg principles (the current running through projectile from contacts doesn't do any work). Unlike a railgun though, I can't use solid rails. One side of the contact can be a solid rail, since it will always be connected to GND, or neg cap bank bus. The other side however, has to have multiple contacts that will be connect coil sections as the projectile travels further down the barrel.

What do you guys think? Is it possible to come up with something that won't just weld the projectile in place? Or should I just give this design up and try for a solid state switching system for enabling coil sections (which is expensive...)?
(P.S. I'm planning to use about 5000uF @ 450V of caps to power this thing, and very thick coil wiring, so we're talking around 1kA for current through the projectile)

Background on design: After playing with FEMM and a spice tool for a while, I found two important details that greatly influenced this design:
1. The best place to put the projectile is right at the end of the coil for maximum force.
2. The current generated in the projectile actually takes some time to decrease to 0 due to the low resistance in a small, solid copper peice(takes about 1.5ms for current to go from 45kA to 5kA, atleast in the setup I was simulating.)

The reason for #1 is the the way the magnetic field lines curve right as they exit the coil. The axial component of the magnetic field generates the current in the projectile as the field ramps up, but the radial component of the field passing through the projectile is what exerts the forward accelerating force. So the goal of this design is such that once the current in the projectile is ramped up (400us) the fact that the coil continually "ends" right behind the projectile as it moves should maintain a fairly strong magnetic field with a good radial component to it. Hopefully, putting all that current in the projectile to good use =).

Let me know what you guys think.

Fraggle

Fri Dec 12 2008, 12:11AM

Registered Member #1526 Joined: Mon Jun 09 2008, 12:56AM
Location: UK
Posts: 216

I saw one on the internet that used a low current supply switched by the projectile to trigger separate spark gaps.

TodX

Fri Dec 12 2008, 01:16AM

Registered Member #1118 Joined: Wed Nov 14 2007, 09:42AM
Location:
Posts: 5

Fraggle wrote ...

I saw one on the internet that used a low current supply switched by the projectile to trigger separate spark gaps.

Do you have a link to that by any chance?

Electroholic

Fri Dec 12 2008, 01:23AM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

this?

El_Roberto

Fri Dec 12 2008, 01:48AM

Registered Member #1774 Joined: Wed Oct 22 2008, 02:51AM
Location:
Posts: 135

^^ Wow thats impressive, how did he get it to go 225m/s??? He's only using 8x 120j capacitors! Is this just a multistage coilgun with a different triggering method or what? (refering to "induction gun 2")

Camel

Fri Dec 12 2008, 06:57AM

Registered Member #1694 Joined: Sat Sept 13 2008, 09:13AM
Location: Australia
Posts: 108

Induction guns use aluminium or copper projectiles. They work by creating a large electromagnetic pulse, which generates eddy currents in the projectile. The eddy currents create their own magnetic fields which oppose the field of the coil. Induction guns can be a lot more efficient than coil guns because the projectile is close to the coil when the gun is fired.

El_Roberto

Fri Dec 12 2008, 10:44AM

Registered Member #1774 Joined: Wed Oct 22 2008, 02:51AM
Location:
Posts: 135

SO if I put an aluminium projectile in the centre of the coil in my coilgun and fired it, it would work? If so would it work better than using my steel projectile?

WaveRider

Fri Dec 12 2008, 02:07PM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

SO if I put an aluminium projectile in the centre of the coil in my coilgun and fired it, it would work? If so would it work better than using my steel projectile?

It is not likely to work to well. Induction guns require very fast rise times on the current to function well. In a normal reluctance coilgun, you may see the projectile move a little, but there will be no sound-barrier breaking!

Induction guns have the potential to be more efficient than reluctance guns because they are not limited by saturation of a magnetic material (like iron). They are limited by the achievable rise times (meaning very high voltages compared to reluctance guns) and the conductivity of the projectile (do the induced currents make it melt..!!!).

The required coil current rise times are a function of the conductivity and thickness of the projectile metal. Generally, if the field penetration is less than 20-50% or so by the time the projectile has exited the coil, good efficiency can be had. This would equate to:

0.5 * radius * radius/ (sigma * mu_0) =approx t

"radius" = projectile radius
"sigma" = projectile conductivity (in S/m)
"mu_0" = vacuum permeability (4*pi*10^-7 H/m)
"t" = expected current rise time needed for good efficiency.

In many instances, the current rise times will be on the order of 10-250us for induction guns compared to reluctance ones.
Cheers!

Fraggle

Sat Dec 13 2008, 01:47AM

Registered Member #1526 Joined: Mon Jun 09 2008, 12:56AM
Location: UK
Posts: 216

Yep, that`s the one.

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