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My coilgun, I need a lot of theory

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LAN_403

Arowana

Sun Jun 10 2012, 08:41PM

Registered Member #5059 Joined: Tue May 29 2012, 06:28AM
Location:
Posts: 4

I made a coilgun, I use a simple design because I need it in order to do some experiment for my studies.
I use a transformer, a diode bridge, a resistor, a capacitor bank and a coil.

http://imageshack.us/photo/my-images/854/20120105192609.jpg It's a photo without the wire, because now it's a mess

I didn't find what I expected on the Internet, I want accurate theory, I only found a lot of curves wich look to pop up from nowhere.
I need you're help, I started a first physical analysis, I only know the laws of Maxwell, so I have calculated the Foucault current induced in my projectile, but I had to do a lot of approximations (radius of the projectile small in front of the radius of the coil...). Moreover, I'm not sure of the relative permeability that I use.
I'm a bit confused, please give me some advices.

If anyone master the theory I will be glad to mail what I already do in maple (a mathematic software).

PS: ask if you want more details of my design or other photos

Sulaiman

Mon Jun 11 2012, 12:27PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

You would need a LOT of theory to model a coilgun,
start with the basics here

Arowana

Mon Jun 11 2012, 03:20PM

Registered Member #5059 Joined: Tue May 29 2012, 06:28AM
Location:
Posts: 4

I've already read everything in this website. I'm able to evaluate the current in the RLC circuit, I don't want to use tools in order to calculate the field, I want to use my own program, I want to understand each formula I use, not graphics from others.

I proceed like that :
1. First I take the formula of the field in the axis of a coil.

2. I made an approximation of the radial field thank's to the local laws.

3. I found a formula of the Foucault current induced in the projectile thank's to integral laws of Maxwell-Gauss.

4. Then only the radial field create a force wich throw the projectile. Then I calculated the Laplace force.

5. I made a program in maple, wich do an differential integration of the dynamic principle. Then I find the speed and the postion at each time.

The problem is that my results are far from reality, I think I'm missing the saturation of the projectile and maybe the magnetic force

Sulaiman

Mon Jun 11 2012, 07:20PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Are you also considering eddy currents,
and Lenz's law etc?
e.g. if the projectile is conductive but not ferromagnetic
it will move away from the coil, not towards it.
Never ignore Murphy's law also.

Forty

Wed Jun 13 2012, 05:13AM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

He's calling the eddy currents "Foucault currents" apparently. Never heard them called that.
Also never heard of a Laplace force. Do you mean Lorentz?
(Have I drank away my physics degree knowledge already?)

You'll probably never get your results to match to reality. It's a rather complex problem.
you'll have to take into account:
-coil dimensions and turn count to determine exact inductance and resistance (which is temperature dependent, so it will change shot to shot depending on the time between shots and heat dissipation of the coil)
-projectile material, dimensions, moment of inertia, starting position, magnetic permeability, resistivity (eddy currents will be induced in it,) and saturation magnetization
-barrel friction coefficient (and resistivity if you use a conductive material, because eddy currents will also be induced in it)
-connective wire inductance and resistance
-voltage, capacitance, ESR, and ESL of capacitor bank
-switching method used
-angle of the coilgun when firing
-position of the moon, etc. you get the idea.

Pinkamena has done some great simulation work here on the forums for coilguns. Although they will never match reality, they're a good way to comparatively see what might be best in a setup (at least for some variables)

Arowana

Wed Jun 13 2012, 03:36PM

Registered Member #5059 Joined: Tue May 29 2012, 06:28AM
Location:
Posts: 4

Thank's for your answers.

Foucault current is an other name for eddy current (wikipÃ©dia).
The Laplace force is the force wich involve the eddy current and the field :
dF = j.B.dV where j is the eddy current, B the orthogonal field and dV the small element of volume.

Forty : I already take account of the coil dimensions and variables of the circuit. But it's harder to find the relative permeability and the conductivity of the material I use for the projectile.

I think that force of friction is very small in front of the Laplace force.

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