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Induced permament magnetic field in projectile and coils

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Pinkamena

Sun Apr 22 2012, 03:58PM

Registered Member #4237 Joined: Tue Nov 29 2011, 02:49PM
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Posts: 117

Hi!
I was wondering about something... Considering the strong magnetic fields that the projectile and metal around the coils will be experiencing, is it not a real chance that they'd become permanent magnets? And would this not be bad for the effectiveness of the coilgun? I can't seem to find much to read about this problem, so I wanted to hear what the people who's got experience with coil guns have to say. Is it a real problem, and what can be done to remedy it?

Forty

Mon Apr 23 2012, 11:22PM

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

I've never noticed any permanent magnetization of projectiles except when I used a magnetic strip to hold them.
I guess theoretically it should happen a bit with some materials. It might actually help a little at first if the projectile is magnetized in the same orientation as the coil (I think we were discussing this recently)

I usually shoot at hard targets so that probably helps to scramble any residual magnetization.

To remove (or at least reduce) residual magnetism, I believe impact, heating, and magnetizing in the opposite orientation can be used.

Off topic: hows your coilgun sim/design coming along? I was working on mine for a while but the optic sensors got fried or blocked, and the charge regulation failed yet again

Pinkamena

Tue Apr 24 2012, 07:32AM

Registered Member #4237 Joined: Tue Nov 29 2011, 02:49PM
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Posts: 117

Yeah, intuitively, a slightly magnetized projectile would be good for efficiency. Just gotta remember to have current flow in the same way in all the coils!

I don't want to shoot at hard targets at first, it might deform the projectile. I guess I could make "de-magnetization"-coil that I'll stuff the projectile into and magnetize it in the opposite direction until all the residual magnetism is gone.

About my coilgun, it's still in planning, but all I need now is to get the projectile made. Got the charging circuit done (2,8kW with variable shutoff voltage), a 1ÂµS resolution laser gate for measuring the speed of the projectile, and of course the optical triggers. Sorry to hear about your project. I hope you'll be able to finish it! How many joule will it be?

Forty

Tue Apr 24 2012, 04:06PM

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

I was just looking at your charger circuit the other day. I might try the relay method soon as well. Oh and you'll need to disconnect the power to your charger before you fire or else your scr won't close.

Mine is just a small pistol that uses up all my camera flash capacitors I've collected. It's about 300J.
also have a box mounted one for demos that can go up to 576J (can also attach induction launcher coils to it, but I haven't tried that at full charge yet, not sure if the 3 paralleled scr's can take it)
I've got plans for another pistol at about 400J over 2 stages, and eventually a rifle at about 1.5kj over 3 stages. I like the added challenge and cool factor of making them portable.

I think any residual field could probably just be ignored (like most people do already.) The projectile material will most likely be a soft magnetic material, so the residual field should be weak compared to the one that created it.

Pinkamena

Tue Apr 24 2012, 04:52PM

Registered Member #4237 Joined: Tue Nov 29 2011, 02:49PM
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Posts: 117

Yeah, I'll not bother with the residual then. And I have also done a few changes to the charger circuit so that I can completely shut off the charger when I want to.
Also, I see that you choose to use 3 stages for 1,5kJ. I'll use 9 for 3,8kJ, so I guess I got a little more than needed. Oh well, extra efficiency :P

Regarding your own project, why not just buy some caps off ebay? I got 20 5800ÂµF 350VDC for 30 dollars.

jnbrex

Wed Apr 25 2012, 03:37AM

Registered Member #3950 Joined: Wed Jun 15 2011, 12:45AM
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Posts: 51

Permanent magnetization of a projectile is certainly an issue, and a minor cause of efficiency loss.

Since most projectiles that are used in coilguns are soft magnetic materials, their magnetic coercivity is relatively low. Hard magnetic materials, on the other hand, can remain magnetized almost to the point of saturation after the external magnetic field is removed and are used to make permanent magnets.

The reason that efficiency goes down when materials become magnetized is that permanent magnets have potential energy. This potential energy would otherwise have been converted to kinetic energy, so efficiency is lowered. Because soft magnetic materials have a low coercivity, they do not retain a significant B field after the external H field is removed and have a small amount of potential energy. For the average hobbyist, the effects of permanent magnetization can be safely ignored as they do not detract significantly from optimal efficiency.

I'm not an expert on the subject, but I'm also confident that a projectile with no magnetic hysteresis -not permanently magnetized- will be the most efficient projectile.

Forty

Wed Apr 25 2012, 06:39AM

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

wow for $30? that's quite the deal.

I have 10x 2400uf 450v that I got for $5 each. A few other similar sized ones as well. I just chose to use the camera caps for the one pistol because I already collected them awhile ago and don't have any other use for them.

Pinkamena

Wed Apr 25 2012, 09:37AM

Registered Member #4237 Joined: Tue Nov 29 2011, 02:49PM
Location:
Posts: 117

I see. By the way, what's a good efficiency estimate for the first stage, if that stage has flux linkage? 1%? 2%? I'm gonna write a program in mathematica that can tell me what the best energy distribution over the stages will be.
Oh, and thanks for the explanation, jnbrex.

Forty

Wed Apr 25 2012, 05:00PM

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

Well it depends on a lot of factors (mostly the pulse length you design the first stage to have, and the projectiles initial position) but the usual numbers I've seen for first stage are about 1-4%. 1 is pretty poor, and 4 is a little hopeful, so i usually estimate 2 or 3 for any calculations. With the extremely thin and close fitting (with the projectile) barrels, external iron, and high quality projectiles (solenoid plungers) I use, I could probably afford to estimate a little higher but, without any hard data, I don't.

Adding a rough accelerator before the first stage can increase that a couple percent (of course the coil needs redesigned for a shorter pulse length then)

Different quenching schemes can improve the efficiency quite a bit by reducing suckback (v-switch, series diodes, diode+resistor, etc)

Wish I had a photogate so I could run a whole bunch of experiments and really give a good answer.

Pinkamena

Thu Apr 26 2012, 01:47PM

Registered Member #4237 Joined: Tue Nov 29 2011, 02:49PM
Location:
Posts: 117

Making a potogate isn't very hard. The one I'm making costs me about 55$. 18$ for the digital counter, 15$ for laser and photogate, and the rest is just various electronic components. I can give you the schematic if you want to.
Oh, and where do you get your projectiles? As it stands now, I'll likely have to make it from a bar of pure iron, but solenoid plungers will probably be more efficient.

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