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External Iron: Help Needed.

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LAN_403

MArked One

Fri Mar 18 2011, 07:18PM

Registered Member #3688 Joined: Mon Feb 14 2011, 07:39PM
Location: Europe
Posts: 38

Greetings.

I have a question about external iron:

I have a 880j , 400V rig coming up, and I was wondering about improving the efficiency. I have achieved some results (penetrating two layers of thin acrylic) with a test coil I cobbled together, but I want some brick-busting performance for my final design.

I have quite a lot of the following iron laminations:

_______5cm_________________________
             |                                   |
             |     ______           ______      |   5cm
             |    |      |         |      |     |
             |    |      |         |      |     |
             |    |      |         |      |     |
             |    |      |         |      |     |
             ''''''      '''''''''''      '''''''''  
                   1cm       2cm       1cm
and 
                       _________________________________
                      |                                   |   1cm
                      |                                   |
                      '''''''''''''''''''''''''''''''''''''
                                5cm

I was wondering about two things:

Is it feasable to use this material at such high energies? And if it is, then:

Will my efficiency suffer terribly if I make a thin coil (A la Barry's mark 4)?
And if it will, will cutting out the middle leg defeat the purpose of using the iron in the first place?

My Apologies for the wall-o-text.

Kind Regards,
Ivan Kozlov.

Turkey9

Fri Mar 18 2011, 10:02PM

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

The best thing you can do is make your coil about 1.25 times the length of the projectile in the shape of a simple cylinder. Then cover the outside with iron as best you can. That seemed to help my coilgun pretty well. There's probably a lot going on that you could simulate in FEMM to get the best geometry. What you're looking for is to have the field intensity in the center of your coil as large as possible. I would cut out that middle leg if I were you and make a 4cm long coil and create the projectile from there.

Iron should be fine at that energy I'm pretty sure, especially since it is laminated.

pauleddy

Sat Mar 19 2011, 12:21AM

Registered Member #2909 Joined: Wed Jun 09 2010, 12:31AM
Location: fort belvoir, Va USA ( south of DC)
Posts: 145

I am using two 1kg lamated iron cores like the bars in your text pic that I tape in to one 2kg projectile and it works wonderfully. My system is a 800v system at 37mF and my coil is a pig secondary.

Turkey9

Sat Mar 19 2011, 01:59AM

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

Oh my god that coil must be huge!!! A pole pig secondary? 2kg projectile? 37,000uF? Pics?

pauleddy

Sat Mar 19 2011, 03:51AM

Registered Member #2909 Joined: Wed Jun 09 2010, 12:31AM
Location: fort belvoir, Va USA ( south of DC)
Posts: 145

correction its 21.6mF, it was 43.2mF back when i had at 400v. number got mixed up.
16 400v 2700uf caps
her you pics as wished

the bank will eventuly used for a rail gun project but right now its a nice magnetic mortar

Turkey9

Sat Mar 19 2011, 05:09AM

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

Wow that's a beast. Is there a reason that you didn't cover the entire thing in external iron other than the large amount required? If that amount was experimentally the best that might help out Marked. Although I'm almost positive that more would increase performance. Sorry for the slight thread hijack there Marked

pauleddy

Sat Mar 19 2011, 11:30AM

Registered Member #2909 Joined: Wed Jun 09 2010, 12:31AM
Location: fort belvoir, Va USA ( south of DC)
Posts: 145

I dint have that much iron these are jus tops of a large solenoid relay from a power box but I am collecting more cores fo this purpose
Ps that iron on top is my projectile

Barry

Sat Mar 19 2011, 07:13PM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

Q: How much external iron do you need? A: A little more than the projectile's iron.

Since we want the projectile to saturate first, all we need is enough external iron to be sure that happens as desired. Any more external iron than that will have little further advantage.

So calculate the cross-sectional area of the projectile. (Length doesn't matter, all we care about is cross-section.) Then use enough layers of external iron so that it has a larger cross-section than the projectile. Mission accomplished, lol. This is assuming both materials have a similar B-H curve; if the external iron has higher permeability then you can get by with proprotionally less of it.

By the way, it can be helpful to keep the fingers of the E-shaped layers fairly short. If the fingers are extra long, and the empty space not needed for filling with coil windings, then magnetic leakage occurs in the empty gaps and can waste some magnetic flux.

Cheers, Barry
Bacteria: The only culture some people have.

MArked One

Sun Mar 20 2011, 02:40PM

Registered Member #3688 Joined: Mon Feb 14 2011, 07:39PM
Location: Europe
Posts: 38

Greetings.

Thank You for All the info. I'm away from home at the moment, but will return soon, and test out the external iron. Kudos to all who helped out!

I'll update this post with pics, when I finish the design.

Kind Regards,
Ivan Kozlov.

EdIT: I have searched the forums, and have not found anything: if I'm using a simple current limiting resistor to trigger my SCR, what kind of MTTF am I looking at, on this power level?
And more importantly, is it safe to trigger the SCR, with no voltage in the cap bank?

Marko

Sat Mar 26 2011, 04:31PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hi guys,

It's interesting to me this has been pointed up, because I've been thinking about it for long time. What I thought about was designing a coilgun while not thinking of it in a traditional way, but rather as a linear motor, either a synchronous or induction one. The basic idea is to find a considerable amount of those E transformer laminations, and arrange them into a like 1m long 'stator' with coils around every pole, and the projectiles could consist of I core parts glued together. I'm not sure whether a LSM or LIM would work better. A LIM might offer more force before the iron saturates, but I'm no expert on there. It's likely that an optical feedback and IGBT drive for every coil would be required which is costly.

In any case I suspect a design like that could have great efficiency (50%+) and extreme output energies if done right.

Marko

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