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Zuckerstange47

Sat Jul 07 2018, 12:54PM

Registered Member #61926 Joined: Wed Nov 22 2017, 04:08PM
Location:
Posts: 34

When reading Barry's Coilgun Design Site I came across the section about external iron. In many coilgun-projects that I have seen on this forum or on youtube, it seems that this technique was not implemented. I wonder if there are people who could pass on their practical experience regarding the use of external iron - how did it influence the performance of the coilgun, which design works best?
Also I wonder, if other materials apart from iron are possible, too: Can you use steel instead of iron (I think it is not easy to buy pure iron pipes, washers ect)?

Assumed steel would not be a good alternative, do you think it would be possible to use a mixture of iron powder (which is available easily) and epoxide resin, which would also have the benefit of getting a perfectly fitting shell aound the coil?

DerAlbi

Sat Jul 07 2018, 02:50PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

When i started i did a lot of tests with this. I can tell you that iron powder with epoxy will not make an iron-powder core. In fact the irons reaction to a magnetic field changes with particle size. the smaller the worse its magnetic property becomes. Iron-powder cores are sintered which is a much more intimate bond than gluing.

If iron helps or not depends a lot on your application, geometry and energy transfer goals (will it saturate?).
You should simply try it.

Sulaiman

Sat Jul 07 2018, 03:22PM

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

My own experimenting with coil-guns was very disappointing, so I may be wrong :
. the rate of flux change is high in a coilgun
. solid iron/steel in a magnetic path does its best to resist rapid flux changes by heating up due to 'eddy-current' heating.
even at 50 Hz steel needs to be at least laminated.
How significant this effect actually is I can't say as my own tests were shite

OTOH most projectiles are made of solid steel.

Zuckerstange47

Sat Jul 07 2018, 09:42PM

Registered Member #61926 Joined: Wed Nov 22 2017, 04:08PM
Location:
Posts: 34

I think I will try to concentrate on other parts first, because it seems much more complicate than I hoped, and the chance of succeeding rather low. My understanding of physical laws also is not good enough to have a thourough understanding of terms like saturation and the effects of laminated metal, or which different 'energy transfer goals' could be differentiated. Probably I will do some testing with simple steel washers anyway, without understanding the math behind it

At the moment I am beginning to wind a coil for first experiments, starting with printing a winding device...

Nevertheless I would appreciate if others would like to share their experiences, too.

_Eugen_

Wed Jul 11 2018, 05:45PM

Registered Member #57984 Joined: Thu Nov 19 2015, 09:44AM
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Posts: 58

I did some calculative assesments of the influence which a core takes for one- and multistage coilguns here. The conculsion is indeed negative.

DerAlbi

Thu Jul 12 2018, 06:16PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

An easy way to test the effectiveness of external iron is simply by measuring the inductance with and without projectile.
The higher the ratio between L_proj / L_empty, the higher is the potential energy transfer capability of the coil. If external increases the ratio, its worth doing. If not, not.

Sulaiman

Thu Jul 12 2018, 10:49PM

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

Provided that you do the measurements at frequencies equivalent to operating frequencies then I agree,
external iron WILL increase low frequency inductance,
but with coilgun pulse waveforms/frequencies I can imagine external solid iron decrasing performance, (eddy losses and effectively, shorted turns).

DerAlbi

Fri Jul 13 2018, 11:44AM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

complicated. if you stzart like that you should also consider upping the current to working levels to include saturation effects :-/ - and here we are right at "just try it".
External iron does not increase the inductance that much (if its just washers at the end). But it can lead to better coupling of the outer turns because they are directed towards the projectile.. but thats just theory.

A continuous waveform also creates different issues btw. Eddy currents in the projectile and stuff. there is never a ideal measurement setup. A coilgun is a bitch to analyse

I measured at 1kHz and my findings were spot on by the L_proj/L_empty metric - thats all i can say.
And it directly comes from the force equations of the coil <-> projectile.

hen918

Fri Jul 13 2018, 02:25PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

This has been tried, but the major problem is saturation. If you've got hundreds of amp-turns and a small amount of iron, the permeability tends to Âµ0. Then you have eddy currents and hysteresis losses and suddenly you have made things worse.

DerAlbi

Sat Jul 14 2018, 05:12PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

Not definitely sure, but while the differential Âµr (dB/dH) converges towards 1 the overall Âµr should still be somewhat active. I mean, even saturated iron functions magnetically somehow, if not, a coilgun wouldnt work if saturation occurs (with an Âµr = 1, there wouldnt be any force anymore).
My point is, the portion of the flux below the 2 tesla, is still helpful.
So optimizing that portion should always be beneficial, neglecting eddy losses.
Please correct me if i am wrong.
I honestly think that most effects are from the changed current- and force- waveform. This has mostly be done with SCR designs. It would be interesting to see it under more optimal circumstances.

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