External Iron
Zuckerstange47, Sat Jul 07 2018, 12:54PMWhen 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?
Re: External Iron
DerAlbi, Sat Jul 07 2018, 02:50PM
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.
DerAlbi, Sat Jul 07 2018, 02:50PM
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.
Re: External Iron
Sulaiman, Sat Jul 07 2018, 03:22PM
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.
Sulaiman, Sat Jul 07 2018, 03:22PM
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.
Re: External Iron
Zuckerstange47, Sat Jul 07 2018, 09:42PM
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.
Zuckerstange47, Sat Jul 07 2018, 09:42PM
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.
Re: External Iron
_Eugen_, Wed Jul 11 2018, 05:45PM
I did some calculative assesments of the influence which a core takes for one- and multistage coilguns here. The conculsion is indeed negative.
_Eugen_, Wed Jul 11 2018, 05:45PM
I did some calculative assesments of the influence which a core takes for one- and multistage coilguns here. The conculsion is indeed negative.
Re: External Iron
DerAlbi, Thu Jul 12 2018, 06:16PM
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.
DerAlbi, Thu Jul 12 2018, 06:16PM
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.
Re: External Iron
Sulaiman, Thu Jul 12 2018, 10:49PM
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).
Sulaiman, Thu Jul 12 2018, 10:49PM
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).
Re: External Iron
DerAlbi, Fri Jul 13 2018, 11:44AM
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.
DerAlbi, Fri Jul 13 2018, 11:44AM
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.
Re: External Iron
hen918, Fri Jul 13 2018, 02:25PM
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 µ0. Then you have eddy currents and hysteresis losses and suddenly you have made things worse.
hen918, Fri Jul 13 2018, 02:25PM
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 µ0. Then you have eddy currents and hysteresis losses and suddenly you have made things worse.
Re: External Iron
DerAlbi, Sat Jul 14 2018, 05:12PM
Not definitely sure, but while the differential µr (dB/dH) converges towards 1 the overall µr should still be somewhat active. I mean, even saturated iron functions magnetically somehow, if not, a coilgun wouldnt work if saturation occurs (with an µr = 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.
DerAlbi, Sat Jul 14 2018, 05:12PM
Not definitely sure, but while the differential µr (dB/dH) converges towards 1 the overall µr should still be somewhat active. I mean, even saturated iron functions magnetically somehow, if not, a coilgun wouldnt work if saturation occurs (with an µr = 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.
Re: External Iron
WaveRider, Wed Jul 18 2018, 12:00PM
Many years ago, did some modeling of the effects of external iron and projectile eddy current/saturation effects. The first GIF movie shows an iron projectile with iron shielding with no eddy currents. Note two things: 1) the instant penetration of field lines into permeable material and 2) the saturation of the iron shield as seen in the form of field lines that "escape" the iron. Time elapsed during simulation ca. 2.5-3msec.
The second GIF movie shows a simulation where eddy currents are significant. The field lines require some time to penetrate the iron. In fact, careful observation of flux lines at the initiation of the pulse shows the field lines completely expelled from the conductive iron.
Another interesting effect (that I did not have much time to investigate) was the projectile flux-dragging effect when losses were present. This is seen in the second GIF as the projectile exits the coil.
The thing to take away from this is that for pulses of the order of milliseconds, eddy currents will be present, but iron sheaths will help performance more than it hurts. Once you start using pulse widths less than 1ms or so, the use of iron sheaths will deliver diminishing returns. One can use the skin-effect formula to give an order-of-magnitude estimate of penetration into a linear permeable material D = sqrt{delta_t / (sigma * mu_r * mu_0)}. This will likely be an underestimate because of the non-linear saturation effects, which effectively reduce permeability.
Also, when the core armature saturates, the force on the armature becomes almost linear with respect to applied coil current. This is because the core behaves as if you had used a permanent magnet (a body with a constant magnetization) as a projectile. The force is proportional to the _square_ of the current in a non-saturated permeable armature because armature magnetization depends linearly on the applied field.
Take a look at this for a detailed picture of what is going on.
WaveRider, Wed Jul 18 2018, 12:00PM
Many years ago, did some modeling of the effects of external iron and projectile eddy current/saturation effects. The first GIF movie shows an iron projectile with iron shielding with no eddy currents. Note two things: 1) the instant penetration of field lines into permeable material and 2) the saturation of the iron shield as seen in the form of field lines that "escape" the iron. Time elapsed during simulation ca. 2.5-3msec.
The second GIF movie shows a simulation where eddy currents are significant. The field lines require some time to penetrate the iron. In fact, careful observation of flux lines at the initiation of the pulse shows the field lines completely expelled from the conductive iron.
Another interesting effect (that I did not have much time to investigate) was the projectile flux-dragging effect when losses were present. This is seen in the second GIF as the projectile exits the coil.
The thing to take away from this is that for pulses of the order of milliseconds, eddy currents will be present, but iron sheaths will help performance more than it hurts. Once you start using pulse widths less than 1ms or so, the use of iron sheaths will deliver diminishing returns. One can use the skin-effect formula to give an order-of-magnitude estimate of penetration into a linear permeable material D = sqrt{delta_t / (sigma * mu_r * mu_0)}. This will likely be an underestimate because of the non-linear saturation effects, which effectively reduce permeability.
Also, when the core armature saturates, the force on the armature becomes almost linear with respect to applied coil current. This is because the core behaves as if you had used a permanent magnet (a body with a constant magnetization) as a projectile. The force is proportional to the _square_ of the current in a non-saturated permeable armature because armature magnetization depends linearly on the applied field.
Take a look at this for a detailed picture of what is going on.
Re: External Iron
BigBad, Mon Sept 03 2018, 02:15AM
External iron can work, but you need to use something like high-mu powder cores that can take very high frequencies and flux density. Simply using laminated iron will only give you a few hundred hertz (down to about 1ms) or so.
Note, also though, that iron is destabilising, laterally, or longitudinally, or both. With care, it's only destabilising longitudinally, but then you can electronically restabilise it by measuring the position of the projectile optically or whatever.
BigBad, Mon Sept 03 2018, 02:15AM
External iron can work, but you need to use something like high-mu powder cores that can take very high frequencies and flux density. Simply using laminated iron will only give you a few hundred hertz (down to about 1ms) or so.
Note, also though, that iron is destabilising, laterally, or longitudinally, or both. With care, it's only destabilising longitudinally, but then you can electronically restabilise it by measuring the position of the projectile optically or whatever.
Re: External Iron
V2006, Wed Sept 05 2018, 09:17PM
External iron has no decisive significance - the most important is the accelerating magnetic field of optimal form and tension
V2006, Wed Sept 05 2018, 09:17PM
External iron has no decisive significance - the most important is the accelerating magnetic field of optimal form and tension
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