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Idea for a coil gun

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Gregary Boyles

Wed Apr 08 2015, 03:31PM

Registered Member #9039 Joined: Wed Dec 26 2012, 03:31PM
Location: Epping, Victoria, Australia
Posts: 117

I was reading about how air core coils are very inefficient at transferring energy from the current to the projectile.

And I saw some one had crudely stacked some E-pieces from large transformer, drilled a hole through the stack to insert the tube and then put the windings around the stack.

I was thinking, surely a better way to do it it would be to get an iron core from a toroidal transformer, put the windings through the toroid and then stick the tube through the hole in the toroid.

As it happens I found a discarded electrical device that contained such a toroidal transformer. I am removing the windings and will tuck the core away for future use.

Does any one have any thoughts about the pros and cons making a coil gun via this method?

Also I had some thoughts about the projectile and about eddy currents decreasing the efficiency of the transfer of magnetic energy to kinetic energy.

What if you were to araldite a series of steel washers (I know soft iron is the best material but it might be hard to come by washers made of iron rather than steel) together and then put this assembly in a bullet shaped mold and fill it with liquid epoxy.

The principle would be the same as using a laminated iron core in a transformer to minimise the eddy currents.

Does any one have any further thoughts about this?

Skynet

Wed Apr 08 2015, 04:10PM

Registered Member #11734 Joined: Thu Mar 21 2013, 08:44PM
Location: Brno Czech republic
Posts: 35

IÂ´ve made some simulations in FEMM. It helps to better magnetic induction but bigger coil inductance negatively affects the switching speed.

1428509411 11734 FT170413 Cvka Se Shieldem

Gregary Boyles

Wed Apr 08 2015, 04:20PM

Registered Member #9039 Joined: Wed Dec 26 2012, 03:31PM
Location: Epping, Victoria, Australia
Posts: 117

I assume you mean that the magnetic field cannot build quickly enough with an iron core to generate the acceleration for the projectile.

Perhaps a large ferrite core would be better, as they are meant for high frequency operation.

The rectangular cores out of TV flyback transformers spring to mind - I have several of those stashed away.

I could make a multi-stage gun in that case.

DerAlbi

Thu Apr 09 2015, 09:12AM

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

I was reading about how air core coils are very inefficient at transferring energy from the current to the projectile.

You are a victim of people who think they know much, but dont. This wrong selfreflection leads to a lot of guys doing tutorials without actual background knowledge. You can watch a lot of those guys on youtube.
All that external iron does is to change the shape of the force vs projectileposition F(x). This leads sometimes to better efficiency in primitive Coilguns, however thats only due to better timing where the Integral( F(x)*I(x) ) is larger at the end. But thats nothing you can generalize. What you can say is that air cores are not the best thing for SCR-switched coilguns... but air coils transfer energy quite happily if used correctly.
As Skynet said correctly the larger indurctance reduces switching speed. Thats why you will end up with less turns for the same original inductance. Less turns means less resistance which is interpreted by people that it must result in higher efficiency. What they dont see is the picture that Skynet posted: the actual positions where the projectile is accelerated becomes actually shorter because the field extrends less. Less distance means shorter traveltime and less energy transfer. That means in the end youve got less resisitve losses, but also lower energy transfer. It basically results in the same efficiency.
Efficiency is not controlled by the coil, but the timing.

Also I had some thoughts about the projectile and about eddy currents decreasing the efficiency of the transfer of magnetic energy to kinetic energy.

Thats a useless thought. What makes a coilgun efficient is complex circuits, but not avoided eddy-currents. Ive got 30%eff with a solid projectile. Where all those tricks people do leas maybe to the increase from 2% eff to 4%eff... thats kind of not worth the effort at all (from my point of view).

Perhaps a large ferrite core would be better, as they are meant for high frequency operation.

Now its getting pathetic. Ferrite saturates so early that it will just be like air. You just add weight and complexity without gaining anything.

as they are meant for high frequency operation.

DUUUUUUDE STOP IT! a coilgun is NOT a high frequency device. You getting completely esotheric here, thats not helpfull at all. If you say its high frequency, then pleas proofe it first. Tell me a actual frquency you expect and why you expect this frequency and then why this would be too high for normal iron...

Gregary Boyles

Thu Apr 09 2015, 10:04AM

Registered Member #9039 Joined: Wed Dec 26 2012, 03:31PM
Location: Epping, Victoria, Australia
Posts: 117

Ferrite cores....

My understanding is that they are designed for switch mode power supplies that operate efficiently with kHz frequency DC.

But Iron cores are better suited to low frequency AC.

In a coil gun the coil is energized within milliseconds or microseconds....hence my assumption that ferrite cores might be better.

But I will take your word for it that a core of any sort is not necessary.....it was just a thought.

I will fix the primary coil of my iron toroid, re-wind the secondary and use it as a transformer instead.

DerAlbi

Thu Apr 09 2015, 11:03AM

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

within milliseconds

is acutally realistic... now: 1ms is 1kHz. multiple milliseconds is lower than 500Hz. Ferrites:

with kHz frequency

But dont take me wrong: if you plan a SCR-Design Coilgun, externail iron can do something. Its just not generally true, that iron improves the energy transfer of an aircore-inductor.
I just corrected such wrong generalized stuff because it causes wrong thinking and understanding of coilguns. Efficiency is all about the right timing.

Skynet

Thu Apr 09 2015, 05:55PM

Registered Member #11734 Joined: Thu Mar 21 2013, 08:44PM
Location: Brno Czech republic
Posts: 35

Here is bachelor's thesis from my university dealing with magnetic shielding/effiency.

https://www.vutbr.cz/www_base/ zav_prace_soubor_verejne.php?file_id=88073

DerAlbi

Thu Apr 09 2015, 08:35PM

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

Well i would say that your reasoning and evaluation is far from complete, but i can agree with
8.3.3. "Results shows mostly an insignificant difference"

What did you get for this experiment-outcome-presentation-style-work ?

Skynet

Thu Apr 09 2015, 08:58PM

Registered Member #11734 Joined: Thu Mar 21 2013, 08:44PM
Location: Brno Czech republic
Posts: 35

I find out that for achieving major changes in efficiency is useless to experiment with projectile materials or coil shielding. And in my project, I focus on precise switching control

DerAlbi

Thu Apr 09 2015, 09:01PM

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

..and thats perfectly right! Halfbridges rock like hell.

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