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multistage coilgun coil placement.

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LAN_403

rp181

Fri Jul 11 2008, 04:11AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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Posts: 1529

I was wondering which setup would be better in a 3 stage coilgun. I plan on using 500J-1KJ for each speed, highest effeciancy is the goal. Triggering will consist of a LM339 (not sure if thats the correct number, its a quad comparator, 2 for triggering, 2 for speed detection) hooked up to a phototransistor, with infrared LED opposite of it. SCR's will be used.

black is barrel, red is detector, grey is coil.

OZZY

Sun Jul 13 2008, 09:24AM

Registered Member #511 Joined: Sat Feb 10 2007, 11:36AM
Location: Somerset UK
Posts: 55

This is not a simple question and there are different ways to approach the problem. From the magnetics point of view it is best to have the coils close together (if you have the current flowing in the same direction in each coil). This is because the projectile will be magnetized by the first coil and it will take a finite time for it to de-magnetize. So if the second coil is close to the first you can save some energy by not having to re-magnetize the projectile.

If you want to use photogates to trigger the second and third stages you will need gaps between the coils. The most important aspect of timing is that the pulse should end when the projectile has filled the coil. With SCR switching the pulse length is fixed so you need to adjust the point when you fire the SCR so that the projectile is in the right place when the pulse ends. To have some adjustment you either need to move the photogate w.r.t. the coil or set the photogate some distance from the coil and use an adjustable time delay to fire the SCR. Both these methods will take up a lot of space between coils.

Ozzy

...

Sun Jul 13 2008, 03:34PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

On the contrary depending on how large of a projectile you are planning to use you are probably better off putting some space between the coils to let the field die down to prevent saturation. I am not sure how long you need to space them out, but I think even a few ms should be long enough for the projectile to 'reset' itself.

OZZY

Sun Jul 13 2008, 05:31PM

Registered Member #511 Joined: Sat Feb 10 2007, 11:36AM
Location: Somerset UK
Posts: 55

... wrote

On the contrary depending on how large of a projectile you are planning to use you are probably better off putting some space between the coils to let the field die down to prevent saturation. I am not sure how long you need to space them out, but I think even a few ms should be long enough for the projectile to 'reset' itself.

I disagree, I believe the idea that the projectile must "reset" is a myth. If you want to prevent saturation, the best way is to use less ampere turns and hence less input power. If you want high acceleration then you need a high flux density in the projectile and it will be saturated.

In order to exert a force on the projectile you must put flux into it, this requires energy to overcome eddy currents and hysteresis losses. If you let the projectile reset then you must do that work again for every stage.

Ozzy

youngcoilgunner

Fri Aug 15 2008, 03:06AM

Registered Member #1633 Joined: Tue Aug 12 2008, 04:21AM
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but then there is always the problem of the subsequent coils acting as windings on a transformer; a good amount of energy will be inducted into the coils, killing the total possible energy imparted to the projectile

Barry

Fri Aug 15 2008, 07:24PM

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

OZZY wrote ...

I believe the idea that the projectile must "reset" is a myth. If you want to prevent saturation, the best way is to use less ampere turns and hence less input power. If you want high acceleration then you need a high flux density in the projectile and it will be saturated.

Ozzy is correct, imho. There is no advantage in letting the projectile's field die. All it would do is delay the time until it is magnetized again. We want the projectile to be fully magnetized so it improves the coupling to the next coil. Indeed, the process of un- and re-magnetizing the projectile results in energy loss and heating effects.

Now, what about "the problem of the subsequent coils acting as windings on a transformer"? I don't see a reason for major concern. Although one coil will induce a voltage on another, no current will flow while its control circuit is open. You might need a higher PIV rating on the SCRs to block it.

Take for example two closely-coupled windings in an air core transformer: no energy is lost in the second coil if it has an open circuit. Although it induces a voltage (proportional to the winding ratio) it cannot lose energy because no current can flow. It's just a potential problem in one special case: while coil A is changing current while coil B is turned on. But um, if the winding directions are chosen properly, this can be made into an advantage, right? One coil could help ramp up current more quickly in the next coil?

cheers, Barry

youngcoilgunner

Sat Aug 16 2008, 02:33AM

Registered Member #1633 Joined: Tue Aug 12 2008, 04:21AM
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and how would we make the configuration that would boost current rise?

Barry

Sat Aug 23 2008, 03:01PM

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

youngcoilgunner wrote ...

and how would we make the configuration that would boost current rise?

Hey no fair, I asked the question first! lol

My malformed unfinished wild-assed guess of an idea was that if two coils were close enough together for good magnetic coupling, and if the timing worked out correctly and if the winding directions were chosen correctly ... Then as the current ramped down on coil A it induces a current that helps it ramp up on coil B.

Can this work? Would it help enough to make a measurable difference?
Barry
High On Donuts

rp181

Sat Aug 23 2008, 03:19PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

ile give it a try =)
BTW barr, i love the comments you always put after your name =)

rp181

Sat Oct 04 2008, 09:34PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

Si i finally got around to making it, heres a pic of the layout. The coil setup is next to the hymu 80 rod (1ft.). I figured it wasn't worth the time to get it to trigger right with the close setup. Its way easier to delay then make it trigger faster.

Coil is of 12AWG copper wire.

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