Coil length / Electromagnetic Projectile Accelerators / Forums

Forums

4hv.org :: Forums :: Electromagnetic Projectile Accelerators

« Previous topic | Next topic »

Coil length

1 2 next

Move Thread

LAN_403

Gregary Boyles

Sun Apr 12 2015, 02:46PM

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

There are many examples of coil guns with multiple stages with fairly complex switching mechanisms.

But what if you were to have just one long coil, say 30cm.

The distance to the centre of the coil would be long, along with the available time for the capacitors to discharge.

Would that not mean you could use larger capacity capacitors and get greater energy transfer over a longer period of time?

Signification

Sun Apr 12 2015, 03:34PM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

The typical answer to this question is something like "no, don't waste your time with this", but I recently need exactly the same thing and believe it can work--well! I will post on a bit later since I am working on it NOW! I --usually-- design the projectile to be between 50% and 100% of the coil length, but, IMO, there are many other valid possibilities.

Please give more details of your setup plans and design parameters...wire size, capacitance, SCR shutoff, etc.

Gregary Boyles

Sun Apr 12 2015, 05:11PM

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

No concrete plans as yet. Just trying to understand the concepts and toy with thought experiments.

I wonder if it would work best with thin wire, high voltage and low current or thick wire, lower voltage and high current.

I guess you would you are effectively doing is creating a tesla coil like winding as opposed to a flyback transformer like winding.

It would be interesting to compare the simulation of the magnetic fields in both cases.

Do you know of any good tutorials on how to use that femm software?

I downloaded it but I can't quite figure out how you are supposed to use it.

DerAlbi

Sun Apr 12 2015, 05:30PM

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

Long coils come to mind to everybody who starts with CGs... its discussed here over and over. Just scroll down a little bit and read "Coilgun - 1 long 1 layer coil as long as the barrel? Good, Bad?"-Thread to get a start of the disadvantages. No reason to avoid the forum search or Google search for coilgun concepts. There are tutorials out there.

I wonder if it would work best with thin wire, high voltage and low current or thick wire, lower voltage and high current.

Its both the same.

I guess you would you are effectively doing is creating a tesla coil like winding as opposed to a flyback transformer like winding.

The coils timeconstant is afaik better f the coil is multilayerd insteads of long and singlelayered.

Do you know of any good tutorials on how to use that femm software?

The turorials out there arent so nice but they teach you how to use the shitty userinterface. When it comes to simulations: you must know the physics and should read about the math behind it.. its all about the border-conditions. However for a start i just would google for pictues of simulation results. this stuff is so basic, it should be done before. Hiowever i dont expect you to read any information out of it.

You should really start to read ALL of Barrys work. This guy is amazing! He presents all his experience and explains the results and everything. Just read. Yes, that takes hours, but it will save you many more time than spent after you worked though the pages.

Signification

Sun Apr 12 2015, 05:41PM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

I was thinking of very thick wire, perhaps even paralleled layers, to increase the current, and very large capacitance to slow the discharge giving the B-field time to 'grab' the slug, overcoming it's inertia. Maybe tuned for a half-cycle discharge to allow that huge SCR pulse-current and easy shutoff at the end of the cycle. But right now, these are just 'blind thoughts'--but have good gut-feelings.

Ash Small

Sun Apr 12 2015, 06:52PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I seem to remember repeatedly readingthat the best coil shape for a CG (and a lot of other applications as well) is when the coil diameter is three times the width.

You can arrange several of these in line along the barrel, and use an optical trigger to trigger each one to 'fire' as the 'slug' enters each coil.

This is obviously a 'multi-stage CG'

Others can probably give more detail.

EDIT: typo corrected

Signification

Sun Apr 12 2015, 11:38PM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

Ash Small wrote ...

I seem to remember repeatedly readingthat the best coil shape for a CG (and a lot of other applications as well) is when the coil diameter is ee times the width.

Sorry, but I seem to be missing something here: what do you mean by:
"when the coil diameter is ee times the width."?

Uspring

Mon Apr 13 2015, 09:56AM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

I looked at some stuff in Barrys work. So here my first question:
He states in the chapter about saturation:

The magnetic saturation point puts a limit on the maximum effective strength to drive your coil. There will be no benefit at all if you exceed this, only wasted heat.

Force on the projectile is m * dB/dx. When saturated, m won't increase anymore with rising B, but dB/dx still does. So why is a larger B completely wasteful?
AFAIK, saturation occurs at some tens of amp-turns per cm for e.g. steel. Even a moderately powerful coil seems to be much beyond that. So all of these are wrongly designed?

DerAlbi

Mon Apr 13 2015, 10:00AM

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

actually yes. without saturation, the force increases quadratically with current, with saturation it rises only linear.
Of course resistive losses rise by square-law..so you want the force to rise the same way... and with saturation you dont have that.

Its all a tradeoff... if you want to be a cool kid that shoots stuff on youtube that shatters easily anyway then you want maximize outputpower... in a basic design this leads to bad efficiency. I f you want to be cool on the number-side,you wont be recognized for it, but you will win efficiency

Ash Small

Mon Apr 13 2015, 10:31AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Signification wrote ...

.Sorry, but I seem to be missing something here: what do you mean by:
"when the coil diameter is ee times the width."?

I meant to type 'three'. Sorry about the typo. Not sure what happened, some kind of wireless keyboard 'glitch'. Thanks for pointing it out. I should check my posts more carefully.

There is a 'three to one' optimum ratio for a lot of coils, but I don't remember all the detail. It's been discussed here before several times.

1 2 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob