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Coils - contract or expand?

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LAN_403

Camel

Sat Mar 28 2009, 06:42AM

Registered Member #1694 Joined: Sat Sept 13 2008, 09:13AM
Location: Australia
Posts: 108

Hi,

I've read on many websites that the coils have a tendency to crush barrels due to the coils contracting when they are fired. If this is the case, why is it that pan cake coils can expand and deform outwards?? Is the contracting coil thing just widespread disinformation? Or is the force on the coil due to the type of projectile, since pan cake coils are often used in induction launchers and coilguns fire ferrous slugs?

Backyard Skunkworks

Sat Mar 28 2009, 06:55AM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

Colis do indeed crush themselves while firing.

I think the simplest explanation for why pancake coils crush outwards is simply because they are being crushed down upon by the force of accelerating the projectile (think pancaked) in addition to constricting inwards.

Dr. Slack

Sat Mar 28 2009, 10:39AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Each conductor in the coil feels a force due to every other conductor. If the currents line up, it's attractive, if the currents are in opposite directions, it's repulsive. And that's with or without a projectile present. This means a thick pancake will try to get thinner, while getting wider. Consider the current induced in the projectile, and do the same sums to find the force.

There are several ways to remember which sense is attractive and which repulsive. You've heard of the "pinch" effect, where current in a plasma compresses the conducting channel thinner, that's just attraction between the parallel filaments. Similarly, the kink instability is the tendency of any loop to grow as its opposite sides repel. Another more interesting way to remember is that, left to its own devices, a system will try to increase its inductance, which means increasing the area of coils, or reducing the thickness of single conductors.

Signification

Mon Feb 02 2015, 12:14AM

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

If you consider adjacent strips, current is traveling in the same direction giving the 'attractive' force.
If you consider the strips on opposite halves of the coil, current is in opposite directions giving a 'repelling' force.

I would assume the repelling force is MUCH weaker due to the greater separation. Remember for each of these forces, the 'number of conductors' is the same -and- all currents are equal.

BigBad

Mon Feb 02 2015, 03:54PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

Both crushing and expansion happen in most coils.

Longitudinally the parallel turns attract each other, whereas radially, one side of the coil repels the other.

So when firing coils are always trying to expand radially, and contract longitudinally.

Pancake coils are already as flat as they can get so they mostly only expand (although a group of conductors on one side of the coil will see a mutual attraction, so there's some pinch effect there as well)

DerAlbi

Mon Feb 02 2015, 06:48PM

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

Signification.... you have the tendency to wake up really old threads. Adding information to long (6 YEARS!) forgotten content doesnt help anyone...

Signification

Tue Feb 03 2015, 11:19AM

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

DerAlbi,
Well, Since you are talking about threads that are years old, and considering I have just joined, is it really a big deal?...should I really read all posts starting from the year 2006 before posting one myself?? Anyway, it seems, so far, my posts have had very positive replies.
Maybe: a few things have changed over the years
Maybe: a few were never answered (which, BTW, if you look back yourself, you will find this to be the case)

Anyway, When I post information, I DO look at the thread first, and only reply if I think my information is new and helpful to the members involved.

klugesmith

Thu Feb 12 2015, 04:33PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Back to question in OP, and expanding on Dr Slack's answer.

In a solenoidal can crusher, or pancake coil disk launcher, we have a shorted turn that's magnetically coupled to the wound inductor. The induced countercurrent repels the winding current, so in radial direction the can contracts and coil expands. Or in a coin shrinker, explodes.

As for the dL/dx method, the closer the (nonferromagnetic) projectile, the lower the inductance as viewed from power supply. Not hard to measure using small signals, or simulate with SPICE etc.

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