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cduma

Wed Aug 12 2009, 08:59PM

Registered Member #1822 Joined: Fri Nov 21 2008, 08:04PM
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Posts: 300

I have access to a large capacitor bank (45uF 35KV) that a group of engineers use to shrink quarters. The problem is the electromagnet always explodes and no, they do not charge all the way to 35KV. Usually somewhere around 25KV. My idea was to reinforce the magnet with strips of metal and epoxy. This imposes several problems; Added inductance and short circuits being the largest issue. How do I find out the voltage difference across each turn of the coil? How do I find out the effect of the added inductance? Is this even worth pursuing? Should I just put the coil in a gallon or two of Epoxy?

hvguy

Wed Aug 12 2009, 10:30PM

Registered Member #289 Joined: Mon Mar 06 2006, 10:45AM
Location: Conroe, TX
Posts: 154

Quarter shrinker coils are generally meant to be disposable. If you do decide to pursue this I would consider edge wound copper strap potted in a g10 case with a strong epoxy. It is not actually the current that destroys the wire in a quarter shrinker; it's the HUGE magnetic repulsion force that tears it apart. That's why all the wire bits left over have sharp edges... Do not use metal.

klugesmith

Wed Aug 12 2009, 11:37PM

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

Many people with extensive engineering knowledge have sought re-usable quarter shrinker coils,
and I've seen no reports of success.
The Bitter coil configuration with a specially strong conductor material might not be thoroughly explored.
Somebody at Hackerbot Labs posted pictures of a not-too-serious Bitter coil attempt, which failed.

Your gallon or two of epoxy would surely be wasted.

aonomus

Thu Aug 13 2009, 01:55AM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

The whole idea of can/coin crushing is that the coin metal is repelled from the coil, and since there is a equal force on the coil outwards as there is force on the coin inwards, the coil usually fails. I would expect that if you used substantially over-rated wire with additional insulation, you *might* be able to encase it, however after a single shot the potting material would shatter. I believe TDU tried something involving ice encased work coils with no luck.

I think the actual failure mode is 2 fold, first due to the coil being expanded, second due to the expanding coil having thinner wire, more heating, and eventual breaking. I don't know how much heat would result from a small coin shrinking coil having no output, but I don't think that the coil would hold up for repeated shots, the inconvenience of a extremely long cooldown time between firings for a reusable coil isn't worth it. Just put a blast shield over the coil and blow up a dollar worth of copper wire...

cduma

Thu Aug 13 2009, 05:41PM

Registered Member #1822 Joined: Fri Nov 21 2008, 08:04PM
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Posts: 300

There is not too much heat. I have never seen oxidation of the metal and after the 45 seconds or so it takes to render the machine safe the coin is able to be handled by bouncing it around like a hot potato. I have seen (not in person) coils that have been encased in ice succede.

My main question is how would I find out the voltage difference between each turn of the wire?

Myke

Thu Aug 13 2009, 05:48PM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

Find the total voltage across the coil and divide it by the number of turns to get the voltage per turn.
I also think that it is kind of silly to try to keep the coil together because there is a ton of force involved with shrinking the coin. I the video of quarter shrinking at 100,000 frames per sec, it seems like there is lots of plasma floating around right after the shrinking of the coin. I'm thinking it's either from the wire blowing it self up or from the current path being interrupted.

klugesmith

Thu Aug 13 2009, 05:53PM

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

cduma wrote ...

My main question is how would I find out the voltage difference between each turn of the wire?

Divide the capacitor voltage by the number of turns.
It's an inductive voltage divider, and we can choose to neglect the inductance of capacitor and interconnection.
You could throw in a factor of 2 for magnetic field differences between center turns and end turns --
I'm hereby making a SWAG that presence of a conductive workpiece has a leveling effect.

cduma

Thu Aug 13 2009, 06:02PM

Registered Member #1822 Joined: Fri Nov 21 2008, 08:04PM
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Posts: 300

I figured that would be the case but, it seemed to easy to just divide the voltage by the number turns.

What is a good spray/brush on insulator to use? The wire I bought had a very brittle insulator and there may as well be none left.

cduma

Thu Aug 13 2009, 06:08PM

Registered Member #1822 Joined: Fri Nov 21 2008, 08:04PM
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Posts: 300

I am going to document my adventures as best as I can and post the results. Hopefully on friday evening I will be able to use the capacitor bank. Its tempting to go full power and fire the coin at 28KJ but, I will try and resist and instead only do 4KJ. I will also place some coils nearby with very fragile diodes and see how far away the EM field can reach.

klugesmith

Thu Aug 13 2009, 06:43PM

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

cduma wrote ...

What is a good spray/brush on insulator to use? The wire I bought had a very brittle insulator and there may as well be none left.

The insulator will be strongly compressed. During the shot, each turn is attracted to its neighbor with the same magnitude of force that repels it from the workpiece and (less strongly) from coil axis. How 'bout a very thin sheet of fiberglass as a spacer?

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