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DC magnetic arc quenching

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Cain

Tue Dec 25 2007, 01:15PM

Registered Member #624 Joined: Tue Apr 03 2007, 07:50AM
Location:
Posts: 59

i have been toying with the idea of using magnets to help quench the arc from my dc coil...

but haven't found much info on the subject can anyone help?

like what direction should the poles face? can one use 2 poles facing each other, and if so should the poles be like or opposing?

Nik

Tue Dec 25 2007, 02:31PM

Registered Member #53 Joined: Thu Feb 09 2006, 04:31AM
Location: Ontario, Canada
Posts: 638

The only time i have seen magnets to quench an arc it take a long time compared to a TC spark gap. The magnet is used to force the DC arc to get longer (by moving it up some rails, like a jacobs ladder) untill it cant maintain itself and it goes out. It will work like TDU's Jacobs Bi-Ladder.

CT2

Tue Dec 25 2007, 08:41PM

Registered Member #180 Joined: Thu Feb 16 2006, 02:12AM
Location: Ontario, Canada
Posts: 187

I've heard you put the magnets same poles facing each other, perpendicular to the SG electrodes. The field is soposed to snap the arc out, don't know how well it works or anything though.

Steve Ward

Tue Dec 25 2007, 09:49PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Get some neodymiums and see! Its basic Lorentz forces going on here, where moving charges will "loop around" the magnetic field lines. Intuitively i would say you want opposite poles facing each other (N-S) so you get a decently uniform B-field in the gap between the magnets. In fact, i would think a nice horse-shoe magnet with the spark gap in the magnets gap would work best (though might not be practically possible). Remember, you can generate a pretty powerful B-field with common steel transformer cores (up to about 2 Teslas) but you need a hefty DC power source to drive it. I dont remember how powerful Nd magnets are now though (anyone care to comment?) and if 2T from a electromagnet even compares.

Honestly though, blasting the gap with air is usually easier and more effective (there is a reason you dont hear much about magnetic quenching).

J. Aaron Holmes

Wed Dec 26 2007, 07:10AM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

In large HV circuit breakers, magnetic arc quenching is sometimes employed. Fixed magnets are not used, however. Instead, the arc current itself feeds a field coil that produces the magnetic field, in turn causing the arc to spiral to great length, forcing it to extinguish itself. I have a book which touches on the workings of these breakers: Transmission and Distribution Electrical Engineering, by Colin Bayliss. You might see if it can be found at a local library. There are almost certainly other books that provide a much more comprehensive treatment of the subject, too. I have often wondered about the application of this principle to quenched spark gaps in Tesla coils.

Cheers,
Aaron, N7OE

Cain

Wed Dec 26 2007, 12:43PM

Registered Member #624 Joined: Tue Apr 03 2007, 07:50AM
Location:
Posts: 59

i have a few books talking about using magnets in large circuit breakers..

they are old books and they all use fixed magnets and vents to break the arc but none tell me what way to have the poles facing its more about installation of these units...

Steve Ward

Wed Dec 26 2007, 06:36PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Think about it... if you have like poles facing eachother, the B-field will go to zero (not to say there isnt a gradient!) in the center of the magnetic gap. So i dont really see the point of putting like poles towards eachother. Still... it should be pretty simple to just try and see. You are unlikely to find an exact recipe for a magnet quenched Tesla coil spark gap (one that works anyway!) so you will probably just have to do trial and error like most experimenters do.

Shaun

Thu Dec 27 2007, 12:31AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

I have a large 2" dia by 1" thick N50 cylinder shaped NIB magnet, and 1cm from the surface of each pole I measured peak field strength to be ~1.25T (with commercial gauss meter). I used just the one to quench an SGTC gap, and I have to say an old vacuum motor gave much better results.

I wouldn't want to try it with 2 of those monsters; with my luck I would get distracted for a split second and end up with a snapped finger. (plus I don't think I could afford another one!)

EDIT: couldn't afford another magnet, I mean. Not finger.

Coronafix

Thu Dec 27 2007, 01:55AM

Registered Member #160 Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938

Tesla himself used magnetically quenched spark gaps, covering the poles with mica to insulate them.
There is plenty of stuff about it in "Inventions, Researches, and Writings of Nikola Tesla".
The poles were also shaped into a point, presumably to lessen the chance of the arc sparking to them.

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