Phil, Steve, and all,Sorry for the damage to your secondary, Phil. The videos were excellent and will hopefully lead to good discussion here and effective preventative measures for higher power systems.
A thick coating of 2-part epoxy or polyester may accomplish a similar function as PMMA tubing at significantly lower cost. When uniformly coated, errant strikes to the secondary will "splash" outward across a relatively wide area of the outer surface of the coated secondary. The coating dissipates spark energy by spreading it out and capacitively conducting it to the secondary without creating damaging hot spots. In practice, I've seen hot primary-secondary flashovers that left NO permanent or visible effect on thickly-coated secondaries. These would have undoubtedly caused melting and turn-turn shorts on an unprotected secondary.
Your system also has a comparatively large topload OD compared to primary OD. You may want to consider adding one or two additional, larger-diameter, strike rails to spread out the E-field "footprint" of the base. These will alter the E-field between the topload and base, making it more vertical (in the space between the two) helping to reduce strikes to the secondary. The larger strike rails could be an add-on that can be removed before transporting the coil, and they would would require any changes to your existing primary winding.
You may also want to consider adding a smaller toroid under the top toroid. By elevating the top toroid, you'll increase the outward "throw" of sparks while further reducing hits to the strike rail. In the latest video there were a number of hits to the strike rail that came quite close to flashing over to the lower half of the secondary even at 100 BPS. The hotter 200 BPS strikes combined with thermal rise seems to have tipped the balance, unfortunately.
Good luck and best wishes, Bert Steve White wrote:
I have also had this idea for some time. I would love for somebody to try this but even if it works I don't know if any of us could stand the expense. Steve ----- Original Message ----- From: "Phillip Strauss via Tesla" <tesla@xxxxxxxxxx> To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx> Sent: Wednesday, July 4, 2018 4:02:13 PM Subject: Re: [TCML] Fixing secondary strikes Re: Bad strike to a 12 inch traditional coil (somewhat terminal) Hello Jim, Although very costly in the UK, I was considering a 350mm diam cast acrylic 1 metre long tube (that's how it comes) with 5mm wall thickness to go over my 300mm (12") diam secondary, it would cover about the first two thirds of the tube, a good few inches higher than the previous strikes. Your comment on a dissipative tube caught my eye for that particular reason but I don't understand the concept of loading or that my idea would work, any explanation and/or prediction would be much appreciated.I'm contemplating your other suggestions (which are totally novel to me) with interest. Regards,Phillip. _____________________________________________________________________________ 14 Broad Street, Stamford, Lincs PE9 1PG Tel: 01780 753008 On Wednesday, 4 July 2018 21:26:39 BST, jimlux <jimlux@xxxxxxxxxxxxx> wrote:On 7/4/18 11:40 AM, David Rieben wrote:Hi Phil, My deepest condolences for your loss. I know it's JUST a secondary coil, but as a fellow coiler, I most assuredly feel your pain. On the bright side, at least you did manage to capture some truly spectacular footage of this secondary coil mishap. I have had this happen on rare occasion with the operation of my big coil, though fortunately, none of my mishaps turned out quite that severe! Only once did I actually have to repair some damage to the side of my coil and was able to get it back into full functioning mode via the repair. Since I must operate mine outside, I did have one occasion where the wind actully "blew" one of the streamers back into the side of my secondary coil, too. Lesson learned - although refraining from outdoor operation during rainfall is an obvious good rule, non-starters in windy conditions are also well advised. I suppose this is a risk, that although may be small with a well-tuned and efficiently operating coil, is never completely absent. :^/I wonder if we could figure out a way to segment a large coil vertically, so if a segment gets damaged, you can just rewind that segment. Just off the top of my head, I'm thinking about something like segments with a hundred turns or so. Could we come up with a way of making the connections in a "good" way. I'm almost thinking about how you using field grading rings on a Van de Graaff. You don't want a complete shorted turn, but you could terminate the winding in some sort of flat terminal on the "mating face" of a segment. You'd stack the segments, and then put some compression on it (threaded fiberglass rod?) The other idea that comes to mind is if there is some way to "spread" the energy of the secondary strike.. Say your secondary were coated with a resistive (but still conductive) coating. Would that spread the current density enough to prevent burning through the insulation? Or a dissipative tube covering the secondary - not enough to "load" the secondary, but enough to "take the hit". OR, what about a second helix, space wound, that extends the length of the secondary, with some suitable resistive conductor, so the voltage profile matches that of the secondary (so no protective helix to secondary arcs), but so it doesn't enter into the resonant circuit. The protective helix would be mostly capacitively coupled to the secondary, establishing the voltage. What about something like a helix wound with wire, but with small gaps along the length.. the gaps don't break down normally, but if a streamer strikes, the gaps break down and provide a conductive (but lossy) path to the base. _______________________________________________ Tesla mailing list Tesla@xxxxxxxxxxxxxxxxxx https://www.pupman.com/mailman/listinfo/tesla_______________________________________________Tesla mailing list Tesla@xxxxxxxxxxxxxxxxxx https://www.pupman.com/mailman/listinfo/tesla _______________________________________________ Tesla mailing list Tesla@xxxxxxxxxxxxxxxxxx https://www.pupman.com/mailman/listinfo/tesla
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