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Re: Burned Secondary... (fwd)
On Thu, 11 Sep 1997, NTesla wrote:
At 07:15 PM 9/10/97 -0500, you wrote:
>
>From: Thomas McGahee[SMTP:tom_mcgahee-at-sigmais-dot-com]
>Sent: Wednesday, September 10, 1997 3:31 PM
>To: Tesla List
>Subject: Re: Burned Secondary...
>
>
>
>>
>> From: Dan Kline[SMTP:ntesla-at-ntesla.csd.sc.edu]
>> Reply To: ntesla-at-ntesla.csd.sc.edu
>> Sent: Monday, September 08, 1997 10:58 AM
>> To: tesla-at-pupman-dot-com
>> Subject: Burned Secondary...
>>
>> Hi all,
>>
>> I made a couple of changes to my system the other day and promptly
>> had several turns weld together on the bottom 1/3 of the secondary. I
>> cut that part off, retuned, and had the rest of the secondary burn the
same
>> way. I don't mind this though...Last time I jumped up a level in coiling,
>> it was because I was burning secondaries (arcing through the inside of the
>> form), but it's not that way this time. Here's the system before the
>> changes:
>>
>> Power Supply: 14,400V, 1.5kVA Distribution xformer
>> Current-limit: approx 750W space-heater
>> Primary cap: .025 uF 20kV Plastic Cap brand
>> Gap: 3 series gaps
>> Primary: 16 turns #6 rubber-covered, stranded cable in saucer-shape
>> at 15 degrees, 3/8" spacing, tapped at about turn 11
>> Secondary: 6.25" lucite form wound 22" with #22 double-covered
>> formovar magnet-wire
>> Discharger: *2* toroids stacked, Al dryer ducting, both 24" x 4"
>> Performance: Occasional 36" strikes, double and triple forked,
>> blue-red in color
>>
>> Replaced the 3 series gaps with 4 electrodes on an 8" lexan disc,
>> 3300 rpm motor, but worked best at about 1700 RPM
>>
>> Replaced the space heater with the primary of a microwave-oven
>> xformer.
>>
>> Everything else was left the same.
>>
>> Performance: White bolts all over the place, like being inside a
>> thundercloud. Assistant screaming in fear and awe ;) Bolts too
>> long for room, but no strikes to the system itself. Assistant heading for
>> bathroom, soon to make ten-foot-pole for flipping main system-switch ;)
>> After shutting down, the system wouldn't run again, due to shorted turns
>> on secondary.
[snip]
>> I never saw any inter-turn arcing, but pin-point hot-spots shorted
>> the secondary turns in several places.
>You might have a wire insulation that is defective. At ANY time did
>your wire undergo a sharp bend? An example would be a metal guide
>attached to a coil winder. During handling, was the wire subjected to
>excessive tension? In winding the coil, did it touch any metal
>object? Was your coil wound particularly FAST? Any of the above may
>have caused pulling or abrasions or microsopic cracks in the
>insulation.
Right, as a matter of fact I'm rather fanatic about good winding, and other
than using a motor to slowly turn the coil-form, all guiding and and laying
of wire is done by hand. So I would guess no bends, stretches, cracks or
anything.
>Also, since we want to touch all bases, is this new wire? Old wire?
>USED WIRE? Wire subject to excessive sunlight or heat or cold can
>have the insulation jeapordized. Used wire is almost certain to have
>microscopic fractures in the insulation due to bending and
>re-bending.
Well, the wire does stay in the shop, but I'd hardly call our winters or
summers excessive, as far as temperature is concerned...I do have a newer
roll of wire though with a different insulation...I'll try *it* this time. :)
>You say that there was no visible inter-turn arcing, just these
>minute pinpoint hot spots. Being over-coupled would definitely tax
>the insulation in even the best case. If you are going to use the
>same type wire to make your next coil, you might want to liberally
>seal the secondary with several coats of a good insulating material.
>If you apply such insulating material AS the coil is being wound so
>that it is even between turns, that should help a lot.
Now that is interesting: For insulation, I used a great deal of spray-on
Q-Dope sorta stuff...I'm wondering if maybe there wasn't some sort of
solvent in it that attacked the insulation on the wire...I'll look into
that. :) If formovar is styrene-based...hmmmm
>Last-ditch method that you can use is to get some thin string or
>monofilament line and SPACE wind your next coil. You wind it bifilar
>with the string, and then as you remove the string you coat the coil
>with varnish/whatever liberally so the wire stays put. THAT should
>stop the inter-turn sparking! There are other methods of space
>winding that do not require using a string as a bifilar element. Most
>of these other methods require a winding apparatus that is fairly
>exact, and involve feeding the wire across the length of the form so
>that it forms an open spiral. Have fun!
Heh, I built a thing like that using two threaded rods in parallel, driven
by a common v-belt over a pulley on the end of each rod. The lower rod had
a big nut that was the wire-guide mount. The turn spacing was determined by
the pulley-diameter ratios. Since the wire-guide-mount-nut was kept from
rotating (because of the wire-guide), the nut rode down the rod as the rod
turned. If the rods were 1/2" x 13, and the upper pully was half the size
of the lower, two revolutions of the upper rod would mean one revolution of
the lower rod, and I'd get 7.5 turns per inch of nut-travel. For equal-size
pulleys, 13 turns per inch of nut-travel. If the upper pulley was twice the
size of the lower pulley, I'd get 26 turns per inch of nut travel. (It's
hard for me to explain, much less to try to visualize without a drawing :)
But by making my own pulleys, I could get any spacing I wanted. But it's
been a few years since I've space-wound a coil.
>Have you had problems with this particular run of wire before? It
>just sounds to me like a defective insulation problem.
Nope, always used this same wire, from this same roll, never a problem, but
now, I bet it was the insulation-coating that attacked it :)
>Hope something here helps.
>Fr. Tom McGahee
It did, and thanks :)
Dan