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Re: Burned Secondary...
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.
Dan,
The original blue-red arcs are indicative of the resistive ballasting
method you used. When using purely resistive ballasting your
transformer cannot supply enough current. When you changed to an
inductive ballast you allowed the unit to get surges of current
without creating a serious primary overload condition. The inductive
method does not throw power away as the resistive method does, and it
allows the system to process large amounts of power, but with some
peak control built-in.
>
> 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.
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.
Remember that in Tesla coil operation we tax the insulation to the
max at times. Very often it is physical causes that lead to wire
destruction, because the physical abuse has weakened the integrity of
the insulation in one way or another.
> So I'll wind an identical secondary soon, but before I do, I'd like
to know
> what burned me up. :) I can guess over-coupling, so I've purchased
1/4"
> copper tubing to make a flat-pancake primary, spacing 1/4" if it's
over
> coupling.
>
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.
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!
> Another thing is that, before we replaced the space-heater with the
> microwave xformer primary, the coil ran as it had before I replaced
the
> static gaps with the rotary, with only a slight improvement in
performance.
> But I kept hearing this whummmm-whummmm-whummmm sound that I
thought might
> be a beat frequency or some sort of phasing response.
Coil-performance
> seemed unaffected by it, however.
Could have been a beat between the 60HZ mains frequency and the
preferred resonant charging frequency of the transformer/capacitor
pair. Harmless.
>
> The real destruction and performance-increase took place when the
ballast
> was changed to the microwave xformer primary.
>
> Running the distribution xformer without ballast promptly blew
> circuit-breakers, btw :)
>
But of course, Dan! Yeah, even though we KNOW what the result is
going to be, we just HAVE to try it anyhow. Sometimes with disastrous
consequences, and sometimes with interesting effects that we actually
learn something from. Hope the neighbors enjoyed the resulting light
show as their voltages bounced around and around :(
> So. Any ideas what caused the shorting on the secondary? Again,
these were
> pinpoint-hotspots and were *not* caused by arcing across the
secondary from
> one turn to another.
>
> Thanks,
>
> Dan
>
>
Have you had problems with this particular run of wire before? It
just sounds to me like a defective insulation problem.
Hope something here helps.
Fr. Tom McGahee