Coil coating: (was Re: Fiat PVC, Fiat Lux)

To: tesla-at-pupman-dot-com
Subject: Coil coating: (was Re: Fiat PVC, Fiat Lux)
From: "Reinhard Walter Buchner" <rw.buchner-at-verbund-dot-net> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
Date: Tue, 06 Jul 1999 08:19:08 -0600
Approved: twftesla-at-uswest-dot-net
Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme
Hi Bob, Yuri, Grayson, all,

I agree with most of what you wrote to the question that
Yuri Markov asked, except for the comments below:

> Original Poster: Bob <misiura-at-nccoast-dot-net>
> Yuri, There is no magical substance use nothing at all, or
>perhaps a light wipe down with some acetone to remove
>any grease or fingerprints.  Acetone is a weak

Snip

The PVC pipe I used, was "brand" marked with hot letters,
which result in the PVC being raised around the imprinted
letters. These should be sanded down. Not only will it
result in a less bumpy apperance (cosmetics), but it will
also keep entrapped air to a minimum.

> A little time preparing to wind a secondary helps.  First, build a
>jig to hold the form.  Make something that will allow you to turn
>the coil with one hand and lay on wire with the other.

Total agreement. I would NEVER wind a coil on my lathe. Esp. not
if I would be coating it with epoxy, etc (running the lathe to prevent
sags and runs). A lathe is an (expensive) piece of high tech
equipment and should not be abused for such purposes. It only
took me about 2 1/2 hrs to build a electrical winding rig out of
scrap parts, I had laying around. It was a most beneficial aid in
secondary construction. Another problem with winding a coil on
a lathe: This is EXTREMELY difficult to do, if you wind the coil
alone. You will never be able to stop the lathe in time (mine
goes down to 60 rpm), if the wire tangles, etc. If the wire gets
caught or tangles somehow, it WILL snap before you can stop
the lathe. My lathe spins down slowly. It doesn´t have a quick
stop. (I´m not sure if any lathe has a quick stop, because you
have high inertial energy "stored in the lathe" esp at low (high
gear ratios) turning speed).

> An outer coating is not necessary if the winding is fair (smooth,
>tight, no overlaps).  It will make the coil more rugged. I generally
>use one, but it does detract slightly from performance.  Use a
>clear oil-based, varnish, shellac, or epoxy coating.  You can sand
>between layers, if you want to achieve a thick, high-gloss, coating.

And here I TOTALLY disagree. Here is how I prep´d my secondary.
The bare form was sanded inside and out. Then, I washed it down
with water and soap with a clear rinse to finish it off.  I force dried
the former, wiped it down with acetone and coated it on the inside
with PU varnish. This dry, acetone, PU process was repeated for
the outside of the former. Then I wound the wire on the former. I
used a thin epoxy resin (24 hr hardening time) to coat the wire/
former. After it was hard (I actually waited 48hrs), this was
sanded smooth and I applied a second coat of resin. This, too,
was sanded smooth. Then I applied two more coats (with sanding
inbetween) of PU varnish. Due to the thin nature of the resin and
PU varnish and the constant turning, I have almost no air bubbles
(you will need a magnifiying lens to see them) and the coating
is very smooth and glass hard. The reason I used epoxy AND PU
varnish is easy to explain: The epoxy is thick (and will really embed
the wire) and very hard, whilst the PU (on the outside) is slightly
resilent and won´t crack if you pick up (or maybe even bump
something into) the coil. Now to the performance aspect (which
is the place where I really disagree with you): My coil is powered
by (a) 7.5kV neon(s). At the time I am running 1275VA and getting
continious 57" sparks with an occasional 63 incher. Input Joules
are 3.77J (~16nF cap for 15kV coilers). I use a flat primary with
a 11.8" inner diameter. This means I have around 2" clearance
between primary and secondary. The primary is RAISED ~4.5"
ABOVE the lowest secondary turn. While I do still have to
measure the true coupling factor, I am pretty certain it is on the
high side. During one run, the primary tilted and I got hefty
flashover between primary and secondary. While this only lasted
a short time (I shut down immediately), I examined the coil and
found not the slightest mark. If the coil had not been coated or
just a single coat (to keep the wire in place) of PU varnish, I
am most certain, it would have been trashed during this run.
I have had this happen with a non coated coilform (2.5"), run
on much lower power (200VA) and the wire (cotton covered
magnet wire) was charred, more or less, instantly. The
measured (!) Q of my coil is ~200 (196 to be exact).

My bottom line on coil coating: I don´t think a coating will ruin
/ hurt the coil´s performance at all. As a matter of fact, I
would even consider it a must, esp if one runs at high kVA
and or coupling levels. It WILL definately save the coil wire
and former from injury during a mishap (which will eventually
happen).


Coiler greets from Germany,
Reinhard