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Re: [TCML] Lacquer
Thomas Schmit wrote:
What do you folks use for the lacquer on the secondary coil? There
are special lacquers available at motor winding shops, etc that are
rated at something like 2000 volts per mil. They are a little bit
pricey and was hoping for an easier solution - i.e. something from
Lowes, Home Depot or other "big box" store.
Any advice?
Thanks,
Tom
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Hi Tom,
Most folks use multiple coats of high gloss polyurethane, marine spar
varnish, or a single, thicker coating of two-part polyester or epoxy
that polymerizes to a hard, clear coating. These materials are all
compatible with a variety of coil forms and wire insulation systems.
These coatings will create a tough, protective barrier, but polyurethane
will take many many coatings to achieve a smooth, deep coating that you
can obtain with a single coating of a two-part system.
A thick polymer coating helps to more evenly distribute the electrical
field around the secondary winding. More important, the coating is more
resistant to accidental strikes or racing sparks. The thick coating
tends to spread out a stray secondary strike into a wider region so that
energy is distributed (through a spiderweb of smaller streamers and
corona) instead of being focused on a single spot that burns, or melts,
the underlying wire. With either finish, you'll want to slowly spin the
form while applying and during the drying/curing process in order to
prevent unsightly sagging of the coating. You may wish to read the
"Envirotex Lite coating tips" thread, especially the postings by Terry
Fritz and Scott Hanson:
http://www.pupman.com/listarchives/2006/Jun/msg00741.html
Some coilers and professionals have also reported good results using a
specialty product made for HV use and corona suppression such as Glyptal
or Dolph's AC-43 synthetic varnish. These coatings may be harder to
locate in small containers, may be more expensive, and may take longer
to dry. However, these products are specifically designed for other high
voltage applications, and they may be more resistant to long-term corona
damage and tracking than other, more commonly available, finishes.
However, I am not aware of any clear cut results that prove that these
products perform better than polyurethane, polyester, or epoxy coatings
for Tesla Coil secondaries. You can find more information by searching
the Pupman archives for "Dolph's" or "Glyptal".
Older vintage coils were often coated with multiple coatings of shellac,
but the previous coatings have better insulating characteristics and do
not have the characteristic yellowish tinge of shellac. Shellac works
quite well with cotton-insulated magnet wire on phenolic or cardboard
forms when reconstructing or replicating older coils. However, do not
use shellac on clear polycarbonate or acrylic coil forms as the
alcohol-based solvent may cause stress crazing, cracking, and complete
failure of the form.
I would not recommend using any lacquers, since the aggressive solvents
may attack some types of magnet wire insulation or even the coil form
itself. I'd also recommend using new 200C polyamide-imide
(Polythermaleze, or similar) double-build or armored (two layers of
insulation) magnet wire to provide the best in turn-turn voltage
withstand capability and resistance to corona degradation. The
combination of polythermalize wire and a thick protective overcoat will
result in a robust secondary that should last for years.
Good luck!
Bert
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