[TCML] Lacquer

[Bert Hickman]

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
>
> _______________________________________________ Tesla mailing list
> Tesla at pupman.com http://www.pupman.com/mailman/listinfo/tesla
>

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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