Re: [TCML] secondary windings burning?

[Bert Hickman <bert.hickman@xxxxxxxxxx>]

Dex and Jim,

Most hobbyist coils should use close winding to maximize inductance per 
unit length and average voltage/unit length. Very large coils typically 
do not use magnet wire, but instead use stranded wire with thicker 
polymer insulation - this "effectively" space winds the secondary. This 
is done for a variety of practical reasons such as increased winding 
flexibility, better resistance to handling or environmental damage and 
sometimes to reduce voltage stress per unit length.

Many different types of magnet wire can be used with varying degrees of 
success. However, for demanding coiling applications, heavy build 200C 
polyamide-imide (PAI) - Nylon and Kapton - is recommended. Heavy build 
240C polyimide (PI or Kapton) magnet wire can also be used, but it tends 
to be significantly more expensive and offers no additional benefits to 
coilers. Polyamide-imide wire is designed for applications where high 
voltage transient voltage stresses and/or high temperatures are 
expected. Although other types of double-build magnet wire can also be 
used, your results may vary. The dual insulating layers combine to form 
a tough, low friction, abrasion-resistant coating that's also resistant 
to most oils and solvents.

Most important to coilers, heavy build (also called double build, heavy 
coat, armored...) PAI insulation is partial discharge and 
corona-resistant. This wire typically has a breakdown strength that 
exceeds 10 kV(!), so it can withstand exceptionally high turn-turn 
voltage transient and RF stresses without suffering breakdown. A 
close-wound single layer winding subsequently coated with a 2-part epoxy 
or polyester coating not only looks great, it will also perform well and 
be extremely resistant to corona or strike-related damage.

The wire should meet ANSI/NEMA MW35C. Some commonly used names or 
abbreviations for this kind of wire includes Heavy or Armored 
Thermaleze, Heavy or Armored Polythermaleze, APTZ, HPT, HAPT, HPAM, 
HTAI, HTAIH, GP200, MR200. Some of these designations are older terms, 
but may be of some use when purchasing off eBay.

Hope this helped...

Bert
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Dex Dexter wrote:
> Is it better to use space wound coils or close wound coils then?
>
> Dex

<snip>

Jim Mora wrote:
It may be useful to state the type of wire preferred. I used to think
Formvar was king - way off. However, The other day I landed on a 
reputable Mag wire site. It seems Polyamide rules. It has very high temp 
and highest dielectric strength by substantial margins.


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