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Re: Primary coil configuration



Original poster: "D.C. Cox" <resonance@xxxxxxxxxx>




Don't use "junk wire". Use 200 degree C. wire from a reputable mfgr to start with. Good engineering solves many problems long before they become problems --- an ounce of prevention is worth a pound of cure!

Dr. Resonance



Commercial magnet wire is 100% tested for insulation pinholes, and usually for other defects like blisters, lumps, and minimum insulation thickness. You will find "reject" magnet wire sold on eBay as "hobby wire".

The amount of damage sustained by a secondary during a "racing arc" incident can be very severe. A few years ago at the NorCal Teslathon, someone had a unique coil (with around an 8" X ~50" secondary, I think) that had a truly monstrous toroidal topload. It also had an unusual motor-driven rotating primary, with a fixed "wheel" as the tap point, to adjust tuning.

During his initial attempt to run the coil, the operator slowly cranked up the power and simultaneously "adjusted" the tuning. The spark gap was firing continuously, but the toroid was so large that there was nothing really happening as far as a discharge, excepr corona starting to develop everywhere. My perception that the whole system was on the verge of a massive breakdown. Suddenly, there was a blinding arc from the top of the secondary down to a point near the bottom of the secondary. There was a fireworks-like spray of sparks and glowing molten copper, and then all was silent. If I remember, there was about a 3/4" diameter spot on the secondary where multiple turns of the heavy magnet wire were completely vaporized ..... simply "gone". There was a large smudge of carbonized insulation and vaporized copper deposited in a zone at least 6" in diameter around the arc spot. This coil would have required a lot of work to salvage.

This also points out the need for EVERYONE around a large Tesla coil to wear eye protection, or to stay back at least 20-30 feet from the operating coil. We always talk about the danger from HV transformers, charged capacitor banks and disintegrating rotary spark gaps, but rarely is a spray of molten metal discussed.

As for the ability of insulation applied over the secondary windings to prevent or reduce surface arcing or "racing sparkss", I'm convinced that it can, at least on small secondaries. I overcoat all my secondaries with a very heavy single layer of clear epoxy resin. This provides mechanical protection to the windings, and definitely increases the resistance to insulation breakdown. For instance on a small desktop coil with a 2" dia X 11" tall secondary, I use a very tightly coupled helical primary. During initial testing at night, there was a very obvious "bloom" of corona forming at a single tiny point about 1/3 of the up from the coil base. Increasing the input power induced surface arcs from the top of the coil to this point where the corona was seen. The secondary was then examined under a stereo microscope, and a tiny bubble (literally microscopic) was found in the epoxy overcoating. There was no defect in the magnet wire insulation, just a thin spot in the overcoating. This spot was repaired, and on retest the corona formation was eliminated, and no corona or surface arcing at this spot could be induced at any power level.

I epoxy overcoat all of my secondaries, and have never had a secondary damaged by racing sparks or insulation damage. Early on I did have a secondary that suffered arcing and breakdown on the inside of the coil form, but since then I always use one or two internal baffles and this has completely eliminated this source of failure.

Regards,
Scott Hanson

----- Original Message ----- From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Saturday, April 22, 2006 8:37 AM
Subject: Re: Primary coil configuration


Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>

Hi Gary,

Interesting. I also think that one can buy "virgin" wire and find that somewhere there will be pin holes in the insulation. It is a matter of probability. Seems like if one coats the coil with polyurythene, these "pin holes" will be covered. If a racing arc causes a "permanent" problem, seems like one only needs to remove any carbon tracking and re coat. I have heard of people repairing their secondary coils this way.

Gerry R.


Original poster: "Lau, Gary" <Gary.Lau@xxxxxx>
(SNIP)

I don't know that it's ever been conclusively demonstrated that any
secondary sealing technique will render it invulnerable to racing
sparks.

Regards, Gary Lau
(SNIP)