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Re: [TCML] Primary Support Materials
Bert,
Great explanation and info.
Thanks for sharing!
Dan
http://www.easternvoltageresearch.com
Class-E Plasma Speaker Kits
-----Original Message-----
From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On Behalf Of Bert Hickman
Sent: Tuesday, August 24, 2010 3:49 PM
To: Tesla Coil Mailing List
Subject: EXTERNAL: Re: [TCML] Primary Support Materials
Hi Gary,
Unfortunately, PVC tends to track more quickly than many other polymers
in the presence of a corona discharge. Corona is typically generated at
"triple points" - where a conductor, air, and insulator all come
together. The E-field is enhanced in these regions, and if allowed to
create corona, the insulation system will ultimately fail. In PVC, the
combination of UV, ozone, and heat in a corona discharge cause some of
the PVC to break down, liberating carbon and hydrogen chloride. The
latter combines with water vapor in the air, condensing onto the nearby
surface of the PVC (and anything else in the vicinity).
Partially-conductive chloride ions on the surface lead to development
and accelerating growth of carbonized tracks. Once begun, these evolve
into branching, black, Lichtenberg-like figures that culminate in the
complete failure of the insulator. However, IF you can prevent initial
corona formation or arcing near the surface, PVC can be an excellent HV
insulator. It's just not a very forgiving one.
Polymers that don't contain halogens are considerably less affected by
small amounts of corona. Common solid polymers with much better corona
and tracking resistance include polyethylene (LDPE, HDPE, non-conductive
UHMW), Plexiglas (PMMA), Polycarbonate (PC), and glass-epoxy. Flexible
polymers with excellent corona and tracking resistance include silicone
rubber and EDPM rubber.
Here's an example of tracking that I encountered within a high energy
spark gap (poor design/material selection on my part). Although initial
tracking began on the surface of the white PVC spacer, it progressed
along the surface of the nearby polycarbonate as an HCl film was
deposited onto its surface. The PC surface was roughened from sanding
away a previous set of tracking patterns - it didn't fix the problem.
Replacing the spacer with one made from PMMA completely eliminated the
problem.
http://en.wikipedia.org/wiki/File:Tracking1.jpg
Bert
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Gary Lau wrote:
> It's not clear that folks are in agreement on whether PVC is prone to
> surface tracking. Here's my thoughts.
>
> If PVC were inherently prone to surface tracking, we'd all be in trouble, as
> pretty much all of our secondaries are wound on it, right? Surely a
> secondary coil has a far higher voltage gradient than a primary support.
>
> I think Ralph's account of a PVC antenna support succumbing to carbon
> tracking is not surprising, given that it occurred when it was covered with
> dew.
>
> FWIW, I had carbon tracking on some (dry) Lexan primary supports - I had too
> many unused primary turns adjacent to a grounded strike ring. Anything will
> surface track, then carbon track, if it's carbon-based.
>
> That said, it's hard not to notice the discoloration of white PVC near a
> spark gap. If that brown stuff is more conductive than PVC (don't know),
> perhaps there is a higher risk of surface tracking on PVC near the gap,
> although I've never heard this reported. But secondary coils are
> sufficiently shielded or distant to not be affected. I'm guessing the same
> is true for primary supports as well.
>
> Regards, Gary Lau
> MA, USA
>
>
>
> On Tue, Aug 24, 2010 at 9:35 AM, David Rieben<drieben@xxxxxxxxxxx> wrote:
>
>> Hi Greg, all,
>>
>> Yes, I certainly agree that PVC is farily prone to
>> tracking, especially when used in key insulative
>> components of a fair sized VTTC (personal ex-
>> perience with this). I've also discovered that it
>> does not fair too well over the long term as the
>> containment tube for a multiple "RQ" style sta-
>> tionary spark gap, as white PVC tends to de-
>> velop a yellowish-brown tinge, which is brittle
>> and tends to be somewhat conductive, in the pre-
>> sence of the strong UV radiation from the SG.
>> However, I have had absolutely no trouble along
>> these lines using it (gray electrical grade PVC) as
>> primary support rails in my large coil, which is now
>> about half way into it's 7th year of existance. And
>> I don't think that it's really THAT ugly ;^))
>>
>> David Rieben
>>
>>
>>
>> ----- Original Message ----- From: "G Hunter"<dogbrain_39560@xxxxxxxxx>
>>
>> To: "Tesla Coil Mailing List"<tesla@xxxxxxxxxx>
>> Sent: Monday, August 23, 2010 10:12 PM
>> Subject: Re: [TCML] Primary Support Materials
>>
>>
>> I'll second that motion. I've been using PVC pipe and zip ties for my
>>> primary supports since the 1990's. I've never noticed any particular
>>> propensity for PVC to break down and carbon track--not any worse than any
>>> other plastic. This is the quickest, cheapest way for the non-machinist to
>>> build a primary support. If you require an appearance-grade coil, do
>>> something else. The PVC pipe method isn't pretty.
>>>
>>> Greg
>>>
>>
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> Tesla@xxxxxxxxxx
> http://www.pupman.com/mailman/listinfo/tesla
>
>
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