Surface Gradients (was A&E Tesla coil) (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

Moderated and approved by: Gerry Reynolds <greynolds@xxxxxxxxxx>



---------- Forwarded message ----------
Date: Sat, 23 Dec 2006 12:45:27 -0800
From: Greg Leyh <lod@xxxxxxxxxxx>
To: tesla@xxxxxxxxxx
Subject: Surface Gradients (was A&E Tesla coil)

Hi Steve,

The 8000V/inch is a working number I use for a reasonably maintained, smooth cylindrical coil.  I've pushed new, clean, smooth HDPE surfaces to 12kV/inch with no problems under pulsed ringdown waveforms.  I've also experienced surface flashover failures on the 120L50K coil, down the pine 2x4 verticals, at 4500V/inch.  However, the 120L has sat for years in dusty warehouses, and has been on many overseas voyages in shipping containers.

Electrum operates at a peak output of about 1.3MV, calculated from the base current and assuming a transfer efficiency of about 40%.  This results in an average gradient of 8300V/inch along the 154" winding, which is somewhat higher than I would prefer.  However, they have had no tracking problems on the coilform in nine years of operation (knock on fire-resistant fiberglass).  Since Electrum is near saltwater, before each run they hose down the tower, then measure for residual surface conductivity using a 1kV megger.

The design limit for surface gradients along structural members for the 120ft twin coils will be 4800V/inch.

I suspect that your coil can produce a longer spark for a given voltage since the output waveform envelope does not immediately decay.  However, while studying loss mechanisms on my new 1:12 scale model coil I've recently observed that a substantial portion of the exponentially decaying output waveform contributes to arc length; at least down to 1/3 of the peak starting voltage.  Based on measurements of the arc-stem on Electrum, I had imagined that only the first few highest peaks contributed to growth.  Using solid-state switches of course also makes a world of difference.

-GL


---------- Forwarded message ----------
Date: Fri, 22 Dec 2006 01:39:51 -0600
From: Steve Ward <steve.ward@xxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: A&E Tesla coil, and Mind Freak. (fwd)

Interesting bit of information there Greg.  Have you measured this
yourself?  Im curious as to how you did measure this.  Also, im
curious as to the failure mode presented in real tesla coils where
this gradient is exceeded.  Is it a localized arc-over, or does it
present itself across the entire coil?  Im thinking back to when i was
pushing my 45" long secondary to 145" sparks, where at one point i had
a bright white spark jump down the entire coil surface.  At 8kV per
inch, that would suggest my coil can support no more than 360kV.  This
is about what i'd expect from this coil (produces 10-12 foot sparks).
Its a DRSSTC if that matters to anyone.

Steve

On 12/22/06, Tesla list <tesla@xxxxxxxxxx> wrote:

>> Original poster: Gerry Reynolds <greynolds@xxxxxxxxxx>
>>
>>
>>
>> ---------- Forwarded message ----------
>> Date: Thu, 21 Dec 2006 15:33:06 -0800
>> From: Greg Leyh <lod@xxxxxxxxxxx>
>> To: tesla@xxxxxxxxxx
>> Subject: Re: A&E Tesla coil, and Mind Freak. (fwd)
>>
>> I did not.  However it must have been an impressive secondary coilform, given that 8000V/inch is about the steepest gradient a standard coilform can reliably support without breakdown in normal use.
>>
>> -GL
>>
>>
>> Original poster: Gerry Reynolds <greynolds@xxxxxxxxxx>
>>
>>
>>
>> ---------- Forwarded message ----------
>> Date: Thu, 21 Dec 2006 12:33:49 -0500
>> From: Slurp812 <slurp812@xxxxxxxxx>
>> To: tesla@xxxxxxxxxx
>> Subject: A&E Tesla coil, and Mind Freak.
>>
>> Did anyone else see the Mind Freak episode where he gets zapped by a 3
>> million volt tesla coil? Any thoughts? Does anyone here have anything to do
>> with that coil?
>>
>>
>>
>>
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