Re: A Couple of Questions

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Bert Hickman by way of Terry Fritz <twftesla-at-uswest-dot-net>" <bert.hickman-at-aquila-dot-net>

David and all,

Pulse capacitors are rated with an expected lifetime of "X" shots at
"Y%" voltage reversal, where Y is typically 20% of the rated Volts DC
rating for plastic-cased caps. If the particular application requires a
higher % reversal (such as in a Tesla Coil), the expected lifetime of
the capacitor may be significantly shortenned. This can be seen by
examining the Life Expectancy Multiplier versus Voltage Reversal
Coefficient graph in the Maxwell's specs for plastic-cased caps. 

But what is the failure mechanism that's shortenning capacitor
lifetime?? All HV capacitors (with the exception of vacuum caps) have
problems handling large, rapid, voltage swings. While there can also be
mechanical reasons for failures, particularly during extremely high
current pulse discharges, the main reason for life reduction in
capacitors stressed by oscillatory discharges is "partial discharge"
damage. When subjected to rapid voltage reversals which significantly
exceed the rated VDC of the cap, low energy partial discharges (sudden
impulse-like corona discharges) can occur on the surface of the
dielectric along the outer edges of the capacitor plates. Over time, the
dielectric in these areas is increasingly damaged, leading to
progressive tracking in branching tree-like fractal patterns called
Lichtenberg figures. Finally the dielectric completely fails, shorting
the cap. 

As long as you've designed your system such that the expected
peak-to-peak swing (after the gap fires) is no more than 120% of the VDC
rating of the pulse cap, you should obtain the stated life expectancy
for the cap. You CAN choose to exceed this without directly overvolting
the cap, but you WILL shorten its life. For example, a 40 kV Maxwell cap
can conservatively operate with a peak-to-peak swing of 1.2*40 = 48kV.
This implies that you could operate in a Tesla Coil tank circuit with a
firing voltage of up to 24kV assuming 100% voltage reversal. This
provides a degree of design conservatism, since a Tesla Coil primary cap
will normally see voltage reversals in the range of 75-85%. 

Hope this helped!

-- Bert --
-- 
Bert Hickman
Stoneridge Engineering
Email:    bert.hickman-at-aquila-dot-net
Web Site: http://www.teslamania-dot-com


Tesla list wrote:
> 
> Original poster: Tesla729-at-cs-dot-com
> 
> Hi Terry, all,
<SNIP>
> Also, on the Maxwell cap specs page that Jim Lux so gracious-
> ly made available to us (thanks Jim), there was some spec
> about 20% reversal current, or something like that. Can some
> one explain this to me? I would appreciate any responses on
> this to help better educate this dum-dum. :^)
> 
> Sparking in Memphis,
> David Riebe