Re: Hmmmm...(was CDE 942c20p15k Group buy)

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

Original poster: Sean Taylor <sstaylor-at-uiuc.edu> 

Kreso (and others),

>Why i do not belive in GG caps. MY maxwell was rated 35 kVdc and it blew on
>a 10 kV transformer. And i cannot figure off why should a capacitor rated 14
>kVdc hold for years? ( and only 3.5 kVac rated).

First, the AC rating on the GG capacitors is 630 V.  The reason for this is 
a higher AC voltage will cause corona formation at the dielectric, and the 
dielectric will SLOWLY break down, so 7 caps in series will give an AC 
voltage rating of approximately 4.4 kV.  The reason this works in coils 
with a transformer at 10 kVAC is that they will never see enough use or 
long enough run times to cause serious degradation of the dielectric.  On 
top of this, it is a very conservative rating (Terry has shown that the 
caps won't punch through until over 5 kV, IIRC) and the caps are self 
healing, so one punch through (or several) event won't harm the 
capacitor.  So, for our typical use and run times, the DC rating works 
fine.  If you are going to use resonant rise, and work on the edge like 
that, then you'll run into problems - with both an MMC and a commercial 
pulse cap - the main difference is once the pulse cap is gone, you can't 
replace a $3 element and have it running, nor can you pinpoint the cause as 
easily.

This whole debate has gone on many times before, so I won't say much 
more.  Personally, I'd rather depend on a brand new product that I know 
I'll have a (plentiful) source for rather than a used product that has 
questionable life left (since I can't afford a commercial pulse cap new!).

Because of the voltage reversal rating and lifetime expectancy of the 37xxx 
series Maxwell caps, many people have recommended that they be run on a 12 
kV transformer or less.  The lifetime is a function of voltage to the 
fourth (!!!) power, so running them at lower voltage will make them last 
much longer: half the voltage, 16 times the life!  This is why it is 
important to use an LTR size and not approach the voltage limits of the 
capacitor.  The rated voltage is not a sudden cutoff between working and 
nonworking - the capacitors have a finite life no matter how you use them 
(for some uses it may as well be infinite though).

I think (complete opinion, no way to back this up, right now at least), 
that the Maxwell caps at their rated voltage, are closer to self 
destruction than an MMC is at it's rated voltage.  After all, the Maxwell 
capacitors have a similar contruction to an MMC - several capacitors in 
series, and are certainly subject to the same rules such as corona 
inception, which is why the rating is in kVDC, and the voltage reversal 
rating is much smaller - the lifetimes are stated at 20% VR - so an 
effective AC rating of less than 7 kV for a 35 kV caps (if it works that 
way, probably not, since the VR is counting on a exponential decay).  I'm 
not sure what kind of AC rating they would take continuously.

Anyway, I hope that helps, and clarifies things.

Sean Taylor
Urbana, IL