Re: MMC cap bank

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

Original poster: "Malcolm Watts by way of Terry Fritz <teslalist-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Dr R.

On 4 Jul 2003, at 12:37, Tesla list wrote:

 > Original poster: "Dr. Resonance by way of Terry Fritz 
<teslalist-at-qwest-dot-net>" <resonance-at-jvlnet-dot-com>
 >
 > My info comes direct from Beau Meskin, Pres. of PCI in Chicago, who has
 > visited my lab and commented on the MMC project.  He said they would work
 > for a while but would ultimately fail due to excessive corona at the caps
 > edge plate surface.  He said the only way to prevent this from occuring is
 > to immerse them in oil, usually silicon oil, which is the way all of their
 > commercial caps are mfg.
 >
 > His standard .01 and .02 MFD units are rated at 20 kV Erms and 60 kV DC peak
 > which represents a 3:1 ratio.  I know this ratio is not necessary for
 > average experimenters work but I was just pointing out a potential problem
 > with the experimenter running 14. 4 kV Erms on a 9 kV Erms bank.
 >
 > Beau has been experimenting with RF caps since we were all knee high to a
 > grasshopper (or less) and is a very respected professional in this area.
 >
 > PCI, CSI, and Maxwell all use silicone oil in their caps.  Short cuts would
 > only prove problamatic at a later date.  I know we can run the MMC hard but
 > we always use at least a 2:1 or 2.5 ratio of DC to AC to prevent problems.
 >
 > And, it seems the problems usually occur right when you want to impress
 > someone --- Murphy's Law of course.
 >
 > MMC's and Terry's work with them is terrific.  I only meant to point out
 > that failure would be a probable mode when operated as the poster posted on
 > the Tesla List.  Running at DC ratings in a non-oil cap just won't work for
 > extended periods of time.
 >
 > I have seen the research but it can't be shared publically --- a sort of
 > "mfgr's secret".  And the big boys guard these well.
 >
 > Dr. Resonance
<snip>

I think that is a fair assessment of what to expect based on units
I've constructed. Since the capital cost is not insignificant, I like
to be conservative in all electronic design and build things for
robustness and longevity. I could say a lot about equipment I service
every day where manufacturers have considered it too expensive to do
so. The MMCs I've constructed are built to a set a guidelines which
do ensure their survival - a single instance of HV puncture would put
paid to the working parameters of the units and to date none have
failed. Ratings are conservative - I consider that doing otherwise is
a false economy. I want these things to last. Do it once, do it right.

      It's been instructive to read of cap failures where a "failed"
capacitor which has finally revealed itself in an obvious fashion has
had its dielectric peppered with punctures.

Malcolm