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Re: MMC cap bank
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
> 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
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.