Re: Maxwell AC vs. DC ratings & failure modes

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

Original poster: "Malcolm Watts" <m.j.watts-at-massey.ac.nz> 

Hi Scott,

On 7 Apr 2004, at 8:02, Tesla list wrote:

 > Original poster: "Scott Hanson" <huil888-at-surfside-dot-net>
 >
 > Terry -
 >
 > Although several list members have claimed that they have had multiple
 > commercial pulse caps failed in Tesla coil service, only one list
 > member has actually come forward with a commercial pulse cap for
 > failure analysis. Some time ago, Bill Vayno (are you still out there?)
 > sent me a Maxwell capacitor that is (suspected) to have failed while
 > in use in his coil. Bill's cap is a Maxwell model 37321 in the
 > single-ended plastic case. This capacitor is rated .06 uF at only 30
 > KV. While the label gives the original "measured" capacitance as .061
 > uF, it now measures .077 uF on my Fluke 87 III, which is quite a
 > difference from the initial value.

That sounds remarkably like the failure one of my commercial CP caps
showed (the company replaced them under guarantee). It sounds as if
one of the sections has shorted.

Malcolm

  The white plastic case is slightly
 > swollen, as are many of the "used" Maxwells I have seen. All I know
 > about Bill's setup was that it used a 15 KV, 180 ma NST bank for
 > power, and he supplied up to 140 VAC input to the NST bank. This could
 > have charged the cap to 24,750 volts, assuming no resonant rise. This
 > did not leave much of a safety factor, given the capacitor's 30 KV
 > rating.
 >
 > Now for the "problem" with this capacitor: I have not yet been able to
 > confirm that it is in fact bad, which is why I have not opened it up
 > for analysis. Although I'd really like to see the internal
 > construction, I am holding off on any exploratory surgery until I can
 > confirm that the cap has truly failed. Resistance measures off-scale
 > megohms with my multimeter, and I have not yet had a chance to take it
 > in to the lab and check it with a 1KV megohmmeter. After that check, I
 > will measure leakage current with a 25KV hi-pot tester. If all still
 > looks good, I'll functionally test it by using it to replace the (2)
 > .06 uF, 40 KV series-connected Maxwells (P/N 31642) I have been using
 > to drive my 6" coil (serious retuning obviously required).
 >
 > If testing confirms that the cap has failed, I'll open it up and
 > submit the photos to the list Moderator for archiving at
 > Hot-Streamer-dot-com.
 >
 > Regards,
 > Scott Hanson
 >
 > ----- Original Message -----
 > From: "Tesla list" <tesla-at-pupman-dot-com>
 > To: <tesla-at-pupman-dot-com>
 > Sent: Monday, April 05, 2004 10:31 PM
 > Subject: Re: Maxwell AC vs. DC ratings & failure modes
 >
 >
 >  > Original poster: Terry Fritz <teslalist-at-twfpowerelectronics-dot-com> >
 >  > Hi Scott, > > I will also be interested in what truly breaks these
 >  capacitors.  I think > we can rule out current ;-))  But that leaves
 >  us with pure over voltage or > dielectric failure do to ionization.
 >  I find it hard to believe that > ionization would do it and 35kV is
 >  pretty high for a breakdown voltage.  I > just don't see why these
 >  super robust commercial caps pop like pop-corn > when hooked to a
 >  simple NST...  I guess we would also have to consider > mechanical
 >  stress, but these are multi kamp caps... > > Cheers, > >
 >  Terry > > At 03:49 PM 4/4/2004, you wrote: > >Mike - > > > >Thanks
 >  for the additional info on the failed Maxwell capacitors. As I >
 >  >suspected, there were no measurements made on the actual operating
 > voltage
 >  > >when the caps failed.
 >  > >
 >  > >Just because the system was connected to a 15 KV NST does NOT mean
 >  that
 > the
 >  > >voltage across the capacitor was 15 KV; it may have actually been
 >  2X or
 > 3X
 >  > >or higher, depending on resonance effects, safety gaps, etc, etc.
 >  Please > >post the results of your failure analysis when you open up
 >  the
 > capacitors.
 >  > >If you like, I'll pay shipping costs for you to send one of the
 >  failed
 > caps
 >  > >to me and I'll perform an intensive failure analysis in my lab at
 >  work,
 > and
 >  > >post the photomicrographs & findings at hot-streamer-dot-com for
 >  everyone to > >review. We've seen several failure analyses of MMC
 >  caps that died in
 > Tesla
 >  > >coil service, but nothing on a commercial pulse cap that died
 >  under
 > similar
 >  > >conditions.
 >  > >
 >  > >As for the Tesla coil driven by the 90 KV X-ray transformer, a
 >  posting to > >this list from Mike.Marcum-at-zoomtown-dot-com dated Sat, 04
 >  Oct 2003 > >05:09:29 -0600 went into great detail describing the
 >  system, with the
 > X-ray
 >  > >transformer cooled by a car radiator & pump system, driving a
 >  giant MMC > >array of five-hundred (!!!) Cornell-Dubilier 943
 >  capacitors, with the
 > whole
 >  > >system drawing 24KVA (!!!!) and being capable of operation at this
 >  power > >level for a period of 10 minutes before "the neighborhood
 >  browns out".
 > This
 >  > >posting is archived at www.pupman-dot-com.
 >  > >
 >  > >Likewise, the claim of a 50 KVA Jacob's ladder running in your
 >  backyard
 > (two
 >  > >34.5 KV, 50 KVA pole pigs):
 >  > >
 >  > >"Don't know what the actual power draw was, but it was popping
 >  800A worth
 > of
 >  > >breakers (!!!!) in 5 seconds or so (made the drop wire from the
 >  pole
 > quite
 >  > >warm in the process). The arc spread over 17 feet before the
 >  breakers
 > popped
 >  > >(which was over 4" thick and bright as a welder". This also is
 >  archived
 > at
 >  > >Pupman-dot-com, posted by Mike Marcum -at- zoomtown-dot-com, on Sun, 23 Nov
 >  2003 > >22:04:59 -0700. > > > >Are there somehow TWO different Mike
 >  Marcums -at- zoomtown-dot-com?  Could there > >somehow be an "imposter"
 >  posting incredible claims under your name? > > > >Regards, > >Scott
 >  Hanson > > > > > >----- Original Message ----- > >From: "Tesla list"
 >  <tesla-at-pupman-dot-com> > >To: <tesla-at-pupman-dot-com> > >Sent: Sunday, April
 >  04, 2004 1:48 PM > >Subject: RE: Maxwell AC vs. DC ratings & failure
 >  modes > > > > > > > Original poster: "Lau, Gary" <gary.lau-at-hp-dot-com> >
 >  > > > > > Some information about the operating voltage might be
 >  inferred if you > > > could supply just a few more details.  What
 >  size NST was used (voltage > > > AND current), what size cap was used
 >  (I assume just the single > > > .03uf/35kV), gap type (sync/async),
 >  and safety gap/MOV used? > > >.......... >
 >
 >
 >