Re: Maxwell AC vs. DC ratings & failure modes

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

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. 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?
 > > >..........
 >