Re: Maxwell AC vs. DC ratings & failure modes

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

Original poster: Terry Fritz <teslalist-at-twfpowerelectronics-dot-com>

Hi,

I would like to ask members that if they have any commercial caps fail, 
lets try to find out why.  Shipping an exploded oily mess through the mail 
these days is real tough with all the national security stuff.  However, if 
we can get a better idea of what makes these expensive caps fail, may be we 
can avoid the failures.  Although commercial caps don't fail often, it is a 
big deal due to the cost.  Kreso just posted some very interesting 
pictures!  I wish I had grabbed Chip's old Halloween Caps...  Failed 
commercial data points are so rare, but so important!!

Cheers,

         Terry


At 12:45 AM 4/7/2004, you wrote:
>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?
> > > >..........
> >
> >
> > ---
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> >
> >
>
>---
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