Re: Maxwell AC vs. DC ratings

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

Original poster: "Christoph Bohr" <cb-at-luebke-lands.de> 

Hello all.

This is only a little bit related to the topic, but anyway:

Is the impregnant in the 37667 caps flammable?
I wanted to mount the caps right above th RSG but now I
fear this might be a serious fire hazard in case the caps spill oil mist
during a cap failure.
I have run these caps with a 6MOT ( guestimated 12KV ) without failure
but I had only short runs ( < 30 sec ) with this setup.

regards

Christoph Bohr

----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Saturday, April 03, 2004 11:38 PM
Subject: Re: Maxwell AC vs. DC ratings


 > Original poster: Terry Fritz <teslalist-at-twfpowerelectronics-dot-com>
 >
 > Hi Bert,
 >
 > At 11:21 AM 4/3/2004, you wrote:
 > >Tesla list wrote:
 > >>Original poster: Terry Fritz <teslalist-at-twfpowerelectronics-dot-com>
 > >>Hi All,
 > >>Although we commonly use DC ratings as AC ratings in MMCs, I was
 > >>wondering about commercial caps like the Maxwell 37667.  We are running
 > >>about 60Hz AC across them and they probably have far fewer caps in
series
 > >>internally than an MMC.  They are rated for "35 kV".  I was wondering
 > >>what people thought about the voltage they should be run at for Tesla
coil use?
 > >>I would guess that 15kV RMS or 21kV peak ( 42kV peak-to-peak) would be
ok?
 > >>Cheers,
 > >>     Terry
 > >>
 > >>.
 > >
 > >Hi Terry,
 > >
 > >I'd strongly suggest that the tank cap's DC rating be a MINIMUM of 3X the
 > >HV transformer's faceplate RMS voltage, and I personally use a minimum of
 > >4X RMS to provide margin for "the unexpected" and long life. The above
 > >factors take into account peak versus RMS voltage, voltage
 > >reversal/ringing, and overdriving at 140 or 280 volts. A 3X factor
 > >is  marginal, while 4X provides comfortable margin, especially for used
 > >caps with an unknown history:
 > >
 > >Estimating the peak voltage stress seen by the capacitor's dielectric
 > >system (with no additional cap/ballast resonance effects):
 > >15 kV*1.414*(140/120)*2 (for voltage reversal) ~ 49,500 volts
 > >
 > >Designing a system with no/negative margin is a recipe for premature
 > >capacitor failure. Some coilers have blown these "eBay special" caps in
 > >their 15 kV NST-driven systems. Pairs of these caps should connected in
 > >series for use in 15-20 kV RMS systems.
 > >
 > >Best regards,
 > >
 > >-- Bert --
 > >--
 >
 > I never realized these caps were so fragile!!  I sold those two I have
from
 > that Lichtenberg figure experiment (wouldn't have worked...) to Kreso and
 > we were wondering about this.  I have always played with MMCs, not
 > commercial caps ;-)
 >
 > Cheers,
 >
 >          Terry
 >
 >
 >