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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?
>>     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 ;-)