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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?
>>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