Re: Maxwell AC vs. DC ratings

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

Original poster: Bert Hickman <bert.hickman-at-aquila-dot-net> 

Tesla list wrote:
>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
>
>.

Terry,

You aren't trying to stir up a MMC-Maxwell controversy would you?  :^)

I really wouldn't call the Maxwell caps fragile - they are quite robust 
when properly designed into systems within their rated specifications. BTW, 
I also would never size an MMC so that it operated at an RMS voltage equal 
to it's DC voltage either. But heck, that's because I'm a crusty, 
conservative old electrical engineer - I sleep better knowing I have a bit 
of design margin in my systems. :^)

BTW, regarding Lichtenberg Figures, some research on dielectric space 
charge indicates that the average internal E-field just before the Plexi 
breaks down to form a Lichtenberg Figure is in the range of 5-6 million 
volts/cm... kind of tough to recreate in your home lab using either MMC's 
OR Maxwell caps. We are planning to create a new batch of Figures using an 
e-beam accelerator in mid June - some will be as large as 12" x 12", and 
others as thick as 2". The latter will be irradiated on multiple surfaces 
for (hopefully) some special 3-D effects.

Best regards,

-- Bert --
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