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 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 --
-- 
--------------------------------------------------------------------
We specialize in UNIQUE items! Coins shrunk by Ultrastrong Fields,
Lichtenberg Figures (electrical discharges in acrylic), & Scarce OOP
Technical Books. Stoneridge Engineering -- http://www.teslamania-dot-com
--------------------------------------------------------------------