Re: CAPACITOR INFORMATION NEEDED. Also, magneforming...

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

Original poster: "Bert Hickman by way of Terry Fritz <twftesla-at-qwest-dot-net>" <bert.hickman-at-aquila-dot-net>

Gary, Tom and all,

Those are some really BIG caps! As Gary indicates, because of their large 
electrical size and high voltage, it's not likely that they were used in an 
RF tank circuit but they may have been part of a high voltage supply, a 
pulse forming network (PFN), or voltage or current surge generator. And, as 
Gary also suggests, unless the rep rate is high, a variety of alternative 
dielectric systems are quite usable for non-RF pulsed power ("mad 
scientist") applications. These alternate pulse capacitor dielectric 
systems include mylar-oil, kraft paper-polypropylene-oil, and castor oil 
soaked kraft paper.

For low duty cycle high voltage reversal (High-Q/ringing) applications a 
paper-oil dielectric system can actually be superior to polypropylene! 
Because of this, kraft paper-oil and mylar-oil caps are often used in Marx 
generators and other low duty cycle surge generators. And, during 
industrial electromagnetic forming (Magneforming), a robust work coil is 
normally not destroyed during the forming operation. As a result, the 
energy storage capacitor is subjected to a high current underdamped 
oscillatory waveform where the voltage reversal may reach 90%, and the 
capacitor's dielectric system will "see" a transient voltage stress that's 
up to 190% of the initially stored voltage.

For these applications Aerovox used to make a "PM" series metal cased 
paper-oil dielectric capacitor that was rated for 85% voltage reversal and 
150 kA discharge current. However, the rep rate was a very leisurely pulse 
every two minutes. The reason that kraft paper and oil are better for this 
application has to do with the response of the dielectric system to 
overvoltage conditions and the impact on reducing expected lifetime of the 
cap. The equation that relates capacitor lifetime versus operating voltage 
is often described as a power law equation:

   Life at V2 = Life at V1 (nominal 100% life)*(V1/V2)^n

All things being the same (which they never are...), "n" for foil-oil-kraft 
paper dielectric system is about 5 while "n" for a foil-oil-PP system is 
about 15. As a result, the lowly foil-oil-kraft paper cap is significantly 
more tolerant to transient overvoltage stress than a comparably rated 
foil-oil-PP cap. In magneforming, Marx generators, or other high current 
surge generators, this means longer operating life for low duty-cycle 
ringing discharges and less derating.

BTW, for those of you who've read this far, I'm offering Magneformed 
(Shrunken) coins on eBay for an initial price of 1 cent and no reserve. The 
Maxwell caps used to created them are foil-oil-PP construction, rated for 
20% reversal and 300,000 discharges at 100 kA/pop. You can see the coins at:
http://cgi6.ebay-dot-com/aw-cgi/eBayISAPI.dll?ViewListedItems&userid=berthickman

Happy holidays!

-- Bert --
-- 
Bert Hickman
Teslamania, from Stoneridge Engineering
"Electromagically" (TM) Shrunken Coins
http://www.teslamania-dot-com

Tesla list wrote:
>Original poster: "Lau, Gary by way of Terry Fritz <twftesla-at-qwest-dot-net>" 
><Gary.Lau-at-hp-dot-com>
>Since the capacitance is 15 uF, it's far too large to be used in a Tesla 
>Coil (or is a decimal point missing?).  I wonder if the other "mad 
>scientist" applications for HV caps have similar dielectric 
>requirements.  Dielectric losses must be kept low in continuously operated 
>Tesla coils, or else the cap will overheat and die.  But in single 
>discharge applications like can and coin crushers, the losses from 
>one-time events won't amount to much heating.  I don't wish to go too far 
>off topic, but are low-loss dielectrics really important for such 
>single-shot applications?
>Gary Lau
>MA, USA
>---------------------------------------------------------------------------------------------------------- 
>
>Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" 
><CanalRay-at-aol-dot-com>
>Hello all: I am looking for information on some caps I recently ran across. I
>can't find anything on the web from Cornell Dublier, so I will throw it out
>to the masses.
>The capacitors are 43 inches tall with a body diameter of 12 inches. The top
>of the cap has a round metal boss with, I believe a 1/2-13 threaded hole in
>the center. The bottom boss has four 1/2 inch holes drilled in it. Four ears
>on the boss. The body material appears to be a phenolic type tube, reddish
>brown in color. These caps are heavy, they weigh in at 240 pounds, uncrated.
>The value as indicated on the top boss is:15 uF -at- 120 KVDC. I would suspect
>they were intended for a radar unit or some other type of high powered
>transmitter. These are my guesses. The only thing I don't know is what type
>of dielectric they use. Here is the part number: PL7319XR-1. Manufactured by
>Cornell Dublier. Condition is really good. They do not appear to have ever
>been used. So, short of sawing one in half, I thought of asking the Tesla
>group.
>Any help would be appreciated.
>Sincerely,
>Tom Vales        canalray-at-aol-dot-com
>