Variable Capacitors used in a phase converter

On eBay I noticed a Vacuum Variable Capacitor rated at 20-1000pf 10kV 50A
http://cgi.ebay.com/VACUUM-Variable-CAPACITOR-20-1000-pF-10kV-50A-Ideal_W0QQitemZ9726554594QQcategoryZ1502QQrdZ1QQcmdZViewItem
Couldn't adjustable capacitance like this be added to the lines of a tuned (or nearly tuned) phase converter and with some volt meter electronics adjust the variable capacitor to keep the line voltage equal to the line voltage during load changes?
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http://cgi.ebay.com/VACUUM-Variable-CAPACITOR-20-1000-pF-10kV-50A-Ideal_W0QQitemZ9726554594QQcategoryZ1502QQrdZ1QQcmdZViewItem
If you had a 2 watt, 10 kV phase converter, maybe so. (I am kidding).
This capacitance is very small to matter in a real phase converter (low hundreds of volts and multiple horsepower).
For your comparison, IIRC, I have 368 uF of capaticance in my 17.5 HP RPC, or about 168,000 times more than in that capacitor. It seems about right.
i
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Mark Main wrote:

http://cgi.ebay.com/VACUUM-Variable-CAPACITOR-20-1000-pF-10kV-50A-Ideal_W0QQitemZ9726554594QQcategoryZ1502QQrdZ1QQcmdZViewItem
That's 50A at HF, which is a horse of a different color entirely than 50A at 50 or 60 Hz. (HF means high frequency, probably in the gigahertz range for a formerly Soviet radar installation).
That won't work at all for you, sorry.
GWE
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And, I also would not buy anything from a seller with 95%, 795 ebay feedback. (although most are related to slow delivery, but anyway)
i

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Not to mention the $ 45 shipping cost. Is he planning to crate it and send it by truck?
Ignoramus7121 wrote:

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He is apparently shipping from the Ukraine.
i

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http://cgi.ebay.com/VACUUM-Variable-CAPACITOR-20-1000-pF-10kV-50A-Ideal_W0QQitemZ9726554594QQcategoryZ1502QQrdZ1QQcmdZViewItem
    Hmm ... notice the range of capacitance? 20-1000 pF?
    Let's say your rotary converter takes on the order of 30 uF to tune it (more likely higher, but this will be enough to show the "why":
    30 uF = 30,000,000 pF.
    The maximum value of this is 1000 pF
    So -- you would need 30,000 of these at maximum value to reach that rather low value which I guessed at as the tuning capacitor. That would take $300.00 (at the starting bid of $0.01), plus $1,350,000.00 shipping (assuming that there were no deal on shipping more than one. :-)
    It looks as though this is adjusted by a threaded pull stud on the upper left side of the image. (Just inside the envelope from that end cap is a bellows flexible feature which allows one electrode to move relative to the other.
    You would also need a servo motor to control it, along with the appropriate electronics. (You could probably gang up twenty at a time on one servomotor, so you would only need 1500 servo motors and associated drive amplifiers. :-)
    Each one (ignoring the servomotor drive, and assuming that it resides in a square which barely clears it) would occupy 300 cubic inches, or a total of 9,000,000 cubic inches, or 5,208 cubic feet. (A cube about 17'4" on a side.
    Are you *sure* that you have room for this in your shop? :-)
    I've seen similar vacuum capacitors manufactured in this country by Jensen. They also tended to be very high voltage and very low capacitance. The physics of it limits the maximum capacitance you can achieve. And you don't really *need* that 10KV voltage max for this application.
    So -- I would suggest thinking again.
    Enjoy,         DoN.
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On 15 May 2006 05:40:31 GMT, snipped-for-privacy@d-and-d.com (DoN. Nichols) wrote:

that pf/uf/mf thingy gets me by the short and curlies every time.
Gunner
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1 ufd = 1e-6 F
1 ufd = 1 mfd
1 ufd = 0.0000001 F
1 uufd = 1 pf
1 mmfd = 1 pf
1 pf = 1e-12 F
1 pf = 1/1000 mfd
1 pf = 0.0000000000001 F
Note that the prefix "m" which usually means "milli" or 1/1000 is still used in this obsolete notation, likewise the "mm" which is highly archaic but still sometimes seen. Mostly in military surplus stuff from ww2 vintage.
Basically the mfd and mmfd notation harks back to the days of megacycles.
Jim
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Hey! Rarely do I get the opportunity to correct Jim Rozen; but I'm quite sure he'd render me the same courtesy.
Jim was 1 zero off in his depiction of 1 microfarad (1 ufd) and 1 picofarad (1 pf). One microfarad is 1 x 10^-6 and should be written 0.000001 F; one picofarad is 1 x 10^-12 which is 0.000000000001 F.
Bob (smug, gleefully gloating) Swinney
Gunner says...

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    Also on this one:
    1 pF = 1/1,000,000 mfd
    Enjoy,         DoN.
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Indeed. Geeze, two screwups in one post.
Sorry for the confusion! I think I've been too long lost in megacycle land....
Jim
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    That Hertz.
    Oh -- BTW -- the "mili" prefix was never used in capacitance ratings -- the 'm' was for "micro" before the common use of the Greek letter "mu" (looks like a 'u'), and "mm" was for micro-micro -- now pico.
    Enjoy,         DoN.
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I put that one there for you to find, Bob.
:^)
Jim
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    Especially since the difference between uF and mF is only the age of the device. People used to spell it micro-farads before the use of 'u' (actually a Greek "mu") for micro came into common use. pico-Farads, however, are six orders of magnitude smaller than microFarads, with nF (nano-Farads) half way between them, at three orders of magnitude smaller than uF, and three larger than pF.
    And to add to that, the older practice of using '~' or CPS (Cycles Per Second) for what is now "Hz" is another thing which adds confusion. CPS was quite self-explanatory, and '~' made a nice short symbol for that, but the change to "Hz" (Hertz) was the equivalent of "politically correct" in the electronics world back around 1960 or so.
    Enjoy,         DoN.
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Mark Main wrote:

http://cgi.ebay.com/VACUUM-Variable-CAPACITOR-20-1000-pF-10kV-50A-Ideal_W0QQitemZ9726554594QQcategoryZ1502QQrdZ1QQcmdZViewItem
If you had about a 1000 of them. Those are rated as pico farad. Your caps for the converter are in microfarad.
John
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I'm being misunderstood... or said differently, I must be miscommunicating, and so I'll try again.
I wasn't assuming that this is the primary capacitance and so that's why I spoke of a phase converter that was already tuned. I was wondering if these larger variable capacitors had enough capacity to be added in parallel with the other capacitors to add extra capacity needed during a load and then back off when idle.
I'm guessing that the answer is still now given the resounding no answers.
Can someone help me with the math used to determine what the uF, Amps, Volts rating is for this (and other simlar high frequency items)?
Thanks.
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The answer is still no. The amount of varible capacitance needed would still be thousands of times more than the capacitance of the variable capacitor in question.
A practical approach to tuning large capacitance (tens of microfarads), is to use several capacitors, to be switched on and off by zero crossing relays. Making your system decide when to turn on or off various capacitors, could be tricky.
i
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