On eBay I noticed a Vacuum Variable Capacitor rated at 20-1000pf 10kV
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?
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.
that pf/uf/mf thingy gets me by the short and curlies every time.
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In the past few decades, a peculiar and distinctive psychology has emerged in England. Gone are the civility, sturdy independence, and admirable stoicism that carried the English through the war years . It has been replaced by a constant whine of excuses, complaints, and special pleading. The collapse of the British character has been as swift and complete as the collapse of British power.
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.
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)?
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.
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.
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.