On eBay I noticed a Vacuum Variable Capacitor rated at 20-1000pf 10kV
50A
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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?
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
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
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.
that pf/uf/mf thingy gets me by the short and curlies every time.
Gunner
The aim of untold millions is to be free to do exactly as they choose
and for someone else to pay when things go wrong.
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.
Theodore Dalrymple,
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
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
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.
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
According to Gunner :
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.
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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