On Wed, 17 May 2006 21:07:28 +0000, Christopher Tidy
wrote:
Hi all,
Just noticed Mark Main's thread about using a variable capacitor in a
phase convertor. I thought about this a while back and discussed it with
Bob Swinney (Bob, sorry I haven't replied to your e-mail yet - I will,
I've just been busy with other stuff for a while). I meant to do some
more experimentation on the subject before I posted my thoughts here,
but so far haven't got round to it. The following is I think is an
interesting idea, although probably not one of any commercial value.
Traditional air-spaced capacitors aren't going to work, as people have
mentioned. I think I worked out that a small phase convertor would need
an air-spaced variable capacitor with an area of about 6000 m2. But then
I thought, what about a fake variable capacitor? In the form of a variac
and a fixed capacitor?
I tried it. For my experiment I used one 3/4 hp three phase motor
connected to a single phase supply. I didn't feel a second motor was
necessary for the initial experiment. I tried connecting capacitors
through an 8 amp variac between the supply and the third leg of the
motor. It worked. With a really low variac setting the motor won't
start. A bit higher and it starts very slowly. At the highest setting
the motor starts quickly but makes a whine when up to speed. In the
middle there is a "sweet spot" where the motor runs nice and quietly.
So it seems it is possible to make a form of variable capacitor using a
variac and fixed capacitor. I took a few measurements and reached the
following conclusions. (i) It's possible to adjust the voltage between
the third leg and one of the supply legs until it is equal to the supply
voltage (I didn't attempt to measure any phase differences). (ii) The
motor is noticeably quieter when these voltages are equal. (iii) The
voltage across the motor is definitely not linear with variac position.
(iv) Attepting to achieve equal voltages using various capacitor values
it seemed that the square of the capacitor voltage times the capacitance
was a constant under constant motor load.
Then I had another idea. I thought it would be nice if the system
regulated itself, so that a drawing a higher current from the convertor
would increase the variac setting. I figured that it might be possible
by rewinding a three phase motor with a few turns of thick wire and
connecting it in series with the load, then connecting the motor shaft
to the variac and also to a torsion spring. Bigger load - more current
drawn - more torque developed in rewound motor - rotor moves a little
further against torsion spring - variac setting increases. Obviously
this would be far too expensive to build commercially, and hard to
"tune", but fascinating if it could be made it work.
Here's a little diagram of the idea:
http://www.mythic-beasts.com/~cdt22/..._regulator.jpg
Enjoy. My head hurts and I've been puzzling this out for a while!
Best wishes,
Chris
A slightly different way of looking at the problem.
The apparent value of a fixed capacitor can be varied by
feeding it through a transformer. For a 1:2 step up transformer
the voltage on the capacitor is doubled so the energy stored in
the capacitor is multiplied by four (CxVsquared).
With a reasonably efficient transformer, measured at the
primary, this appears at the primary as the primary inductance
shunted by a pure capacitor which stores the same amount of
energy. Because this is at half the secondary voltage the
apparent value is four times the value of the secondary
capacitance.
The capacitance value is multiplied by the square of the
turns ratio.
This can be turned into a true variable C by the use of a
Variac to feed the fixed capacitor. The impedance of the
magnetising current taken by the Variac needs to be much higher
than the impedance of the intended capacitor. This is easily
checked by comparing the Variac magnetising current with the
current drawn by the capacitor at a similar voltage. Because
Variacs are usually step down devices the apparent capacitance
value will be reduced by the step down ratio squared.
With a variable capacitor a fixed transformer ratio can be
used but, if 100uF is needed from a 1000pF variable, the values
are pretty extreme - more than 300:1 stepup ratio and, with 200V
in, more than 50Kv across the capacitor!
Jim
*
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