[TCML] Freau phase controller issues
canihazpolepig at gmail.com
Mon Oct 18 09:56:56 MDT 2010
I've nearly completed the Freau Phase Controller for my coil. A picture of
the nearly completed unit may be found here:
It incorporates several safety mechanisms, including power-on inhibit unless
the variac is in "home" (0 inductance) position, motor in-rush current
protection for the variac, and power cut-out if the motor is not connected
(via mains neutral conducted to motor, then back to controller on a
different wire, which shorts to neutral only when the cable is connected to
the gap enclosure).
Unfortunately, the variac I used probably had a hidden problem. Either
that, our putting it to use as a variable inductor damaged it. I purchased
it as surplus, years ago. Its info plate was missing, but judging by its
dimensions, I judged it to be good for about 6 amps or so.
After about 3 minutes of testing with the gap motor connected, the variac
started to issue white smoke. I could not determine the source. I was able
to measure the current flowing through the variac, which was 3.5A after the
in-rush. I removed the variac from the enclosure, and connected it as it
normally would be for variac service, and it threw sparks. I found that
several turns on the opposite end of "home" were shorted, in the
wiper-contact area of the core.
Even if the ungapped core saturates, the observed current through the variac
is not exceeding 3.5A. I am concerned that the wiring of the variac as an
inductor, caused a flyback like effect, and that when the "active" portion
of windings consisted of only a few turns, the remainder of the windings
acted like a the extended turns of a flyback transformer.
Fortunately, only 2/3 of the variac's range in being used in this
application (beyond that point, the motor loses sync). I trimmed off the
turns beyond the tap used for 140V boost. Now, connected the variac to
mains, as intended, does not result in any unwanted light show. I also
found that the wiper was on the borderline of making contact with the core,
and this may have lead to arcing issues. I extended the wiper by
desoldering the brush (it appears to be plated with copper on the mounting
end), inserting a few 22 gauge wires into the brush holder, to jack-up the
brush, then soldering the brush back in place.
Upon re-assembly, I find everything works again, as it should, but I am
concerned that my design might be fundamentally flawed. Does using the
variac as inductor derate its current handling ability? I was finally able
to identify the variac, via a label bearing patent numbers. It is either a
Powerstat 21 or 22, the difference being the voltage current ratings, which,
IIRC, are (21: 120VAC/5A vs. 22: 220VAC/2.3A). If it is a 22, this would
explain much, as I am exceeding it current rating by more than one Amp.
I am going to try to obtain a replacement. Superior makes a Powerstat with
the same form factor, designed for 40V service, with a much higher current
rating. In this application, it would seem to be an ideal replacement, as
the voltage across the variac should never be as high as 40V. There will
also be a drop in the inductance range, of course, but I think I can deal
with that, by setting the gap motor in its cradle to near optimal phasing.
Any advise or observations will be greatly appreciated.
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