Remote phase adj of sync rotary gaps, tests.

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Original poster: FutureT-at-aol-dot-com 

Hello coilers,

I'm building a new sync rotary gap.  I was planning to rotate the motor
in its cradle to adjust the phase, but I really prefered to use some sort
of external phase control of the input ac to the sync motor.  Now this
problem seems to be solved.

As we know, a variac can be used to adjust the input voltage to the
motor, and this gives a small amount of phase adjustment, but I was
looking for a larger range of adjustment capability.

As a test, I connected a 50uF cap across the sync motor input, and
then connected a 7.5A variac in series with the hot ac lead to the motor.
The variac is arranged as an inductive rheostat.  One connection is
made to one end of the variac winding, the other connection to the 
variac wiper.  (This is the same type of connection as when making
a variac behave as a current limiter or ballast for a TC.)  The variac
is not between the motor and the cap, but is between the cap and
the incoming ac power from the wall.

This setup seems to work great!  I obtained just about a full ninety
degrees of phase adjustment, and the torque seems to remain
strong.  I still have to install the spinning electrodes onto the rotor,
which will cause drag.  But in any case, a larger motor could be
used if needed and the technique will surely work well, if it isn't
quite strong enough now.  But I think it will be OK even using this
small 1/20HP motor with this 3/8" x 6 1/2" G-10 rotor and electrodes.

The only thing undesirable is that the variac makes a slight groaning
noise at some spots, suggesting that some resonances may be 
occuring.  I tried a larger cap, but the variac groaning got worse.
I tried a smaller cap, but the motor loses too much torque.  I don't
think this is a real problem though... it can be lived with.

Different motors will probably require a different sized cap and 
variac for best results, depending on the size and rating of the
motor, the loading, etc. 

I'm using a 3600 rpm motor, so a full phase adjustment range
is 180 degrees, but still 90 degrees is very good.  Once the
motor is rotated to give a ballpark phase adjustment, a 90 degree
remotely controlled range should be plenty.

There is one limitation.  When the motor is started, the variac has
to be set at minimum or near minimum inductance.  This issue can
be solved by using a relay that shorts across the variac when the
motor is started, then drops out.  Or alternatively, one can simply
turn the variac to minimum inductance before starting the motor,
then adjust it for the proper phase setting according to the variac
dial.

It will be great to be able to abandon mechanical phase adjustment
methods once this sync gap is finished.

John Freau