phase shifter helps lock wimpy motors

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>" <FutureT-at-aol-dot-com>

Hello coilers,

I was experimenting with a small sync motor which does not have
enough torque to sync-lock a small rotor at 120 volts.  But the
motor does lock at 140 volts.  Normally, one might use a variac,
or a boost xfrmer to lock the motor.  Once the motor locks, it
stays locked down to 42 volts.

I connected the motor to the phase shifter circuit, and as soon
as I turned the phase variac to the resonant range area, the motor
locked and remained locked for the full phase variac range.
Someone (James?) mentioned that his motor locked on 120
volts using the phase shifter, but did not lock normally at 120 
volts.  So I'm seeing the same results with this small motor.

This behavior makes sense because the phase shifter can 
cause a resonant rise which raises the voltage to the motor.
In my prior postings, I suggested keeping the reso-rise limited
to 3 to 5 volts.  However, in the case of a wimpy motor, it works
well to deliberately select a phase cap which gives a larger
resonant rise (maybe 10 to 20 volts as needed), to lock the
motor.  The motor will lock when the variac is turned to a 
spot that gives resonant rise.

>From the standpoint of safety, it may be wise to set the
rough phase of the motor, such that the coil normally runs
with the phase variac in the non-reso position.  This way
the motor will be running at a lower more normal voltage,
as the coil runs.  The reso variac position is used only to
start the motor.

Another thought comes to mind.  Brian spoke of the problem
with self-braking of a larger motor at power-down.  Simply 
setting the variac at a non-reso position before shutdown may
stop the self-braking?  (assuming the cap is not too large)
Again, here, it would be good to set the rough phase of the
motor, such that the coil normally runs with the sync motor
at a non-reso position on the variac.  This way the power can
be shut down without first resetting the variac to a non-reso
position.

Another trick to help wimpy sync motors to lock up is to
use a larger than normal run capacitor.  For instance if the
specified cap is 3uF, then a 4uF or 5uF cap may make the
motor lock at a normal 120 volts or lower.  Probably the
cap should not be made too large because the thin run-winding
wire may get too hot maybe?  Of course we don't run our
sync motors continuously, so the motor temperature can
be monitored as needed, etc.  

For those who have not seen it, the phase shifter circuit for easy
remote adjustment of the phase of sync rotary gaps is at:

        http://hometown.aol-dot-com/futuret/page3.html

John Freau