Re: Synch motor Phase controller question

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

Original poster: "Gregory Peters by way of Terry Fritz <twftesla-at-qwest-dot-net>" <s371034-at-student.uq.edu.au>

Dan,

Don't forget the variac is acting as a variable inductor, not a
voltage controller. At some point in the variac's range, the
inductance of the variac, the capacitor, motor cap and windings will
be matched just right to cause a 50/60Hz resonant circuit. The
capacitor size will determine just how high the resonance can ring
up. You want the resonant rise to be below 10v above the line
voltage. So you mix and match capacitances until the voltage rise
across the motor is satisfactory. I set mine pretty high (10v),
because my line voltage drops pretty significantly when I draw
10kW :)

I find that the rise will occur no matter what (though it may be too
big), if the cap is very large. If you wind the variac slowly, you
can stop it before it goes too high, then try a smaller cap. See
below.

1. Just hook up the circuit with a cap that "feels"  a bit too large
(look at what sizes John recommends on his page and go from there)
and voltmeter across the motor.

2. Begin to wind the variac slowly, watching the meter.

3. Watch for a resonant rise. The voltage will begin to increase as
you wind the variac. If the rise goes above, say, 10-12v, stop
winding! You don't want to damage the motor. You will find that the
voltage will peak at some point on the variac's range, and then begin
to drop off as you keep turning. Try again with a smaller cap if it
goes above 10v (or whatever rise you want).

4. If you didn't get a rise, use a bigger cap.
5. Iterate until you get the right rise.

Hope these instructions are clear.

Cheers,

Greg.