Re: SRSG remote phase controller

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

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

In a message dated 1/25/04 2:15:56 PM Eastern Standard Time, 
tesla-at-pupman-dot-com writes:

>to determine the capacitor size. Could someone help me out? Also, I have
>a 3.5 amp variac laying around. Is there some reason this wouldn't work?
>the motor only draws 1.5A.
>
>Thanks
>Steve Vance
>


Steve,

Your variac will work fine for the phase shifter.
To select the cap size,  try a certain value for example 25uF
and see how it works.  Measure the voltage across the motor
terminals when the circuit is all built and working and see
if the motor voltage goes 5 volts higher than the line input
voltage at any position of the variac.  In other words sweep
the variac knob slowly through its range and see if the voltage
across the motor ever goes 5 volts higher than the line voltage.
For example if line voltage is 120 volts,  you're looking for
a max of 125 volts across the motor at the point when the
variac gives the max voltage across the motor.  This max
voltage could occur at anyplace on variac dial, perhaps
1/3rd of the way to the end or whatever.  You're looking for
a slight resonant rise.  This helps the phase shifter to keep
the motor locked for the full ranged of the variac knob.  I mean
locked in sync at the sync rpm.  If you get more than a 5 volt rise
across the motor, then use a smaller capacitor.  If you get
less than a 5 volt rise over line voltage across the motor, then
use a larger capacitor.  It's a trial and error thing to find the
correct cap value.   Also the
rise does not need to be exactly 5 volts over the line.  It can
be a rise of perhaps 3 volts to 7 volts.  Some systems even
work with no resonant rise at all.   With no rise, the motor
may drop out of sync before the end of the variac's range.
You have to try it and see what happens.  Every motor and
combo is different.  You have to go though the trials to see.
If the motor loses lock near the end of the variac, some folks
simply limit the knob range so they don't lose lock.  Let
me know if my explanation is still not clear.

John