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Re: SRSG Phase Controller

Original poster: FutureT@xxxxxxx

In a message dated 7/20/05 2:29:24 PM Eastern Daylight Time, tesla@xxxxxxxxxx writes:



Sorry for the delay in replying, been busy of late! I figured the motor
plate specifications (RPM, HP, etc) might not be the best thing to go by.
I'm thinking (and hoping!) there is a more consistent relationship with a
measured parameter, like the actual current (and therefore the
inductance/impedance) the motor pulls under load as an SRSG.



Rob,

Yes, those characteristics may change with the rotor loading, etc.
Perhaps such a formula can be worked out, I don't know.


Correct me if I'm wrong, but in finding the size of the capacitor to use,
its Most important to find a cap that is large enough to prevent the voltage
across the motor from dropping below the threshold where it starts slipping,
while not so large as to overvolt the windings of the motor with runaway
resonance between the variac's inductance and the capacitor. A universal
forumula for the capacitor would have to take into account, then at least:
-the minimum stable voltage, which would depend on the flats ground into the
rotor
-the maximum voltage, probly don't want to push your motor by more than 5V
-motor RPM
-actual measured impedance/amperage of the motor under load



Another thing to consider is that the voltages and currents may be
very spiky.  Not normal looking at all.



First I tried 100uF and that wasn't nearly enough, turning the variac just a
few degrees (from the 0V to 20V position) dropped the voltage across the
motor to below 105V and it starts slipping badly.

Then 200uF got it pretty close, I can adjust the variac from the 0V position
to about the 90V position before it loses sync, and under a flourescent
light I can see the rotor's phase is being affected by up to ~45 degrees. I
don't get any resonant rise though, at minimum inductance the motor has
exactly 120V and it slowly drops as I sweep the variac.



Using 1800 rpm, the mechanical phase will never vary by more than
45 degrees.  So if you want you can use a cap that's a little smaller
than optimal such as the 200uF, and simply never turn the variac too
far so you won't lose sync-lock.  It's not absolutely necessary to
see any resonant rise, and it's not really necessary to be able
to sweep the variac knob though its full range although it's nice
if it can.  Some folks put a stop on teh variac so it can't be turned
past a certain point.


Increasing the cap to 300uF was definately too much, as it goes from 120V at
min. inductance to over 130V by the time I hit the 10V position on the
variac. I've gotta find some more motor run caps, I'm guessing its going to
require somewhere around 220-240uF.



Yes, I seem to remember other folks using capacitance in that range
for 1/2HP motors.

John