Re: SRSG Phase Controller

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Rob Judd" <canska@xxxxxx>

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.

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


I had opportunity yesterday to experiment with my SRSG. My 2A fuses (both
fast and slow burn) blow immediately, regardless of what cap size I use. I
think the startup current of my motor is just too much. Increasing the fuse
to 4A solved that problem.

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.

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.

I'm going to keep experimenting on this, I'll let you know how it goes!

Thanks again!

Rob Judd - canska@xxxxxx
http://homepage.a5.com/~canska/tesla



> Original poster: FutureT@xxxxxxx
> In a message dated 7/17/05 2:50:53 AM Eastern Daylight Time,
> tesla@xxxxxxxxxx writes:
> Rob,
> I had looked a the idea of making a formula but there were many
> variables.  For example if the motor happens to be a cap run
> motor...  (This is a motor that uses a cap across one of the windings.
> This cap is always connected during use.) .... Then less
> capacitance is needed for a given motor HP.  To some
> degree there's almost a direct relationship between the
> amount of C in the cap run cap, and the amount in the
> phase shift cap.  For example if a particular HP motor is
> predicted to need about 67uF, it may only need 30uF if it
> uses a 25uF run cap also.  In one case which used a large
> run cap, not phase shifter cap was needed at all.
> Also the rpm of the motor seems to make
> a difference.  An 1800 rpm motor needs a different amount
> of capacitance than a 3600 rpm motor even if they are of the
> same design and brand and HP.  In some cases I've heard
> of folks needing different capacitances even for motors of the
> same HP, rpm, and type.
>
> So a formula could be made, but it would just be a guide,
> and/or it must take into account the type of motor, and the
> rpm.  Or 2 separate formulas could be made to cover
> the situation.  In many cases it's possible to predict pretty
> closely what value cap will be needed for a particular motor.
> I made up a chart of the known C values for various motor
> types and HP's, etc.  I may not have had room to post that
> at my website, but I had posted some info on this list in the
> past.