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Re: SRSG Phase Control Question



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

In a message dated 8/28/01 10:52:18 AM Eastern Daylight Time, 
tesla-at-pupman-dot-com writes:

> 
>  I seem to only get about 15 to 20 degrees of the SRSG adjustment which is
>  probable sufficient but I had thought I might see more. 

Dave,

Are you seeing 15 to 20 degrees of mechanical range, or 
electical range?  The electrical phase shift should be able to
achieve 90 degrees of shift.  If you are using an 1800 rpm
motor, the mechanical phase shift will be 45 degrees for
a 90 degree electrical shift.


> I tried a 47uf,
>  25uf, and 15uf and found the 15uf worked best and caused the motor to make
>  the least vibration toward the extreme end of the variac's range.

Did you measure the voltage across the motor?  That is important.
It shouldn't go higher than 5 or 10 volts over the line voltage anywhere
within the variac's range.  But it should probably give at least a 2 or
3 volt rise above line voltage somewhere along the variac's range.
The cap value will determine the voltage rise.  You may need to
use a cap value that is midway between some of the values you
tested.  The proper cap value can be very critical for best results.
I try to get it to within 1 or 2uF of the ideal value.

>  I also
>  note that I can only go to about 75% of the variac's range until the 
voltage
>  suddenly drops and the motor ceases to spin. 

This is OK as long as you get the full 90 degree electrical phase
shift (or if you're satisfied with the amount of shift you're getting).
Of course you'll only be using the range where the motor stays
sync-locked.  A voltage measurement across the motor will tell
you a lot.

> I should add that this is a
>  purchased synchronous motor rather than a modified AC motor if that might
>  make any difference.

It shouldn't matter.

John Freau

>  
>  Please let me know if this is expected behavior.
>  
>  Dave