Re: Phase adjust

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

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

In a message dated 12/24/00 4:44:24 PM Eastern Standard Time, 
tesla-at-pupman-dot-com writes:

>  As I start to increase this variac, the motor voltage falls 
>  >quickly and within maybe 20 % of increase, falls below 100 volts and it 
> blows 
>  >the fuse, which is a 3 amp fast blow type.  I tried it again, same 
result.  
> 
>  >Do I need much larger caps with a 1/3 H.P. motor?
>  >
>  >Ed Sonderman

Ed,

My guess is you need at least 120 uF or more.  I'd think you'd
want to be able to reach at least 1/2 variac range before losing
lock, and assuming you get about 90 degrees of phase shift
on the motor before losing lock.  I usually set the cap value to
be able to achieve a full variac range without loosing lock, but
this is not necessary.  As long as you can get the 90 degree
phase shift, it is OK to use just a part of the variac range, if
you're satisfied this way.  Just be sure not to turn the variac
lower than the "lock point" during operation.  I've only tried up
to a 1/10 HP motor in my tests, but a few other folks have
used 1/4 HP motors and used about 100uF.

Yes, as Terry suggested, I hope anyone who tries the phase
shifter, will report on the cap size they needed for a particular
motor size, what the phase and variac range was, etc.  

I like to measure the voltage across the motor, and look for a
max 3 to 5 volt reso-rise above line voltage at some variac
setting.  This reso-rise will vary with the cap size.  Select the
proper cap size to obtain this amount of rise.  Less rise is OK
too, if the motor shifts and stays locked OK.  This works well
with the smaller motors I've tested.  I suppose the same approach
will work well with larger motors.

John Freau