Re: Question about John Freau's phase adjustment circuit

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: FutureT@xxxxxxx 

In a message dated 6/28/05 11:49:34 PM Eastern Daylight Time, 
tesla@xxxxxxxxxx writes:


> Anyway, onto the next question: I just got a stock of variacs in, and I
> wanted to build the phase controller. I have a question about the cap
> value to use. According to the directions on John's site:
>
> "To select the proper capacitor size, measure the voltage across the
> motor and look for a 3 to 5 volt resonant voltage rise maximum at some
> point in the variac's range."


Tom,

When the cap is the correct value, then you'll actually obtain the
3 to 5 volt maximum resonant voltage rise.  If the cap is too small,
you'll get a smaller max voltage rise, or no rise at all.  If the cap
is too large, you'll get more than a 3 to 5 volt max rise.  So the
correct value cap will give the 3 to 5 volt max voltage rise across
the motor.  It's a trial and error sort of thing to find the correct
cap value.  I've had many questions about this.  I should add more
details to the explanation at my website.


> Maybe I'm missing something - how will that help me to determine the
> value of the cap?
>
> It turns out that I'm using the exact same teletype motor that John's
> using in that page, and this line leads me to believe that the 47uF,
> 240VAC cap would be fine:
>
> "This controller is for the new sync gap below, but it will work for
> other sync motors too."


The controller design will work for other sync motors, but if
a different motor is used, then a different cap value may be
needed.  Generally larger motors will require larger caps.  But
since your motor is exactly the same as mine, then indeed
the cap value should be the same too I would think.  A quick
test lash-up will tell you if all is well.

Cheers,
John


> Thanks again!
>
> Tom