SYnc motor mod's and Variacs

From:  Wysock, William C. [SMTP:Wysock-at-courier8.aero-dot-org]
Sent:  Friday, January 30, 1998 7:24 AM
To:  Tesla List
Cc:  ttr
Subject:  RE: SYnc motor mod's and Variacs

From:  Gary Lau  29-Jan-1998 1532 [SMTP:lau-at-hdecad.ENET.dec-dot-com]
Sent:  Thursday, January 29, 1998 3:03 PM
To:  tesla-at-pupman-dot-com
Cc:  lau-at-hdecad.ENET.dec-dot-com
Subject:  SYnc motor mod's and Variacs

>> I'm just about ready to test out my sync RSG with a 15KV/60mA NST.  For a
>> motor, I bought a 1/8 HP 1725 RPM motor for $4.95 from H&R Surplus.  This
>> unit has a 3.5" dia rotor.  I milled four flats onto it, each being
>> 1.00".  While the flats should have been 1.30" to each cover 1/8 of the
>> Gary Lau

<big snip>

I think I see what's going on:  Varying the NST's variac adds a variable
inductance in series with the NST primary and this changes the phase of
the charging cap.  Thus, if I adjust the sync motor housing at 100%
variac, that setting is not valid at less than 100%.  In fact, it
appeared that at one point on the variac, I was completely OUT of phase,
and resulted in near zero cap voltage at each gap presentation and no
firings, not a healthy situation for my cap or NST!

It would appear thet unless I can devise a PLL servo for my motor, sensing
the (now variable) phase of my cap, that the use of a variac could be
hazardous to my cap and NST.  Do others using sync RSG's use variacs, and
if so, do you have to re-tune for phase at each variac setting?

<big snip #2>
Gary Lau
Waltham, MA USA

Gary, all.

In my post I was referring specifically to the use of two variacs
in controlling a NON-SATURABLE CORE transformer; which an NST
is not!  Adjusting power factor, by changing the time lead/lag between
the current waveform and the voltage waveform, is the most efficient
way of regulating the amount of power being fed from a high voltage
transformer into a complex impedence load (which a Tesla coil primary
tank circuit represents.)  This scheme also benefits finding the optimum
point for any given dwell index point on a synchronous RSG.

Bill Wysock
Tesla Technology Research