Re: Power factor correction yes, but where?

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

Original poster: "Bert Hickman by way of Terry Fritz <twftesla-at-uswest-dot-net>" <bert.hickman-at-aquila-dot-net>

Matt,

Excellent question! Unlike most transformers, an NST, MOT, or OBIT is
intentionally designed to introduce a large amount of leakage
inductance, since it's this leakage inductance that limits the short
circuit current so that the transformer can operate for long periods
with the secondary virtually short circuited. 

Now most iron-core transformers are designed to have very low leakage
inductance. This is done so that the secondary voltage won't vary much
as you change the amount of current loading. Transformers designed to
have low leakage inductance include distribution, plate, filament and
variable (Variac) transformers. As a result, your variac adds only a
little bit of inductance when compared to that of the NST. 

But wait... there's more... 
Your variac can actually be viewed as a transformer with a variable
turns ratio. And, any tightly coupled transformer will "transform" the
impedance from the secondary to the primary side as a function of the
turns ratio squared. So, if were to put PFC's on the incoming side of
the variac, you'd likely under compensate at one variac setting, and
overcompensate at another setting. 

That's why you want to place your PFC's directly across the NST's
primaries (i.e., the output side of the variac) and not on the mains
side. 

Hope this helped!

-- Bert --
-- 
Bert Hickman
Stoneridge Engineering
Email:    bert.hickman-at-aquila-dot-net
Web Site: http://www.teslamania-dot-com

Tesla list wrote:
> 
> Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>"
<Mddeming-at-aol-dot-com>
> 
> Hi All,
>         I don't think this has been asked recently. If the purpose of the
> PFC cap is to make the load on the lines look resistive rather than
> inductive, and your have a Variac across the line upstream of an
> NST/OBIT/MOT/ etc., isn't the power source "seeing" the inductive lag caused
> by the Variac rather than that caused by the Xfmr?  If so, should the
> correction be made at the Variac rather than at the Xfmr, or would both be
> recommended?  I don't think I've ever seen the correction equations for
> sizing a cap to a Variac. I have 55 15uf -at- 660 V run caps to play with, if I
> need to use a bunch.
> 
> Thanks,
> Matt D.