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Re: Power factor correction yes, but where?



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

Tesla list wrote:
 
> Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>"
<Mddeming-at-aol-dot-com>

>         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.

	The cap corrects for the 'real load'.  This is some combination of
	the inductances related to the variac and the other transfromers
	and inductors in the power supply 'chain'.  It is useful to recall
	that a transfoerm, as such, is not neccesarily a highly
	inductive load.  The load seen by the power line is 'reflected'
	thru the transformer.  For a simplistic case, if the load on the
	secondary is a nice neat resistor, then the load seen by the
	power line is largely _resistive_, despite the presence of the
	transformer.

	The load in the case of a Tesla Coil is 'complex'.  In any case, the
	best that can be hoped for (and good enough for all practical
	purposes) is an approximately appropriate value.   An exact match
	seems unlikely to repay the effort spent achieving it...  The
	optimum value of power factor cap, for instance will vary with the
	level of output, when the break out has occurred, etc...

	(The usual discussions in text books of 'power factor' tend to assume
	neat inductive or capitive loads.  Neither really applies to a
	Tesla Coil situation....)