# [TCML] PFC Question (again)

bartb bartb at classictesla.com
Tue May 20 19:17:30 MDT 2008

```Hi Ed,

A program can only ballpark the PFC as based on the transformer,
however, system components will cause the ideal PFC to vary from this value.
Here is what Javatc states about the PFC value listed:
---------------------------------------------------------------------- *
Power Factor Cap Size*

The capacitor size is in uF to correct the power factor based on the
transformer.
This is a ball park number as the actual "best" PFC capacitance may vary
above or below this value due to other components in the system.
The PFC Cap is connected in parallel with the transformer input winding
(directly across the low voltage input terminals).
-----------------------------------------------------------------------

PFC cap size for lowest current needs to be measured and played with to
find the best capacitance to use. Javatc is simply throwing out a rough
value for a place to start. It is not intend as a value that is a
"golden" number.

Too often, any value out of a program is assumed to be "the value" to
use. Well, that depends on the situation. This is why Javatc has a link
on each and every output to help identify what this value means.

FWIW, the PFC approximation equation used is: PFC(uF) =
(VA*1000000)/(2*PI*Hz*Vin^2). This is very simple and certainly a
ballpark number.

Take care,
Bart

Ed Phillips wrote:
>    I can't answer the original question here but want to pose one of
> my own which I've asked before without any real answer.  How can ANY
> program determine the "correct PFC" value?  With a low-reactance
> transformer and capacitive load I would think the power factor at the
> primary terminals would already be leading and added shunt capacitance
> would only increase the line current.  It's pretty obvious that when a
> high-reactance transformer like an NST or OBIT is connected to a
> resistive load the input PF will be lagging and shunt capacitance can
> reduce line current but beyond that how can any generalities apply?????
>
>    How much of useage of  power-factor correction is factual and how
> much is urban myth?
>
> Ed
>
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```