# Re: Power Factor Correction weirdness?

• To: tesla-at-pupman-dot-com
• Subject: Re: Power Factor Correction weirdness?
• From: Terry Fritz <twftesla-at-uswest-dot-net>
• Date: Wed, 09 Feb 2000 18:53:14 -0700
• Approved: twftesla-at-uswest-dot-net
• Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme

```At 11:11 PM 02/08/2000 -0500, you wrote:
>
>Can someone explain what I'm seeing when I attempt to power factor correct
>my neons? I've got my DMM in series with the primary of a 12,000 60mA NST,
>and the secondary of the NST is shorted.  With no PFC caps, it reads 7.13
>Amps.  But when I start adding caps in parallel with the NST primary, the
>current reading drops dramatically-- more than I expected it to.  Here are
>the results:
>
>    PFC Caps    Ammeter Reading (amps)
>
>    0                7.13
>    50 uF            4.96
>    100 uF           2.81
>    150 uF           1.14
>
>What's going on here? is this right?
>
>

Hi All,

I am not the biggest expert in the world on power factor correction so I
may be just reinventing the wheel or be totally off track...

I looked at my coils and the PFC caps I use and the basic principle behind
Adam's test seems to be a useful way of selecting PFC cap values.
Basically just correct for the transformer's VA rating.  In the non-linear
operation of a TC there are a few inaccuracies but they do not seem very
significant.

The VA rating on my big coil is 900VA, which implies a PFC cap value of
166uF.  The actual value is 200uF.  On my little coil the VA rating gives
49.7uF where the actual value is about 60uF.  Not exact but close enough.
Thus, I would propose the following equation (unless everyone has been
using it for years and I am the last person on earth the figure it out ;-))

Cpfc = ( Vo x Vi ) / ( 2 x pi x f x Vi^2 )

Where
Cpfc = Power factor cap value in Farads
Vo = Rated NST output voltage in volts
Vi = Rated NST output current in amps
pi = 3.14159...
f = AC line frequency (50 or 60Hz)
Vi = AC input voltage (120 VAC)

For a 15kV/60mA transformer you get:

( 15000 x 0.06 ) / ( 2 x 3.14159 x 60 x 120 x 120 ) = 165.8uF

To get "exact" you have to take all the non linearities, timing, and such
into account which is a few orders of magnitude harder but this equation
seems to do fine...