Re: Power Factor Correction weirdness?

• To: tesla-at-pupman-dot-com
• Subject: Re: Power Factor Correction weirdness?
• From: "Malcolm Watts" <malcolm.watts-at-wnp.ac.nz> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
• Date: Wed, 09 Feb 2000 18:46:33 -0700
• Approved: twftesla-at-uswest-dot-net
• Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme

Hi Adam,

> Original Poster: Adam <adamsmith-at-mediaone-dot-net> 
> 
> 
> 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?

Yes.  If you keep on adding caps, at some point, current will start to 
go back up. With a short across the secondary, the primary flux is 
coupled mostly to the primary so what the mains sees from the 
transformer is almost pure inductance. Try putting a load on the 
secondary equivalent to the transformer's V/I reactance (rather than 
the short). 
You should find that the PFC capacitor value you need to minimize 
primary current is smaller. Don't forget that minimum primary 
current will now be higher than with the shorted secondary since 
power will now be dissipated in the load.

Regards,
Malcolm

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