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Re: Power factor correction yes, but where?



Original poster: "35045 by way of Terry Fritz <twftesla-at-uswest-dot-net>" <free0076-at-flinders.edu.au>



On Wed, 10 Jan 2001, 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 

It sees both. Unfortunately as you wind up the variac the inductance
of the NST seen by the mains goes up due to the transformer ratio changing
so you would really be best to correct at the NST so that it looks
resistive. Then the lowest current will be drawn from the variac and thus
you will run it cooler etc. Also the varying inductance of the NST won't
become a problem since it looks like it has zero inductance if you correct
it properly.

However I don't know how the magnetising reactance of a variac behaves as
you wind it up but I suspect that it would be roughly constant for a given
variac in which case you could correct for the inductance of the variac at
the mains side of the variac also. You probably don't need to worry so
much about the power factor of the variac though if you only want one cap,
just don't attempt to correct the power factor at the mains only as the
inductance of the NST 'seen' by the mains should vary with the variac
setting anyway. (note that the power factor of the NST and everything
after it shouldn't change just because of transformer ratio since the
resistance varies as well. It probably would change though because the
Tesla circuit isn't very linear).

> 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.
> 

Darren Freeman