Re: Calculation of PFC Capacitors using LTR Capacitor

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-qwest-dot-net>" <evp-at-pacbell-dot-net>

Tesla list wrote:
> 
> Original poster: "Barton B. Anderson by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <tesla123-at-pacbell-dot-net>
> 
> Hi Ed,
> 
> Tesla list wrote:
> 
> > Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> <evp-at-pacbell-dot-net>
> >
> > Tesla list wrote:
> > >
> > > Original poster: "Barton B. Anderson by way of Terry Fritz
> > <twftesla-at-qwest-dot-net>" <tesla123-at-pacbell-dot-net>
> > >
> > > Hi Dan,
> > >
> > > The tank cap won't affect the PFC value.
> >
> >         Why not?  Please explain.
> 
> The transformer is the reactive load we (they) are concerned with. The tank
> circuit downstream
> should only be of concern if it was greatly inductive or capacitive in
> nature. However, it's
> resonant. It's the magnetizing current in the transformer which causes the
> reactive component.

	I haven't measured the magnetizing current of a really big transformer,
but it is only a couple of amps for the 115V/12 kV, 60 ma transformer I
use with my little coil.  Hardly seems worth cutting that down to zero. 
I've tried using a capacitor to "tune" that down to about a half amp,
but with the coil running the capacitor didn't make an observable effect
on the primary current of about 7 amps.  Hardly seems worth the trouble
if all you're trying to do with the capacitor is supply the magnetizing
current.  Am I missing something here?

> The reactive component is current which is circulating around doing nothing
> "useful" (but it is
> still working, that is, working at heating up wire upstream in the form of
> I2R losses while it
> sloshes around). The PF capacitor of course brings the PF back into or near
> unity (W/VA).

	Agree.  The capacitor will indeed reduce the no-load line current. 
Still question whether it is worth the bother.

> The
> coil itself still uses the same amount of power regardless. When we measure
> current (before the
> transformer) we also measure the reactive portion along with the portion
> that is doing
> something useful (entertainment mostly).

	I agree with all of that.  Despite what some of the mystics seem to
think, the main purpose of TC's is to make big arcs and lots of neat
noise.

	
> Often when we compare our coils, we think of it in terms of Watts to coil
> output. In reality,
> we compare reactive + real power to coil output. Because the reactive
> component differs form
> transformer to transformer, we really can't do a good job at output
> comparison without first
> correcting the PF. Once done, then we get a real comparison of Watts to
> coil output.

	I think I agree with you there.  But line voltage times line current
won't "equal watts" unless the operating PF is unity.  The input
reactance of a transformer loaded with an LTR tank capacitor (L for
Larger, not for Less) will be inductive, but the "operating reactance"
with the gap banging away can be a lot different from the no-load
"magnetizing reactance", and no fixed value for the PFC can be correct;
it must be determined by hooking an adjustable capacitor across the
transformer and then varying it for minimum line current with gap
firing.  The formulae I have seen seem to give the same value
independent of the secondary load, and I can't see how that can be
correct.

> The funny thing about all this PFC business is that the transformer and
> coil still use the same
> amount of real power regardless of PFC use. It's  really only important to
> everything upstream
> from the transformer and the current we read on the meter. This to me is
> one of those areas I
> still don't quite understand why we worry about it so much. Yes,
> transformers cause reactive
> power, but, I see little use of a PFC (except to identify how much real
> power is entertaining
> us). Just my opinion.
> 
> Take care,
> Bart

	I agree with that last sentence completely, and also with the philosphy
of the paragraph.  The reason I wrote the questions is that I get the
impression from what I read here that a lot of fellows think that a PFC
is a necessity.

Ed