Re: NST question-Why a Tap?

["Tesla list" <tesla-at-xxxxxxxxxx>]

Original poster: "Bert Hickman" <bert.hickman-at-aquila-dot-net> 

Tesla list wrote:
> 
> Original poster: "Metlicka Marc" <mystuffs-at-orwell-dot-net>
> 
> bert,
> could this same setup be utilized on an mot by placing a Pfc. cap across the
> tertiary (i think that's what it's called?) winding? if so then this
could be a
> nice application for those that gang the mot's for psu's, giving an
inexpensive
> Pfc. option allowing more transformers on a 15 or 20 amp circuit.
> marc

Marc,

Possibly... however MOT's show significant nonlinear behavior due to
partial core saturation because they are made so cheaply. As such, the
expected impedance transformation from a smaller PFC capacitor across a
HV secondary "reflected" back into the primary circuit may not work
nearly as well as with NST's. And this technique will NOT compensate at
all for the excessive open-circuit primary current that's due to MOTs'
basic lack of sufficient iron and copper... 

-- Bert --
> 
> Tesla list wrote:
> 
> > Original poster: "Bert Hickman" <bert.hickman-at-aquila-dot-net>
> >
> > Tesla list wrote:
> > >
> > > Original poster: "S.Gaeta" <sgtporky-at-prodigy-dot-net>
> > >
> > > Hi All,
> > >
> > > I have a bit of a mystery here. I recently depotted a Jefferson Electric
> > > 12/60 NST.
> > > I found a pfc cap inside. It is 4 uF at 660V. I found that the
primary has
> > > three connections. The AC line is connected across the 55 mHy
section, and
> > > the pfc cap goes across the entire winding. The extra winding
measures 1.24
> > > Henries. Expressed another way, the cap is in series with an inductor
that
> > > is much larger than the primary inductance, and this LC circuit is in
> > > parallel with the primary.
> > >
> > > Ok I just did the math and found that the LC circuit comes out to 71
Hz. OK
> > > so my math or measurements or both is a little bit off, but this LC
network
> > > is intended to be resonant at 60 Hz (close enough).
> > >
> > > So what is the purpose of this LC cicuit?
> > > I thought that all that was required for pfc is a cap across the
line, and
> > > primary.
> > >
> > > Thanks,
> > > Sue
> >
> > Sue and all,
> >
> > Larger NST's can pe purchased in two varieties - one without, and one
> > with built-in power factor correction (PFC). An NST without PFC can be
> > "corrected" after the fact by placing a relatively large capacitor
> > across the input line circuit. As you have observed, an NST with
> > internal PFC actually has an additional winding which acts as a step-up
> > transformer (or autotransformer). The overall "step-up" voltage across
> > the internal PFC cap is typically around 3.5-5X that applied to the
> > primary. By connecting a PFC cap across the stepped-up output, a much
> > smaller amount of capacitance can be used to obtain the same degree of
> > power factor correction as connecting a larger PFC directly across the
> > primary. This helps the manufacturer to reduce costs: he can use a
> > capacitor with a much smaller physical size and cost to perform the same
> > PFC function, and he can add a PFC winding at relatively little
> > incremental cost during manufacture of the primary winding.
> >
> > The "effective" size of a PFC capacitor, when connected to a step-up
> > winding, will be multiplied by the inductance ratio of the total
> > inductance versus primary inductance, or by the square of the
> > transformer (or autotramnsformer) voltage ratio. In the case
> > of your transformer, you measured 1.24 H for the entire winding and
> > 0.055 H for the primary winding. This inductance ratio implies a turns
> > ratio of about SQRT(23) or 4.8, and your PFC cap is "seeing" about 570
> > VAC (hence the 660 volt rating). In effect, in this configuration your 4
> > uF cap "looks" to the incoming line as though it was a capacitor that
> > was 23X as large, or about 91 uF, connected across the primary.
> >
> > Hope this helps explain the mystery!
> >
> > -- Bert --
> > --
> > Bert Hickman
> > Stoneridge Engineering
> > Email:    bert.hickman-at-aquila-dot-net
> > Web Site: http://www.teslamania-dot-com

-- 
Bert Hickman
Stoneridge Engineering
Email:    bert.hickman-at-aquila-dot-net
Web Site: http://www.teslamania-dot-com