Re: NST question-Why a Tap?

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

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

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