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: "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