# RE: Repost: NST VA Rating and Power Factor

```Original poster: "David Dean by way of Terry Fritz <twftesla-at-qwest-dot-net>" <deano-at-corridor-dot-net>

Hi Bill
>
> Original poster: "Bill Vanyo by way of Terry Fritz
> <twftesla-at-qwest-dot-net>" <vanyo-at-echoes-dot-net>
>
>
>
> Tesla list wrote:
> >
> > Original poster: "David Dean by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> <deano-at-corridor-dot-net>
> >
> > Hi Bill,
> >
> > Why don't you just hook the primaries of the 120V NSTs in
> series so you can
> > use the full 240V? You can still parallel the secondaries
> together to get
> > the higher current.
>
> Forgive me if I'm a little dense, but will that work?  With three or
> four NST's?
>

It will work with two, or four quite easily. Three gets a little more
involved.

As Matt D. pointed out, the NSTs should be fairly closely matched for it to
work properly. For example, lets say you want to parallel two 15/60 NSTs and
hook the primaries in series. You would first determine the amp draw from
the line for each transformer with both its secondaries shorted. If the amp
draw of each transformer were two be near enough, say within 10%, then you
could hook the primaries in series and secondaries in parallel to get 15/120
out and work off of a 240V source. If one of the transformers were to have
an internal PFC, and the other did not, that would screw things up. If one
of the transformers were to be a "super heavy duty bulletproof" model (I
have yet to see one) and the other a "super cheesy die as soon as you look
at it" model (most are that type, IMHO) they will probably not match the
primary currents well enough to work together that way.

To do four, use two strings of two in series, paralleled.

To do three, well that would require two 15/30s in parallel in series with
one 15/60.

Anyway, the whole idea is to get the voltage across the primaries to divide
evenly. If you can do that, you are "in like Flynn", if not, you would have
to limit the input voltage so that the single transformer with the highest
internal impedance never sees more than 140V across its input terminals, or
it will be toast.

> Also, my first two NST's were a 15/60 and a 15/30 (different
> manufacturers), and I remember that I originally tried hooking the
> primaries in series (I read somewhere that that was the "right" way to
> do it), but neither side of either secondary was in phase with the other
> - whereas when the primaries were hooked in parallel, they were in near
> perfect phase.  Don't know if that makes sense, but it's what I
> observed.
>

As far as phasing the transformers, if the secondaries are not phased
correctly just swap the wires on the input terminals on _one_ of the
transformers and it will be right.

later

deano
> 	- Bill
>
>
> >
> > later
> >
> > deano
> > >
> > > I want to use multiple american trannies on a 240 line (we have them
> > > here in the US, for things like electric stoves and elctric clothes
> > > dryers), stepped down to 120, for the sole purpose of being able to
> > > parallel three or four 15/60's (I have three now, looking for fourth)
> > > without tripping breakers.
> > >
> > > > Good luck, sorry aobut the l8ness of my reply
> > > > Jason
> > >
> > > Thanks,
> > > Bill
> > >
> > >
> > >
>
>
>

```