Re: transformer question

Hi Grayson, James (The FCG), all,

> Original Poster: Grayson B Dietrich <electrofire-at-juno-dot-com>
> Otherwise, I think that one could also wire both the primaries and
> secondaries series, for the same effect.

No, you canīt do that AND it wouldnīt work the way you expect. Hereīs
why: all NSTs (known to me) are center tapped. This center tap is
designed to be earthed (see Fa. Tomīs very good explanation as to
why this is done). If you now series two such transformers and earth
the cases like you are supposed to, you will effectively be shorting
out the two "inner windings". Visualize what you are doing: your 1st
earth goes to the 1st NST center tap and the second earth goes to
the 2nd NST center tap, so you have just shorted out one half of the
1st NST and one half of the second NST. While this wonīt harm the
xformers (as they are current limited), you will only have 1/2 of what
you are expecting, which is already only 1/2 of what the NST is really
capable of.

;o( ???;o)

2x NSTs 120V to 9kV-at-30mA each
Series the primaries: 60V per primary
=> Each secondary half (!) gives you 9kV/2/2 or 2.25kV

As you are seriesing 4 windings and shorting out the two
inner ones, this leaves you with 2 x 2.25kV or 4.5kV, which
is exactly one half of what ONE NST can supply in normal
service life.

>That way, each transformer would be running at half voltage, but
>would it still double the amperage? I dunno, so stick with
>paralleling them.

Any time you series two voltage sources, you will increase the
total voltage across the outer terminals. The current remains
the same. The electrons are still being "pushed" by only the
current of one xformer. If the two voltage sources are of unequal
(current capability) type, then the maximum current you can pull
from the seriesed source (within reason) is the lowest capability
of the two.  In the case of seriesing (in general) you DONīT
need the same voltage or the same current ratings.

If you parallel two voltage sources, then the voltage remains the
same and the current increases (simply current "a" + current "b").
You can use different supply currents(!). They do not need to be
equal. That being said, you MUST (esp. with non limited sources)
have equal voltages. Otherwise the two sources will be fighting
each other and you will either get a lower voltage or not current
"a" + current "b" (it will be less).

Maybe you were confusing current and VA, however. To increase
the VA of a certain setup, it makes no difference if you series or
parallel the xformers.

Series Example:
2x 120V to 9kV (non center tapped units !!) at 30mA
Primaries in parallel, secondaries in series:
Each unit: 9kV-at-30mA = 270VA
Series setup: 2x9(=18)kV*30mA = 540VA, which
is composed of 2x the unit: 9kV-at-30mA = 270VA
or 2 * 270VA = 540VA

Parallel Example:
Source: same as above
Primaries AND secondaries in parallel:
Each unit: 9kV-at-30mA = 270VA
Setup: 9kV*30mA* 2 units = 540VA, which, too, is
composed of 2x the unit: 9kV-at-30mA = 270VA or
2 * 270VA = 540VA

The series example will ONLY work with non center tapped (and
grounded) secondaries. This being said, here comes the next
limitation to seriesing high voltage xformers: You can series (and
ground the newly "created" center tap) only two HV xformers,
due to insulation problems. Each HV xformer is built to withstand
(plus some safety margin) itīs own voltage only. If you series two
of these AND ground the center tap, then each xformer never sees
more than itīs own voltage. For example: when you series two 9kV
xformers that way, then you donīt get 18kV above GROUND, but
rather -9kV/0V/+9kV. Of course, you DO get 18kV at the end terminals,
but each is only itīs own (rated) 9kV above ground. If you further
want to series more xformers, this gets very tricky, very quickly,
because you now need to insulate the next set (pair) of xformers
at 9kV above ground with an isolation xformer, which needs to be
rated at 2x the single HV xformerīs VA rating. Believe me, this
ISN`T really worthwhile. This is also one of the reasons, why MOTs
are not a too hot choice as a coiling PSU. What I mean is using
more than 2 plus voltage doubling (and current "halfing"). For
example, if you want to series 6 MOTs, you would need 12 MOTS
total, using 6 of the 12 as the actual PSU and the other six to
build the necessary isolation xformers. Of course, there are "tricky"
guys out there (look at Jim Luxīs website), that have found a
method to get around the isolation xformer problem, but these too,
have their limitations AND more important, their expenses.

Phew, pretty wordy, ha? Still, I hope it helped clear up some of
the mystery, tho.

Coiler greets from Germany,