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Re: just wonderin... expanded, And more thoughts
Hello Ed, Bert,
Comments down below.
>Original Poster: Bert Hickman <bert.hickman-at-aquila-dot-com>
>> Original Poster: "Seekins" <seeked-at-sover-dot-net>
>>
>> To expand on Christopher's Questions,
>> I was wondering if anyone could maybe comment on NST matching.
>> I've seen what seems to be conflicting info. regarding how close the
>> xformers should be.
>> Some seem to be saying that even a few volts difference on the
output(s) of
>> a pair of xformers paralleled together can cause problems.
>> Others seem to imply that they just throw together a couple or more
with the
>> same faceplate ratings and let 'um rip.
>> Anyone have any comments that might illuminate this area?
>> If it is an issue, how much difference in output voltage is
"tolerable"?
>> (recommendations on the poor man's approach to measuring the
difference
Bertīs comments:
>As long as the transformers have the same output voltage faceplate
>rating you should have no problems mixing and matching. They do not
need
>to have the same output current ratings. You should verify that each
>transformer works independently (by drawing arcs from each bushing to
>the transformer's case). Don't connect a transformer that's got a
"weak"
>side, since it'll reduce the output of all the others when you connect
>them in parallel. There's no need to actually measure the transformer's
>output voltage.
>
Edīs question is indeed a good one and not (I think) that easy to
answer. While most coilers probably do as Bert suggests and
get away with it, Iīm not sure if this is THE correct way.
Iīll try and explain what I mean: The problem we coilers face, when
paralleling xfomers, is the spark gap. Huh?!?. Yes, because the
spark gap is either an open circuit or an almost(!) short. So, it
is a non constant load and that is where the problem arises.
This means during non conduction, we soley have to match the open
voltage. If (in a 2 xformer setup) the first transformer only supplies
14kV and the other puts out 15kV (real measured output, I mean),
there is a potential difference of 1kV between the two. This, in turn,
means transformer one acts a load for the second (15kV) xformer.
So, effectively, the true output voltage of this paralleled setup will
be between 14 and 15kV (depending on which transformer is
"stronger").
Now, when the gap conducts, we have a somewhat different case:
The spark does not present a full short. Now the question is which
transformer break down first and then how much. If the 14kV (e.g.)
unit is much stiffer, then the 15kV unit is now the "load", etc. NSTs
are current limited and the current usually isnīt very high (which
doesnīt mean it isnīt deadly), so paralleling NSTs might not be a
true problem in the real world (esp. since coil(er) run times are
pretty low and NSTs are designed for a 24hr/day/year operating
period). However, paralleling a PT or a pole pig, will probably
lead to the problems mentioned above, unless they are of the
exact same type.
FWIW
Coiler greets from germany,
Reinhard