Hi.
The problem is the primaries.
You'll need to either have a third transformer as a step down (from EU to
US voltages) or separate the common (star) point of each transformar
primary, and wire them in series on a phase by phase basis.
But...
Many US transformers do not have much in the way of a safety margin for
elevating a primary well above what was expected by the designer, unlike
most European made transformers.
Also, you need to make suer that they are rated for 50Hz too, or you'll
have to under run them to prevent excessive heating.
Lastly, yes, with care, you can wire the secondaries for 12 point
rectification and reduce the ripple. But, to do that, one must be wired
in Star formation, the other in Delta. Each needs it's own rectifier,
and you then series or parallel connect the DC side.
This is (or used to be) a common thing to do in high power TWT amps, for
the TWT collector supply, lots of power, with next to no energy storage
(limiting the damage when the tube arc's).
Doing that however, does need very close matching between the individual
transformer AC output voltages, that will be a problem with two identical
transformers, as one wired in Delta, the other in Star will produce very
different voltages, even though the phases will interleave nicely.
It's also too easy for one parallel connected transformer, to do most of
the work, and cook as a result.
This sort of thing is usualy done with either a single transformer with
two sets of secondaries, or two transfomers specialy designed to work
together in that way.
Remember, that the voltage ratio between Delta and Star connected 3 phase
supplies, is related by the square root of three, or 1.732.
This was all I could find on the subject, in a short time.
www.pes.ee.ethz.ch/uploads/tx_ethpublications/gong_ICIT03.pdf
(12 point rectification, at 400Hz.)
Take care.
Dave G0WBX.
------- Original Message -------
Date: Tue, 25 Oct 2011 16:10:11 +0200
From: "Carel, pc5m" <pc5m@xs4all.nl>
Subject: [Amps] running two 3 phase transformer serie/parallel
To: <amps@contesting.com>
Message-ID: <002c01cc931f$cf5846c0$6e08d440$@nl>
Content-Type: text/plain; charset="us-ascii"
Hi,
I have two identical 3 phase high voltage transformer (each around 4KVA
/50 kg/ 100 pounds). Issue is they are designed for US 3 phase voltages.
(so 120 / 204V) so can't use them in Europe as is. What I would like to
dois to put the 2 units at the primary side in serie and at the
secondary side in parallel. Primary and secondary in wye config.
Q: Would there be any benefit of connecting the secondary windings of the
two transformers first parallel and than rectify (with only one rectifier
bridge) or the other way around ? For me it is much easier to first
rectify both transformers and than put the rectified voltage parallel
(only one highvoltage interconnect between both the big/heavy
transformer boxes.).
I have seen some constellations with two 3 phase transformer to gain some
benefit in creating additional phases in order to reduce the ripple
voltage even more. Don't know if I could utilize such a thing as well ?
However, if I recall they were placed in parallel at the input.
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