Three-phases transformer to Single-phase isolation transfomer conversion

[segaura]

Hi all.
I have this three-phases 3,5kVA which has star wiring both in primaries and in secondaries and, if I understand correctly, is able to output 3x55V.

I want to convert it to a single-phase, 230V-to-230V isolation transformer, if possible.

I accept to remove the current star wiring to gain access to single windings and then wire them in a different way.

I am looking for the best topology, maybe something like sending 230V AC in the first primary winding and receiving isolated 230V AC from second and third primary windings:
- putting these "outputs" in parallel
- not using the three (once-were-55V) secondaries ...and I am interested in whether this introduces some inefficiency

I am currently not considering cutting the core, to keep only first two sections and get rid of the third one.

Is the whole thing feasible?
Are some of my assumptions wrong?
Do you have any suggestion?

[CaptDon]

If you isolate the windings from each other there is a possible way to do what you want (actually several ways). I would probably not use the section on the far right. Use the left side as your primary and the center section as the secondary. There is a chance that coupling the 55 volt windings of the left section and center section would be an advantage but be aware of correct phase connections of the windings. I don't know if trying to use all three sections would be better or worse for efficiency? You could have two primaries and one secondary or one primary and two secondaries but I am not sure that offers any advantage over one-in / one-out and ignore the third section. Let's see what other folks offer. I does seem likely that with only one core as the primary the total flux would divide half and half across the other two cores, this would represent a huge problem / loss and would mean you will have to use all three cores in some fashion. The two outer cores as the primary and center core as secondary?

[Terry Bites]

yes1 to p2 should b fine

With separated primary and secondary windings, 3x 55V, else the best you will get is 2x55V. Putting primaries in ll will not affect the secondary voltage. The number of primary turns and the flux density are not affected by doing so. I will reduce I^2 R losses in the primary by a factor of 3

Here are all the possible outputs for a 230V input

31.625   63.25   94.875
63.25   126.5   189.75
94.875   189.75   284.625

[segaura]

There is a chance that coupling the 55 volt windings of the left section and center section would be an advantage

If you mean wiring only 55V coils instead of wiring only 400V coils, ok, they have 7 times less turns and probably have 7 times bigger sections:  big section is good, I don't know if low turns number is an issue.

Anyway, if the magnetic flux of the primary splits itself in two equal magnetic fluxes in other cores, I expect the outcome is more like in the "series" attached picture than in the "parallel" one, am I wrong?

[TizianoHV]

I don't know how they connected the voltage taps (400-440-480V) But if you manage to insulate the three primary windings you could connect two 400V in series as primary and use the third one as insulated secondary (see photo).

[segaura]

There are three 480V primaries, one per core, each one has a 400V and a 440V tap, a few turns before the end.
I easily managed to free all coil ends and wire them as I proposed above, the output-coils-series picture.
230V (measured) mains in the first core primary 480V coil.
Second (closest) primary 480V coil shows 193V.
Third (farthest) primary 480V coil shows 35V ...magnetic flux clearly do not split in equal halves.
Second and third in series shows 228V.

I am already good like that but my feeling is that Tiziano's suggested scheme is better, with mains in outer coils in series and output from the center one, where each external core can have its best influence.

Last thing I don't know how to address is to decide if it is better to use the 480V coils or the 55V, clearly biggier section is a good thing, but I don't know if having close to 9 times less turns could be an issue.

EDIT: i tried Tiziano's scheme with 480V coils and the feeling is that the efficiency increased, now I get 229V in the (single) output coil