Re: 3 phase to single phase, transformer wiring VTTC

Original poster: FIFTYGUY@xxxxxxx In a message dated 8/26/06 10:30:38 AM Eastern Daylight Time, tesla@xxxxxxxxxx writes:
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>Yup, 3x current, but limited to whatever the Y voltage is (otherwise
>the core will saturate). Phasing them all right will be the confusing
>part (if ones out of phase that's a short).  3 phase kva = V*A*1.73
>with V and A in relation to gound. But in single phase mode you'll
>end up with 3x original kva rating.

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I gave this some thought, but wouldn't you be limited to twice the current rating of an individual winding? Obviously, no single winding can handle more current than it's rated for. So that puts one limitation on the possibilities. But I think there would be a limitation due to flux density in the core. Since a one-piece 3-phase transformer designed for a balanced load shares the same core for all three windings, running each winding in single-phase would cause a core saturation problem somewhere. For instance, in normal 3-phase mode, each of the three "I" legs carries a flux that is shared by each of the other two legs. But in single-phase mode, with two windings in parallel, the "I" legs of each core support the full flux all the time. If you add the flux from the third leg, there's no "return path" for the flux from that leg. All the 3-phase transformers I've seen for balanced loads are of the "core" type construction, with the three "I" legs plus top and bottom core sections to link them. As I understand it, transformers for UN-balanced 3-phase loads have additional "I" legs to the outsides of the outside windings, and extended top and bottom cores to link the flux to them. Using a bank of 3 single-phase transformers (i.e., three pigs on a pole) is more versatile because each transformer is complete unto itself, and the flux in each phase is never magnetically linked to the other transformers. Such an arrangement would be readily convertible to single-phase operation. However, then you've got the problem of circulating current from imbalances, since it's not "sorted out" in a common core. Hence the issues with balancing chokes on big variacs. I've worked on high current, low voltage (12V 1000A) systems where circulating current problems with parallel transformers and parallel cable runs cause huge imbalances. Obviously not as much of a problem at "normal" current levels, but try to keep all connections and cables identical.
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-Phil LaBudde
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