Three Phase to Single Phase via Transformer

[BergmanJ]

Folks,

There is a way to convert 3-phase to single phase; but, as BT indicated above, the efficiency is questionable, at best.

Here goes: Scott-Tee from 3-phase @ 120 degrees = 2-phase output @ 90 degrees; then series connect the two output windings with center connection isolated and insulated. Output from two "outside" leads at 240 volts (yes, need to "figure-out" the appropriate turns-ratios to obtain 240 here) to center-tapped auto-transformer (re-establishing "neutral" for 120/240 "split phase" from the center tap.

Again, not efficient; and, I have not figured-out the power-split ratios from all 3-phase windings; but, believe they are not evenly split. Don't know how this compares to derated single-phase use from 3-phase gen output, either. Guessing not a good!

This is more or less just an excercise for those interested in transformer phase conversion.

The direct opposite was used decades ago in 400 Hz aircraft power supplies to provide precision power to electro-mechanical gyroscopic navigation systems: Single phase to + and - 45 degree sources (provides 90 degree total differential outputs -- call one "zero" and the other "90") into two power amplifiers, then into scott-tee power transformers for three-phase power output to drive the gyro motors.

Regards, JLB

[armandh]

getting the right voltage and phase relationship is NOT the same as getting the same power output.
the two [zig zag] legs in series use 2/3s the windings and power to produce half the output

[BTPost]

If you choose the right Transformers with enough Copper & Iron, then you will have your correct AMP thru-put in the conversion....

[armandh]

generator too just like an extended stack

OK for a 240/480 delta or 380Y generator transformed to 120-0-120 single phase

but without enough Fe & Cu in the generator, problems still {IMHO}

there is always a weak link in the power chain. by design it should be the breakers or fuse.

no matter how it is configured.....

both the generator engine and the generator it self should handle more than the breaker,

or longevity will suffer.

[TomBall]

So in reading the responses, am I safe in concluding that if you had a three phase generator laying around and someone said you could easily wire it to run single phase power during outages, it is not that easy or efficient?

[Vanman]

Many of them can be reconnected for 120/240 single phase, but all of them will be limited to 2/3 of their original three phase kva. That said there exist sets that are simply equipped with a larger generator, "extended stack" in Onan lingo, that have their three phase rating decreased such that they are rated the same kw for both single and three phase.

[armandh]

Originally Posted by Vanman

Many of them can be reconnected for 120/240 single phase, but all of them will be limited to 2/3 of their original three phase kva. That said there exist sets that are simply equipped with a larger generator, "extended stack" in Onan lingo, that have their three phase rating decreased such that they are rated the same kw for both single and three phase.

"that have their three phase rating decreased"
[because the engine is smaller and matches the single phase output ability of the greater at 3 phase generator]