It is possible to use buck-boost transformers, 3 X 240/32V connected in wye if the OP really wanted. The output voltage will be 471V (On the high-side, the voltage to neutral will be 272V or a phase voltage of 471V if the input voltage on the low-side is 240V). On the problem with grounding, the wye point of the transformer bank solves that.
or even 240 to 277 or vice versa.
It is just another name for "autotransformer". Add a winding in series with another winding and depending on if it is connected in additive or subtractive manner the voltage across both windings will be more or less than what the input voltage is.
Physically yes, but seems like there was a code section prohibiting the former?
No, that doesn't work with a 240V delta source and autotransformers, although it would work with a 416Y/240V wye source.
You could use three buck or boost transformers on each phase, all affecting the same end of each phase winding in rotation. But it can cause ground reference problems.
IIRC they don't like the idea of not having a grounded conductor for a 277 volt circuit for some reason. It does work though.
Let me try again. No, starting with a 240V delta, you can't use 240V/32V transformers as boost transformers to get a 480V delta. [Well, you could, but it would take 14 or 16 of them.] With autotransformers you can't rearrange the voltage vectors from a delta to a wye, as the new voltages you get are already referenced to the existing voltages. You'll need 240V/240V boost transformers to get a 480V delta.
Sure, but you'd need 240V/240V boost transformers, not the 240V/32V transformers referenced in the post I was quoting.
The NEC does not permit that, as per the sections I posted. It would permit using (2) 240V/240V boost transformers to turn a 240V corner grounded delta into a 480V corner grounded delta.
Keep in mind what is typically marketed as buck/boost is nothing more than a 120/240 by 12/24 or 16/32 isolation transformer. It only becomes an auto transformer because the way you connect it and you could connect pretty much any transformer as an autotransformer.
True, and they make good general-purpose power-supply transformers, too.
On difference is that B-B transformers have primary-level secondary insulation.
That's correct. You can't use those as autotransformers to boost 240V to 480V. For that large a boost, you'd need to start with 240V by 240V transformers, then you could boost 240V to 480V using them as autotransformers.
Or wire them as isolation units with the primaries in parallel and the secondaries in series.
That would take 2 such transformers to get 480V 2-wire from 240V 2-wire. While as a boost autotransformer you can get the same maximum current at 480V using one such transformer.
That’s why in the states we use regular step up transformers when we do a big jump like that. I would say overseas, the voltages vary so much country to country, and it makes it easier to have the multi tap transformers when equipment is moved. Most of the equipment I’ve hooked up lately is from Italy, and is connected 400 volts.
Much of that equipment probably is fine when supplied anywhere between 380 and 415 I would guess.
Yeah, but all I have at the plant is 480 and 240, so I have to step it down. The couple of pieces of American equipment is rated for 480.
I’ve used a 480 delta to 120/208 transformer on a 240 volt delta service to get the 600 volts. That was about 30 years ago, and as far as I know, they are still using it. Corner grounded the output.
Still partially allowed now, via Exception 2 of 215.11 and 210.9:
