Thanks. I understand isolated windings are 'better', at least they're inherently safer. Of course the downside is in making the transformer necessarily much larger and heavier, and more expensive.
Auto-transformers exist largely for reasons of economy, and possibly space-saving. In some cases they're perfectly adequate, but certainly not all. It's understood (or should be) that they aren't inherently 'safe', so they ought to be installed and used accordingly. I can't claim to have used ones this large, or 3-phase transformers at all, for that matter, so I appreciate you input and stand ready to be corrected.
The point is that auto-transformers LOSE that advantage as the ratios go up.
Especially if you are trying to keep an RPC 2XX capable for other work and boost it on the 3-Phase side for only one or few loads rather than run the idler at 4XX with a single-winding - or even a pair-of - on the incoming 1-P side.
Used-but-good 2:1 / 1:2 3-Phase dry-type full-isolation transformers are more common than 2:1 / 1:2 3-Phase "auto" transformers, ergo less expensive.
My case, where I cannot re-strap a 3-P motor for 2XX I just replace it.
Mostly, I have not had to.
Or would have replaced them anyway, such as Siemens or similar optimistically-rated featherweight Euro-trash -> Reliance Duty-Master "heavys" conservatively-rated.
Also larger-framed and massing a multiple of what their kleenex-class cousins do, so it isn't always EASY to find the SPACE!
"Wire and forget", though!
I've been buying them new, even if "old stock". Not used!
Yet-another is on its way from NRi, should arrive next week.
5/7.5 HP, constant-torque @ 200+ Lbs and 4-foot pallet?
The freight cost - terminal pick-up, not residential liftgate @ $215-$250- is equal to or greater than the price of the motor @ $215 - $240 or so!
THIS... is why a more common full-isolation transformer can be the better deal as well.
More common means probably CLOSER. Even DIY go-fetch "close" with low/no handling or freight spend.
Mind, that 387 lb Avoir R.E. Uptegraff "elliptical coil" 15 KVA came out of McKean, greater Cleveland area. But I was going after a B&S surface plate as well and still had a staging point in Pittsburgh. So it was a same-day out and back, S15 4-banger economy.
"Retired" folk like to be "somewhere" besides the same breakfast table now and then anyhow!