Advice needed on finding a 220 to 440V 3-phase auto-transformer, 3.5 to 5kVA

I'm possibly going to need to run a 3 HP 440V 3-phase lathe motor from my 230V output 5-horse RPC. That is, via a step-up auto-transformer.

My rough calc says I'll need at the very least 3.5 to 5kVA. 10kVA probably wouldn't go astray if I end up having to up the size of the RPC.

Needs to be close to home (N. CA) for obvious reasons, and used/surplus, if possible. What would be a good type to look for, and roughly what should I expect to pay?

--thanks!

I bought a used 15kva about a year ago, saw on CL, it had been posted for a long time. I think I paid $150 for it- 4 hr round trip drive to fetch it. I was doing a search for "transformer" IIRC.
He also had some 9kva's but they were 208-440 so no good for me.

I am pretty sure you can also use 3 single phase xfmr's to do the same thing. not sure 3 matching ones would be easier to find though?

Saw this one, price is high but also brand new.... maybe feeding it 240 instead of 208 would kick the output from 415 to be close to 480?
15kva transformer, 3ph 208 delta, 380/400/415 Y on wheels -...

Thanks Rob. I see some on fleabay too.
Thanks for the link. I'll need to do a bit more homework in regard to that one.

shaggy said:

I'm possibly going to need to run a 3 HP 440V 3-phase lathe motor from my 230V output 5-horse RPC. That is, via a step-up auto-transformer.

My rough calc says I'll need at the very least 3.5 to 5kVA. 10kVA probably wouldn't go astray if I end up having to up the size of the RPC.

Needs to be close to home (N. CA) for obvious reasons, and used/surplus, if possible. What would be a good type to look for, and roughly what should I expect to pay?

--thanks!

Click to expand...

I cannot advise an "autotransformer" for a 2:1 step-up.

Better off, and easier to find a "full isolation" at 2:1.

You can get Delta-Delta (your RPC output is Delta, as is a Phase-Perfect).

Or Delta-Wye.

Don't overdo it on the "upsizing". Some is good so you don't overly restrict current for starting or responding to heavy load.

Too much and you are wasting power the whole time it is "ON", even if the machinery is NOT drawing power.

Don't forget you will also need '600 Volt Class" rated goods for CB's, fuses, housings, switchgear, outlets.. basically everything on the high side.

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 in all. It's understood, or should be, that they aren't inherently 'safe', so they need to be installed and used with that in mind. I can't claim to have used atfs this large, or 3-phase transformers at all for that matter, so I appreciate your input and stand ready to be corrected.

Point taken on the class of insulation required.

shaggy said:

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.

Click to expand...

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!