What's new
What's new

Questions - Powering a 3-phase 440v 7.5hp lathe starting with 240v single phase


Dec 12, 2021
I know this is a long post but I wanted to provide as much information as I could.....
I may have the opportunity for a lathe that is 440/480v three phase only with a two speed 7.5 hp motor and a small coolant pump that may or may not be used. Full load current rating is about 10.5 amps.
Of course, I don’t have three phase or 440/480 power and am wondering what my options are and if they are affordable enough to purchase the lathe. At this point I am trying to educate myself to see if this may be an option when the lathe comes available (shop is ‘planning’ an upgrade).

I have spent some time searching on the forum and found some helpful and interesting information but not all of it was especially clear and in piecing it together from the different threads, I don’t know if my assumptions are on track either. I have 240V single phase available. Closest/easiest source is a 100 amp subpanel within about 30ft (probably about 50ft wire run)

My initial thoughts were to either build or buy a 10hp rotary phase converter but when using a 7.5 hp lathe as a load in a couple of the commercial RPC manufacturer’s calculators, it specs out a 15 hp RPC. For now only the lathe would be run off the RPC…..I don’t anticipate any additional machines that size or larger in the future and only one machine would run at a time…if any were added.

Since a transformer would also be needed, I am unsure how the demands or losses of that might affect the size of the RPC required or the specific size and specs. of a transformer. A simple calculation of 480vx10.5A= 5040va
x 1.732 (is this needed here since the transformer former is downstream of the RPC?) = 8,729 and I don’t know how much ‘extra’ capacity might be needed for startup loads.

My initial thought was 15kva. But, in looking at different transformers, it gets a bit confusing for me especially looking at the used market. Most of what I see are step down (may be able to hook in reverse?) and new ones on well known vendor web sites are very $$$$ expensive.

I am not an electrical engineer (obviously) so any assistance and advice is appreciated. Regarding transfomers -- since the RPC would produce 240 volt 3-phase, is it likely I would end up with a delta/delta transformer?
Many of the used transformers I am seeing seem to be step down to a 208Y secondary. I haven’t seen any used 480 wye/ 240 secondary that could be reversed, and no 240primary/480 wye used ones at all. Also, I read a thread regarding grounding and bonding neutral on the secondary and don’t fully understand. For my application, if a delta/delta is needed would that mean it should be corner grounded on the secondary side?

I have many more questions, but I need to get some input to see if this is even feasible and before I get too far into the weeds with my assumptions.

Please disregard this thread.....I tried to post this two days ago but I couldn't see it so reposted a new thread earlier today. I don't know how to edit or delete this one.
Moderator, please remove this to avoid confusion....so sorry, apologies!