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Phase converter rated below motor size - how big of a problem is this

notwrkg

Plastic
Joined
Jan 31, 2023
Location
Independence, MN
I have a Phase Perfect phase converter sized for 5 HP motors and it works fine. I am looking at a lathe with a 7.5 HP motor. I don't see me ever using the full capacity of the lathe, in other words spinning very large stock at high speed. Will my current electrical setup work, or am I just asking for trouble. I have looked through other posts and it sounds like it would work, its more a matter of getting the machine up and running, or am I just being too optimistic. If I need to upsize my 3 phase capacity I would probably pass on the machine.
 
I have a Phase Perfect phase converter sized for 5 HP motors and it works fine. I am looking at a lathe with a 7.5 HP motor. I don't see me ever using the full capacity of the lathe, in other words spinning very large stock at high speed. Will my current electrical setup work, or am I just asking for trouble. I have looked through other posts and it sounds like it would work, its more a matter of getting the machine up and running, or am I just being too optimistic. If I need to upsize my 3 phase capacity I would probably pass on the machine.
Call Phase Perfect. Most RPC's are "uprated". Meaning they can handle much higher startup HP loads than the nameplate. On a CNC lathe, the spindle startup is the high current spike. Until you are running at max RPM, or really heavy cuts. We had a 5hp PP, but was on a Wire EDM, so cannot compare to a lathe.
 
Good ideas from all. I'll call Phase Perfect. It will be good to know for the future as well. I have thought about a different motor, I even have a possible replacement sitting on my shop floor so that might be the route to take. I have a few other issues with the machine so no decision has been made yet.

I was hoping someone would say no problem, what you describe is my exact setup. More homework.

Thanks for the input so far.
 
Good ideas from all. I'll call Phase Perfect. It will be good to know for the future as well. I have thought about a different motor, I even have a possible replacement sitting on my shop floor so that might be the route to take. I have a few other issues with the machine so no decision has been made yet.

I was hoping someone would say no problem, what you describe is my exact setup. More homework.

Thanks for the input so far.

The answer is going to be universally -- "it depends". A lathe is a fairly high inertia load, and as such I would not make the assumption it will work. A lower inertia load like a table saw will have higher odds, but again, current is current and if you exceed the limits of the Phase Perfect it will trip, which is good, you want it to protect itself. You don't really want to be marginal anyway, or you may find yourself in the position that your lathe will only start in part of the gear range, or if you have a heavy piece of work in the chuck it won't start. It's a pain when anything is sized marginally for the task at hand, not just phase converters.

All of that said, if you had access to the lathe you could try it. There is not a lot of risk in that, either it will work or the Phase Perfect will trip and tell you the load is too large.
 
Updates. I did call Phase Perfect and they said the 5 hp converter I have will probably fail with the 7.5 motor. And, I cannot combine my unit with their smaller unit. Therefore, if I want to power a 7.5 HP motor I'll need a larger converter. No surprise here.

So with that in mind I went ahead and bought a bigger lathe anyway with a 7.5 hp motor. Good thinking and planning on my part. I did factor in the cost of the power upgrade so while I'm kicking myself, I'm not really too upset by the turn of events

I'll pick up my brand new 1940's era Monarch 14C 14 X 54 lathe next week. I'll probably be selling my new Phase Perfect 5hp rated converter soon.

More grime for removal.
 
Updates. I did call Phase Perfect and they said the 5 hp converter I have will probably fail with the 7.5 motor. And, I cannot combine my unit with their smaller unit. Therefore, if I want to power a 7.5 HP motor I'll need a larger converter. No surprise here.

So with that in mind I went ahead and bought a bigger lathe anyway with a 7.5 hp motor. Good thinking and planning on my part. I did factor in the cost of the power upgrade so while I'm kicking myself, I'm not really too upset by the turn of events

I'll pick up my brand new 1940's era Monarch 14C 14 X 54 lathe next week. I'll probably be selling my new Phase Perfect 5hp rated converter soon.

More grime for removal.
I had a similar issue back in 2004, with my home made 5hp RPC. I got a lathe with 7.5HP, and it would not even try to start. Instead, some sort of voltage surge occurred that created a short inside of the forward motor contactor, and the short was phase to phase. I dismantled the contactor, and could not see any arc point, but the short was there. I had to replace the contactor with a new one.
I procured a 10HP VFD, and did some wire modifications to keep the aux power from the mains (the single phase power that controls the FWD/REV, light, coolant pump) and connected in the VFD to the 3 phase motor contactors. I also got the system to provide run/stop commands to the VFD.
I was able to get the VFD for a good price from eBay, Mitsubishi brand.
 








 
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