lathe motor 3ph vs. single phase and low rpm

[axgold]

I recently acquired a Reed Prentice 14x30 gearhead lathe model b. I don't have 3 phase and am exploring all options. Static phase converter is not recommended from what I understand. I am looking to find a single phase 3hp motor but they appear to be rather pricey as the lathe requires 1200 rpm. Rotary phase converter is another option but I can't seem to get me arms around what to get or build my own. I am open to suggestions to help me figure this out. Thanks!

[Magicniner]

Get an Inverter VFD, you won't get the power pulses associated with a single phase motor and you'll have infinitely variable speed, if you get a Vector Drive Inverter it will sense motor speed and boost current as the motor is slowed to maintain speed and torque, even at lower speeds,

- Nick

[SteveHGraham]

A VFD would be a nice thing to have. In addition to the stuff Magicniner mentioned, you'll be able to reverse in a hurry. If you have to mount it where chips will be flying, you might consider a VFD that comes with its own protective box. If you want to use the VFD to brake the lathe, you'll want a braking resistor.

It sounds like you already have the 3-phase motor, and if you're already looking for a new motor, you probably won't be bowled over by the price of a VFD.

[spro]

Hey Axgold, welcome to the group! You are in for some learning about VFDs/RPCs . If the original motor still works keep it. It is a beast and that lathe is a beast. I've been up close and personal with a Reed Prentice and they are awesome strong machines. This may sound silly but one the size, anyway they can pull you right in. What's the motor Hp ? maybe we can help 5-7 1/2 Hp. ?

[BadDog]

Yes, keep the original motor if you can. Should be a big heavy cast iron monster. If I'm not mistaken, that's a bit heavy beast of a lathe that looks like a Pratt Whitney C. That things got to be at least 5 HP, and even at 5 HP a VFD to run it will be pretty pricey unless you just get lucky. My advice would be to just get a good RPC and be done with it. I've run my shop of RPCs for years now and I've never had complaint other than it is a little noisy (nothing compared to the machines if running). I also have a 2 HP VFD that comes in handy at times, but having an RPC to run my whole shop is a huge convenience.

Regarding building your own, I built my first 2, then the third came with my new property. First was a 2HP build as a demo project with my local machinist club. There were 3 of us building, one guy teaching (Marty here), and the rest observing and/or helping along the way. The second was my own design with staged Baldor idlers at 5 HP and 10 HP (or 15 if combined) that had a nice control case with AB switches and 3 volt meters showing each phase at any moment. That was a nice one. Then my current one is a 30 HP "CNC rated" monster with 200A input and a sub panel and 6 separate 3PH circuits feeding my entire shop. There are plans on the internet for a variety of styles. As evidenced by my own progression, they are not complicated. My staged RPC took a bit of self education on my part, and some of the guys here were a huge help sorting that (as were some on the Practical Machinist RPC sub-forum, my plans are uploaded there).

[Glenn Brooks]

I went through this same decision process several years ago when I bought my 3 phase powered Standard Modern lathe. I selected 10 hp RPC made by American Rotary, over a VFD for two reasons:

1). The larger VFD's are very expensive - equal to or greater cost than a comparable RPC., and
2) I can use the RPC to power other 3Phase machines in the shop. VFD's generally only power one machine each... I installed a separate 220v 3Phase power center in my shop and run separate 3Phase circuits out of the box to each 3Phase machine. My whole 220 3Phase system is isolated from the main 220v single phase system by the 1p input circuit from the shop main leading to the RPC . Very simple and effective.

The biggest disadvantage is that a VFD will allow you to change spindle speeds. Can't do this with an RPC - u must use the machines speed settings...

So it made sense from a financial and operational standpoint for me to select the RPC - and have been very happy with it since.

As a sidebar, rule of thumb for RPC's are to double the HP rating of your motor to buy the correct size RPC. So a 5hp motor needs a 10HP RPC to insure the motor's start up requirements are met. The rotary people are very good at walking you through what you need, if you decide to go this route.

Regards
Glenn

[SteveHGraham]

I didn't realize that lathe had a 5HP motor, because OP mentioned 3HP. KB Electronics sells enclosed 5HP VFD's for around $400. Whether that seems outrageous or reasonable kind of depends on your budget.

[John Evans]

For lathes without clutches Glenn's double the motor HP is spot on . Learned that the hard way 7.5 HP with a 150lb 16" chuck would not start reliably other than the bottom 2-3 speeds. Added a 5 HP secondary RPC and now starts correctly. 6 of my 8 machines are 3 phase ranging from .5 hp to 7.5 all but one are on rotaries,1 BP is on a static which is a low start load where the static is fine. Investment is less that a VFD for the big lathe would have cost.

[BadDog]

Actually, that's cheaper than I've ever seen for a 1PH input 5 HP VFD. I suppose like so many things, they come down in price over time or with larger economy of scale.

Nobody said it was a 5HP lathe, I was just going off what I think I know about the lathe based on the name. The OP said 3HP for a replacement motor, but I expect that lathe came with something larger.

[Glenn Brooks]

Yep, indeed, a very good price. Actually I probably would have gone the VFD route if I could have found one for $400, or even a bit more. Although Still like the ability to power multiple machines, in my one man shop...)

Couple of years ago I think the 3P VFDs I quoted were well over $1k. Maybe even closer to $2000, although don't really remember exactly at this point. Makes sense that prices will subside over time as electronics mature. The 5 hp example I mentioned was actually what I had to buy, as my motor Turned out to be a two stage beast -3hp, and 2 hp, if I remember correctly. The Rotary guy said I needed to add both stages together then double the HP to be safe. It got a lot more complicated than I initially expected when I bought the lathe. But throwing money at it worked out in the end.

Glenn

[BadDog]

That's closer to what I found when I thought it might be better to upgrade the motor of my old Rockwell rather than rebuild the functioning and complete but worn Reeves drive. Once I did all the calcs, just to break even on my existing 1HP while increasing the top speed to around 1700 rpms, I needed a 5HP motor just to come close to break even, and that still sacrificed a little torque down low where it's needed most. To do that using a VFD on single phase power was going to cost close to $2k, or give up on the SVD and loose even more torque and/or range. And that's not even counting the primary drive setup, fabrication of mounts/brackets, wiring, etc. Rebuilding that Reeves started looking pretty good at that point.

And actually with a 2 speed motor all you need to do is use the higher rating. Not sure why he would say to add them. Also, the number I always heard as a rule of thumb min idler size was 1.5. But, as with most such things, higher is generally better. As I mentioned, I now run my shop off a 30HP RPC.

And to be complete, machine matters too. I've run a 1.5 HP Bridgeport with ease on a 2HP RPC. That worked ok because there were no high starting loads, and even though I do push mine harder than I see most pushed in home shops, that was ok too because it was already spinning adding it's inertia. But, starting my 7.5 HP lathe in high gear with a big slug of steel in the 12" 4 jaw (before someone jumps on that, balanced weight in a Buck rated for 2k RPM) isn't happy at all on a 10HP idler. On the up side, I've found big 3PH idlers can often be had cheaper than more modest sized motors. If you don't know someone in commercial HVAC, call around smaller operations until you find a sympathetic ear. These guys tear out old systems to build new all the time. I've gotten brand new motors that were spares for abandoned systems and were on there way to (or had already arrived) in scrap yards. One scrap yard out on the west side of town always seemed to have PILES of HVAC scrap. I got really nice NEMA enclosures, lots of AB switches, name brand contactors, fuse blocks, disconnects, heaters, and of course 3PH motors for less than $1 per pound (seems like motors were $0.40 and everything else was $0.10 depending on where and when). But, the sue happy public combined with nanny state regulations are ending access and it's getting ever harder to do that, and sadly my stash is running low...

[spro]

Stroke when It was hot.