VFD vs phase converter

First post. Just bought SB engine lathe. Ser num 7702 TK X14. I have 1ph power in my shop and obviously need 3ph to run the machine. Thoughts/recommendations on phase converter vs VFD?

For one machine VFD is a slam dunk. Cheap enough these days.

Especially if the motor is modern enough to have all 6 winding connections bought out into the junction box so it can easily be connected in delta. If not digging the star point out is usually pretty easy if you have wiring skills or not expensive at a motor shop if you don't want to try.

Worst case of new motor won't be silly expensive for a machine that size and the old one will have some value anyway.

Despite what cherry picked internet wafflings imply building a rotary phase converter is something of a pain and both harder and more expensive than casual reading suggests. These days its classic "life is too short" thing. Forget static converters, inherently inefficient and, again, surprisingly expensive. (If you had the build knowledge and scrap box access to sufficient components to do a cheap job you'd not be asking the question.)

Clive

Thank you @Clive603. New to forums, can you recommend a good VFD for my machine? It’s a 240v 3ph 9.7A with a 3hp motor

Any recommendations? 240v 3ph 9.6A 3hp motor. Do I need an speed control device as well? Not sure of the year. I posted the serial num in my first post.

EBMetalworks said:

First post. Just bought SB engine lathe. Ser num 7702 TK X14. I have 1ph power in my shop and obviously need 3ph to run the machine. Thoughts/recommendations on phase converter vs VFD?

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What size motor do you have?
Is your lathe a 14 1/2 Inch?

I am a fan of running old machines how they were intended so that means a Rotary Phase Converter (RPC) for the closest thing to how it would have worked originally. Granted it's more expensive and if you got an old lathe to save money that can be a consideration.

A VFD will work and many are happy with that but you will not have the same torque at slower speeds is what I have read.

That's why I asked about the size motor you have. If you have a 1hp motor there may be some decisions to be made based on that.

Also there is an entire section for VFD, RPC, etc in here and it is very helpful to answer your questions.

Any of the known brand name drives should do just fine.

Loss of torque at low speed is true but generally with a modern VFD you won't notice any difference in performance for changes within ± 1/3 of nameplate speed. Unless working the motor hard probably ± 1/2 will be fine. May well be more. Best to keep the standard multi-speed belt drive and use the VFD to fill in the gaps or get a real low speed for tapping, threading et al. In practice not an issue. If I'm buying a motor to fit a machine I go 6 pole for the extra low speed grunt, 'cos I don't need super high speeds got from running faster with the VFD, but its not a necessity.

I'm in the UK so I can't help with suggestions as to which brand is best bang for buck in your neck of the woods.

If its any help I buy from Inverter Drive Supermarket (UK supplier) and have always found anything they offer to be just fine. They do their own quick start guides for some models which are much better than any factory book I've seen. Their rep told me that those are the ones they are most confident of selling to folk who aren't experienced. The guides can be downloaded so may be worth a look.

I imagine there are satisfactory off-brand boxes somewhat cheaper than brand name available but you'd need local knowledge to make an informed choice. Direct from China via Bangood et al is cheapest but risky.

Clive

EBMetalworks said:

I posted the serial num in my first post.

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Most of us will not know serial number identifiers off the top of our head to know basically what your machine is. Most will post as South Bend 13", 14.5", or 16" and the like.

If you have a 3hp motor I'd guess at least a 14.5 or 16". And those mostly came with 2hp at most original.

RPC for the win. Mostly due to speed limitations of a South Bend. A vfd is better when you need variable speed, and cranking speed higher. Not recommended on a South Bend, by pretty much anyone. Keep the top spindle speed around 1000rpm.

Since you won't want higher speeds, the rpc is better and more efficient at maintaining a motor's given hp.

At 3 hp load motor, you want a bare minimum 5hp rpc. I'd recommend a 7.5hp rpc. This will allow you a little room to expand as well, should you get another machine.

Hello. My South Bend is a Heavy 10. I’m not able to identify South Bends by serial number so I’m not sure which one you have, but I’ll show you what I’ve done.

My little South Bend has been with me for a long time. It started out as a 3-phase, 1 hp running off a whole-shop 10 hp rotary phase converter. It has also run as a 1 hp, 3-phase and a 1 hp VFD. Its current configuration is 2 hp Toshiba motor, 2 hp Lenze VFD.

I run it from a low of 20 hz to a high of 110 hz. I’ll show you come pictures of the belt configuration I use. It pretty much stays in this configuration because it offers the most belt traction. There are also a couple of pictures of maximum and minimum rpm in that run position. The lathe has way more power at any rpm than it has rigidity or belt traction.

Bottom line for me is if you are powering many machines, the rotary converter is better. If you are just powering this one machine, VFD is better. I really like variable speed. I hope this helps some.

I’ve included one more picture of a collet tray that I made and find very useful.

If you don't want to rewire your machines and buy a separate VFD for each of them, go with a single RPC that will provide for all of your future machinery. That was my choice, and I've never regretted it.

If you ever need a variable speed or other features of VFDs, you can always add one to a particular machine.

I will point out that the only people I knew with just 1 machine, sold it after a short period of time, the rest quickly acquired more machines. So either put it up for sale, or plan for more.

Thank you all for the info. Very helpful. It is a 14 1/2.

tailstock4 said:

Hello. My South Bend is a Heavy 10. I’m not able to identify South Bends by serial number so I’m not sure which one you have, but I’ll show you what I’ve done.

My little South Bend has been with me for a long time. It started out as a 3-phase, 1 hp running off a whole-shop 10 hp rotary phase converter. It has also run as a 1 hp, 3-phase and a 1 hp VFD. Its current configuration is 2 hp Toshiba motor, 2 hp Lenze VFD.

I run it from a low of 20 hz to a high of 110 hz. I’ll show you come pictures of the belt configuration I use. It pretty much stays in this configuration because it offers the most belt traction. There are also a couple of pictures of maximum and minimum rpm in that run position. The lathe has way more power at any rpm than it has rigidity or belt traction.

Bottom line for me is if you are powering many machines, the rotary converter is better. If you are just powering this one machine, VFD is better. I really like variable speed. I hope this helps some.

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I want to know more about your carriage lock/stop/indicator holder! That looks super functional. I would like to have this on my machine.

tailstock4 said:

...
Bottom line for me is if you are powering many machines, the rotary converter is better. If you are just powering this one machine, VFD is better. I really like variable speed. I hope this helps some.

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This - the man knows his stuff. Only other options is swap in a single phase motor (often the most costly choice) or a phase-a-matic which everyone mostly agrees simply sucks.

Kevin T said:

I want to know more about your carriage lock/stop/indicator holder! That looks super functional. I would like to have this on my machine.

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I believe it is an original South Bend micrometer stop. Others may know more about the particulars of that stop. The indicator is mounted in an adapter held by a set screw and allows for quick change back to the micrometer. Below are some pictures of the simple adapter and the swap. I make this simple stop/dial indicator holder for all my lathes that don’t have DRO’s.

One more thing... The adapter is long enough to allow the dial indicator stem to be fully retracted thereby working as a stop.

Yes at the simple end there's generally two systems that work the best with 3 ph machines and no available 3 ph to where the machine is. Rotary phase converters and VFD's. Neither removes the requirement of changing the belt position or other mechanical methods (revees drive etc) of gaining more torque at lower spindle rpms. A VFD lessens that, but you can't really throw your multi step pulleys or back gear away. As an example, the open pit mine I retired from had basically from our perspective unlimited 3 ph. power, a large staff of certified electricians, and when your power bills are a couple of million a month you can afford and need to optimize what works the best.

Straight mechanical drive only on electrical powered equipment is today ancient technology in almost all industry. That mine had literally hundreds of electrical 3 ph motors from 1/2 hp on up to multi hundreds of HP. Everything in that ore concentrator plant had either a VFD or PLC on it for there fine tuning ability, and with VFD's almost instant braking if needed. Multiple millions to add all that on top of the standard mechanical drives. For us a good 10 + HP rotary ph converter isn't cheap unless you can build one yourself. My basic electrical knowledge prevents that. But neither are multiple decent VFD's. Even if I had machines with 2 or more separate motors I'd still go VFD. Price wise once you get above maybe 10 or more VFD's the costs probably start to equal out. What they can do for built in programmable pre set rpms, acceleration, deceleration, optional braking varies from brand to brand. If your at all interested in going that route, most of those more brand name models have there manuals available online. Research those to figure out what one can do for you or even if the manual is in understandable English before you decide. Just don't buy at the absolutely lowest cost and as others have said, buy one with a reputable familiar brand name. Hitachi, TECO, Toshiba etc. From what I've seen with what a VFD can do, I'd still use a 3 ph. VFD to power a 3 ph. motor even if I had that true 3 ph power right out of the wall. They make 110V to 3 ph. output VFD's I think. Can't say anything about them since I've never used one, mine are the more standard 220v single ph. to 220V 3 ph out of the VFD. And converting to a single phase motor just to make things easy would be a huge step backwards.

Kevin T said:

I want to know more about your carriage lock/stop/indicator holder! That looks super functional. I would like to have this on my machine.

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Another approach to the idea, a few pieces of aluminum scrap and a surplus 1" travel indicator:

jim rozen said:

Another approach to the idea, a few pieces of aluminum scrap and a surplus 1" travel indicator:

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That is a nice unit. I made something similar after seeing what you guys before me had done. Then I got a South Bend micrometer stop and for the 16 it's a big unit.



It crowds the space between where I like to leave it on the dove tail between the chuck and QCGB. Something like the swappable solution seems better but it would take a lot of time to unscrew the mic. On the SB stop I have there are like 2 inches of fine threads! Maybe a different large indicator could help but I'm fond of my trusty 2" travel Starett



Not enough stick out to make it through the housing!

EBMetalworks said:

Thank you all for the info. Very helpful. It is a 14 1/2.

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EBMetalworks said:

First post. Just bought SB engine lathe. Ser num 7702 TK X14. I have 1ph power in my shop and obviously need 3ph to run the machine. Thoughts/recommendations on phase converter vs VFD?

Click to expand...

As info, your serial# represents an 13" lathe (TKX suffix), not a 14.5". A 14.5" serial# would have a suffix of FKR (early small spindle) or FKL (2 3/8" x 6t.p.i. large spindle). Hope this helps when you're looking for parts. PB

I run a Phase Perfect for al machines but do add a vfd for lathe conversions. You have to understand the benefits and limits though. A vfd can add braking if not on the machine and many lathes have a wide distance between speeds on the high end. A vfd is nice for filling in those gaps. When converting a lathe to a vfd, I generally ( not always ) replace the motor with a larger vector duty motor. It is not that you lose torque at low hz but rather you lose hp at both the low and overspeed ranges. I find that TENV vector motors range from 30-90 hz without losing much of anything and that is enough range to fill in even the gaps on lathes with a two or three speed motor which tend to be very wide. You do need to pay attention to the limits within the gearbox and spindle bearings when overspeeding to increase the top end or even when filling in the gaps by overspeeding a mid range gear selection.

In my world the RPC or PP is first choice and a vfd is to improve the characteristics of the particular machine and should be designed with that in mind. A benefit of the PP ( downside is cost ) is that you can run a vfd off it so they don't need to be oversized. Pretty significant if running a large motor. Dave