converting an old lathe to vfd drive

I have an old south bend heavy 10 lathe. It is very limited for speeds especially mine since someone did away with the motor pulleys to change it to low speed. It has a small 110 volt motor, I think it is only 1/3 hp so I am thinking about getting a 1.5 or 2 hp 3 phase motor and put a 110 volt input vfd on it so it will have almost infinite speed changes. I know very little about vfd's so if some of you have some suggestions let them fly but please explain why so I can learn too. The primary concern is money. I do not want to invest a fortune in this old lathe. There are many better ones on the market but I already have this one so I would like to keep the drive conversion under $300 . It looks doable if I get some parts off ebay.

The motor needs to be inverter duty to handle the full range of a VFD. Many will go to 200Hz or higher and that can breakdown the insulation in older motors. You can always limit your frequency to 60Hz then any good quality motor will work. The torque speed curve depends on the number of poles in your motor and the current capacity of the VFD. For something like a lathe, you don't need a lot of bells and whistles. A programmable ramp up and ramp down will lower the current you need to start, and be gentle on the equipment. You probably don't want any breaking as that can unscrew a threaded lathe chuck, like an old SB. Ebay is a great resource for things like that. I have bought several lightly used or NOS name brand VFDs for pennies on the dollar. But there are also a flood of new low cost Chinese VFDs that might be another option. You might want to go with a VFD that is rated to a little more HP than what you plan to use, if you go that route. You should make a list of what options you would like to have. Some have displays that can be programmed to display RPM even taking a reduction system into account, while others just display the frequency or a 0-100% count. If you can power the VFD with 240 single phase, your input amperage will be much less than what it will take running on 120. Also be aware that some VFDs that are 3 phase input can also run on single phase but with a lower HP rating. Do you have a flat belt or a V belt spindle?

There seems to be quite a few folks using treadmill motors for lathes. That will be the route I will go when I get a lathe.

I recently bought a KB Electronics unit for a 3ph grinder. They have a few models with enclosures that make the install quick and easy. Some have rotary pots for quick speed adjustments too.

All good info. I like the idea of a tread mill motor. I will investigate that. My lathe is a flat belt to the spindal and a v belt between the motor and the flat belt jack shaft. I am open to all ideas the cheaper the better. From the little I know about vfd's it looks like I could use a 1725 rpm motor and at the higher speeds it would run at 90 htz to run overspeed. Or I could use a 3450 rpm motor and would limit it to 60 htz. My concern there would be torque at low speeds. I think I would be running low speeds 80% of the time. My lathe does have back gear but I would like to get down to 150 or 200 rpm without using it. Then in back gear I could get down into 10 or 15 rpm for some larger dia. work. I am wanting to compensate low rpm torque by using a 1.5 or 2 hp motor as at low rpm it would only have frational hp. Most of the SB lathes this size are running a 1 hp motor. I have a 1hp capacitor start 1725 rpm motor that has been on the shelf waiting to be used on this lathe but I think it would still limit my rpm choices a lot. Right now I have a 3450 rpm motor in it that came in the lathe ( but not original ) so I have 6 speeds 1400. 700 , 545, back gear 250 , 145 , 55 . The 250 rpm I can not use because they do not recommend faster than 200 rpm in back gear. If I put in the 1725 rpm motor my fastest speed would be about 700 rpm. I think I would need at least 1200 rpm sometimes. But the lower speeds would be much better than I have now. I would like to keep 110 volt input because I do not want to run a 220 line and this way if I change my shop around it could plug into any outlet I have. It might be possible for me to put a 2 step pulley on the motor to increase rpm with the motor I have but then it would take me 10 min. to change it over every time and that would be a hassel that I would probably be too lazy to do most of the time since most of the time when I am making something on the lathe I am in a hurry.

After looking into the use of a treadmill motor for the lathe , I have ruled it out. I could do it cheaper than with a vfd but the motors are open to dust and chips and I do not think it will hold up as well as using a 3 phase motor and a vfd. If I had one laying around so it was free a treadmill conversion would probably be a good option but I don't have one and do not have time to scrounge one.

I have used both for different lathes, and while not an apples to apples comparison, I had one on a South Bend and the other on a Logan. Because of the pulse nature of the DC drive, the surface finish was never as good as the 3 phase. It might have just been the quality of the cheap ebay DC drive I used, as I never measured the switching frequency of that unit. But it is something else to favor the VFD. Chips are not really an issue if you have the underdrive cabinet type setup, but would be for the bench model.

The downside to a 120 volt VFD is you will be limited to around 1HP. The heavy 10" is capable of more, but for most uses 1HP should be fine.

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Originally Posted by Rambozo

The downside to a 120 volt VFD is you will be limited to around 1HP. The heavy 10" is capable of more, but for most uses 1HP should be fine.

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I found a 110 volt vfd that is rated for 1 1/2 hp and For what I do that would be enough I think. My south bend has the motor in the cabinet so chips would not get in it real easy but with the permanent magnets in the treadmill motor I believe they would keep collecting metal chips until something shorts out. It might take a year or so but I think it would happen. The vfd I found is a cheap chinese made but I think I will gamble on it . Now I need to find a cheap 1 1/2 hp 3 phase motor.

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Originally Posted by TheGary

I found a 110 volt vfd that is rated for 1 1/2 hp and For what I do that would be enough I think. My south bend has the motor in the cabinet so chips would not get in it real easy but with the permanent magnets in the treadmill motor I believe they would keep collecting metal chips until something shorts out. It might take a year or so but I think it would happen. The vfd I found is a cheap chinese made but I think I will gamble on it . Now I need to find a cheap 1 1/2 hp 3 phase motor.

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You will need a special branch circuit to get to 1.5HP 3PH from 120V as the input current required is about 24 amps. 1HP is just under 17 amps so it can run on a standard 20 amp circuit. It's a lot easier to run with 240 input since that halves the current and reduces the wire size required. It all depends on how your shop is wired and what makes the most sense for your setup. You mentioned being able to move things around.
Finding a motor should be easy. 3 phase motors are dirt cheap because they are so simple. One great place to look for new ones is www.surpluscenter.com I'm sure you can find a TEFC motor for under $100 or even half that, if you look around a little.

your right about the amps . I could install a light 220 circuit cheap since it could be 14 ga. If I go to 220v then I will look for a 2hp and install a 12 ga. circuit.

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Originally Posted by TheGary

your right about the amps . I could install a light 220 circuit cheap since it could be 14 ga. If I go to 220v then I will look for a 2hp and install a 12 ga. circuit.

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Sounds like the best plan.

Automationdirect will sell you a well regarded 1hp drive for $134 w/ free shipping. A motor from them is similarly priced if you can't find one for free somewhere. The 2hp 1 phase pushes you up to a GS2 which is like $250, so that blows the budget. Treadmill motors are pretty popular on the minilathes but I think it would be pretty wimpy for a 10" SB, and DC speed controls seem to me to be pretty pricey for what they are.

That said, 2hp into a 10" lathe seems like a lot. I would think ~1hp would be plenty. My 10" is only 3/4 hp and I've never come close to stalling it, even in a crash! I do ~90% stainless and sometimes in pretty large diameters. I'll betcha 1/3hp was pretty horrid to use!

Nowadays, drives like the GS1 are super-simple to use. Plug in one side to your power, the other to your motor and set up a few basic parameters. Some of the cool things are that you can (often safely and allowed in the motor's datasheet) run your motor at higher than standard RPM, and mess with torque at lower RPM. Pretty much the only thing "ya gotta know" is not to hook the drive up backwards!

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Originally Posted by Paul Moir

Automationdirect will sell you a well regarded 1hp drive for $134 w/ free shipping. A motor from them is similarly priced if you can't find one for free somewhere. The 2hp 1 phase pushes you up to a GS2 which is like $250, so that blows the budget. Treadmill motors are pretty popular on the minilathes but I think it would be pretty wimpy for a 10" SB, and DC speed controls seem to me to be pretty pricey for what they are.

That said, 2hp into a 10" lathe seems like a lot. I would think ~1hp would be plenty. My 10" is only 3/4 hp and I've never come close to stalling it, even in a crash! I do ~90% stainless and sometimes in pretty large diameters. I'll betcha 1/3hp was pretty horrid to use!




Nowadays, drives like the GS1 are super-simple to use. Plug in one side to your power, the other to your motor and set up a few basic parameters. Some of the cool things are that you can (often safely and allowed in the motor's datasheet) run your motor at higher than standard RPM, and mess with torque at lower RPM. Pretty much the only thing "ya gotta know" is not to hook the drive up backwards!

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Thanks for posting the web site. I will get a GS1 for a 1 hp motor. I am still trying to scrounge a 3 phase motor. I will not be getting an inverter duty motor. From my investigation it seems that if you stay with 240 volt 3 phase the insulation break down is not a common problem. It is when you are running 440-480 volt on a vfd that it is of the greatest concern. The other thing is cooling for low rpm running. I think I will incorporate a cooling fan into my motor compartment to help keep it cool at lower rpm. I seldom use the lathe for more than a few hours at a time anyway ( of course that might change when I get it working real good since it will be much more usable when I am done). I am not in a big hurry so it might take some time to get the motor as a freebe. If I do not find one in a few months then I will buy new. Its hard to make myself buy one when in the past I have seen dozens of them thrown in dumpsters at the job sites I have been on.

No prob. I know what you mean about the motors. Always what you don't need when you don't need 'em, and never what you need when you do.

Don't get too worked up about the heat thing. A lathe is like any other power tool: you know when you're pushing it and check the motor to see how hot it's getting, and the rest of the time you're only running it light. Responsible power tool operation means responsible thermal management. :) Remember that site is geared for industrial, 120% load /24hrs a day with crappy maintenance type usage. A fan would be a real good idea though.

I have some of their smaller "inverter duty" motors and they're a lot larger than a regular sized motor (which I've also used with those GS1 VFDs with no problem at all), which can make fit an issue. They're *way* lighter than they look though.

Good luck on the hunt!

Well the conversion is off. I picked up a 17 in.Cincinnati hydra shift lathe a few days ago. I was dragging my feet on this project because I know the heavy 10 was too small for what I see happening in the future so when this lathe came up at a good price I got it. I ordered a 10hp vfd for it and it should be in in a week or so.