Dc motor conversion on 13" top drive?

I have seen and looked into a dc motor conversion for a Bridgeport, but what about for a 13" top drive. I was thinking make a single flat pulley and run it straight to the largest step on the lathe from as small of one on the motor as possible without too much friction loss and use a speed controller on a 2 hp motor or so. Would I have enough power at the lower rpm without the idler shaft. It's not like I would be using a puny little treadmill motor like people put on 9" lathes. I already have a 1/2 hp and controller, but I doubt it would handle a heavy cut without the idler reduction. It would sure cut down on a lot of bulk. Anyone done this? benefits? drawbacks?

Years ago I did a DC motor conversion on a Chinese 9 x 20 lathe, and it did not turn out as well as I had hoped. Having the variable speed was nice, but on interrupted cuts the controller was not fast enough to maintain a constant spindle speed. The lathe would bog down when cutting, the controller would then increase the torque, then the speed would jump when the cut terminated. The result was a wildly fluctuating spindle speed - it was like the spindle was connected to the motor with a big rubber band. If I was going to do it again, I'd use a 3 phase motor and VFD to vary the speed. Just my experience, FWIW.

Thank you, that makes a lot of sense. I am just gonna scrap that idea and get rid of my old 100 lb. nonreversible, repulsion start motor and get a new three phase and vfd.

I had a 2HP DC motor on a wood-turning lathe. Controlled it with a variable transformer (Variac) and a bridge rectifier. It worked very well.

billzweig said:

I had a 2HP DC motor on a wood-turning lathe. Controlled it with a variable transformer (Variac) and a bridge rectifier. It worked very well.

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Yeah, I also have a couple of setups with variable speed DC motors; one on a drill press and the other on a band saw. Both work well. The problem I had with the DC motor on the lathe was with interrupted cuts - on continuous cuts it was OK.

So, a dc motor with a penta drive dc motor controller would not do well with interrupted cuts? would I have the same problem with a ac motor and a vfd?

I was only referring to my personal experience. It's possible that the 2 HP DC motor and controller I had were of a design not suited to lathe work, but that other designs might be OK. It's been a long time ago and I can't remember the type of controller I had, but I know it was not a Penta drive. Maybe someone with a positive experience can weigh in on this. As far as the 3 phase with VFD, I've heard nothing but good about it.

I would say that how successful you will be with your project depends a whole lot on the DC motor you select. DC motors can be a very complex subject. Just as you can often pick between an 1150, 1750 or 3450 RPM single phase or 3 phase motor, there are a wide range of RPM's that you could pick for your DC motor. And, the higher the rated RPM, the lower the torque for a given HP. IF you could find a nice 2HP DC motor with a rated speed down around 1100 RPM then you might have a chance for success. To further complicate the issue, you might pick a permanent magnet motor or you could delve into the shunt wound motor arena. With the shunt wound motor, you can play with the strength of the field by varying the current in the field. High field current yields high torque and low speed. However, I seriously doubt you want to get into the business of building field current power supplies for this project. Not saying you couldn't though.

But with more or less conventional motors and controllers I'd say you're going to have trouble providing adequate torque to the spindle for any serious low speed heavy machining. Of course, you could always use the back gears to get the speed down but I'd question whether you'll be happy with that arrangement for cases when you want really low speed and high torque such as threading or working large diameters.

A VFD is definitely the way to go. Better torque and a smoother speed. And there is a lot more safety options built into a VFD. I used to have a DC set up on my 9N. In the beginning I always had problems with it. I traced down the problem to heat in the controller. I placed a larger fan on it in a better heatsink, and the problems disappeared. But it was still less than ideal. I'm now using a TECO EV VFD, on a 1HP three-phase motor. And I've never regretted installing it. It's been absolutely perfect for my needs.