DC motor for lathe

[dust_devil]

The 3/4 HP motor on my 9" lathe burned up, and I am considering replacing it with a permanent magnet DC motor and PWM speed control.

From a hardware standpoint it's no problem, but I have never worked with PM DC motors this size.

Anybody here done this or something similar? Any gotchas?

Thanks.....

[JHanulec]

Have you considered a three phase motor with VFD? I have three lathes, a Logan with a single phase 1/2 HP, a Habegger with a 3/4 HP PWM DC drive and a LeBlond with a 3/4 HP three phase with VFD. In my opinion the VFD is the way to go at this time. It holds the speed better than the DC drive and may be no more expensive then a DC setup since three phase motors are so common. Whether you go with the DC or the three phase keep in mind that when running below rated speed that it is a constant torque so the HP is proportional to speed. This means that you may want to start with a larger motor than the original single speed.

[cba]

DC is yesteryears technology. New, good DC motors are 5x the price of a 3phase motor. And a 3ph motor is maintenance free, whereas a DC motor has brushes to check and a collector to clean. If you want constant speed, you may also need a motor with tacho generator. DC motors with permanent magnets may attract ferrous swarfs.... Only go DC if you can source a cheap used motor or intend to build your own speed controller.

Otherwise, a VFD drive is cheaper & better (has plenty of features built in including ramp up, ramp down, quickstop brake, frequency display etc etc, is easy to interface for cnc, and is maintenance free).

Either DC or VFD, keep your belt drive if your lathe has one. Many make the mistake to think a variable speed drive needs no gearbox. You use it less, but it is still necessary (unless you go from 3/4hp to a 2 or 3hp motor). Chris

[philinmt]

Not so fast.....Dont give up on D C motors, thay are old school, but thay have low end power that a vfd will never have. You will need a 1 to 1 1/2 hp but you should find one cheap on ebay or the local scrap yard. The control can also be found on ebay or if you have a little help you could build it for 100.00 or less. The monarch ee uses dc drives(look at the price thay bring) I have most of my machines at work changed over to DC ( some are over 50 hp)...Phil in Mt

[sandman2234]

No very familiar with DC motors, but what voltage would this motor your looking for be?
Reason I ask, is I have a 24v motor from a sweeper. Probably not the same thing, but thought I would ask. David from Jax

[philinmt]

Most motors to use on a lathe should be 90 to 240 dc. A 24 volt motor would need over 40 amps to make 1hp . The drive for that type of motor would be hard to find cheep...Phil in Mt

[Victor_R]

With regards to installing a DC drive motor on your 9" lathe, let me first say that I have a 9" cabinet SBL with a 3/4 HP motor. I've had it for thirty years and in all that time I've never felt the need for variable speeds over the six readily available. (12 if you include the motor pulley)

If you were to replace the AC motor with a DC unit of similar rating (1750rpm/90v) with similar HP, you would be disappointed with the performance. I was reminded of this from an earlier post because I did not elaborate.

A garden variety DC motor when slowed much below rated speed will lose power. You would need to double or triple the HP rating to avoide being frustrated from lack of drive power. Going up to 180v would be necessary with the higher HP. This, of course, complicates things like installation and cost.

There are some DC motors that would probably work OK but these are not so common. A low voltage high current (4 brush) motor such as found on a winch or serving as a windmill generator come to mind. These have good low end torque but require moose current. And the speed rating may not be to your liking. I have used these with simple home built brute force power supplys. (variac-stepdown trans-bridge rectifier)

Choosing a proper DC motor that you will be happy with is somewhat of a challenge. Diligent and persistent peruasl of the surplus catalogs, eBay, salvage yards and other sources is required. And of course, avoid very high speed motors like those overrated tredmill jobs.

So I guess what I'm recommending is to replace the motor with another AC unit from a good manufacture like Baldor, Reliance, GE, Westinghouse, Dayton, etc, etc. But keep your eyes peeled for a suitable DC motor that you might convert to later if you still really want to.

[Steve_in_Mich]

I only just found and read the posts in this thread and cannot let it go without interjecting a couple of points. For the most part DC motors are being considered gernerically here - they are not all alike. Speed versus Torque curves differ for the different winding types. The basic types are Permanent Magnetic, Series Wound, Shunt Wound, Compound Wound and some might include the Universal AC/DC motors. Personally I would only consider Shunt (or possibly a Compound) wound motors for application on a metal cutting lathe.
Brush maintainence has been discussed before. My reply is that a home shop machinist will never need to replace a brush set on a DC shunt wound motor unless they were nearly used up when he aquired the used motor (visual inspection will give you a heads up on this) or there is a manufacturing defect in a new DC motor. Plan for the negative if you like - I prefer to see the glass half full.
Speed control - what makes you think you have good speed control with your AC single phase motor? Have you measured RPM under load/no load conditions? In lathe work the important cut is the finish cut where the load is minimal and constant so speed will follow. If you "want" better speed regulation go the tach feedback route.
My last pet pieve "rant". Variable speed. I'll start with what some may say is a gloat. It was my privilege to attend a "hands on" university (Mighigan Tech) where we were allowed to operate machines instrumented with strain gages, accelerometers and the like while varying cutting speed and measuring motor load. It was these lab experiments that taught me the value of variable speed in metal cutting operations. If one relied on a step pulley setup alone to find these speed troughs, I think you could miss it altogether. Something as simple as a little different grind on the cutting tool gave marked differences in load but adjusting speed could compensate for a lot. You may not have your lathe all instrumented but you know the sound and feel of the machine when things are going right. Your cutting tool tip does too and the payoff (other than less tool regrinding) is seen in the surface finish. IMHO no mater how you acheive variable speed (I'm talking more than descrete steps here - although that is much better than one size fits all) I say go for it in any metal removal processes.