VFD or DC

I have a SBL heavy 10L (1976) and want to put an adjustable speed motor system into it.
Have some questions about VFD or DC motor with adjustable speed and reversing.

1. What is the average cost to set up a VFD system if you have the 3-phase motor?

2. Would the VFD have a full range speeds and be totally reversible?

3. Is the VFD as quiet as DC motor at all speeds?

4. Why do most people choose VFD over DC?

5. If given a choice with no cost differents, which would you, choose VFD or DC and why?

Thanks Chet

i`ll be watching this with interest- i have a new baldor 3 ph 1hp 1150 rpm motor and a teco microdrive that i have yet to hook up on my 9" underdrive. i paid $135 for the teco and $80 for the motor. i chose the VFD simply because it was cheaper. now if i can just get around to hooking it all up...

I am in the same situation with a 10L. It came with a top-of-the-line Boston Gear DC controller and a 1hp DC motor made to Boston Gear's specs. The DC drive has several modules that are shot--custom made for Boston Gear, unobtanium, and beyond my electrical abilities to fab up. The motor is perfect.

To replace the controller with a new one of similar abilities would cost over $4oo, and there would still be some doubt as to whether it would have the same abilities. I can get a VFD and new 3Ø motor for about that.

I think the answer, for me anyways, was that full-capability DC drives, with full torque at low speed in both forward and reverse and dynamic braking, are now more expensive than VFDs with the same capabilities. Simple forward/reverse DC drives are much cheaper. So it depends on how much control you think you need.

I installed a cheap (treadmill) DC motor and drive on a lathe I once had, and while it did give infinitely variable speeds, it was not good at maintaining spindle speed over varying loads. Doing an interrupted cut, when the tool would bite it would slow the motor, and then the controller would overcompensate and spin the motor too fast - I'm not explaining it well, but the end result was like the spindle was hooked to the motor with rubber bands, and I didn't like the effect at all. The motor was also deficient in low speed torque. I don't think it was the result of too small a motor, as the one I used was rated at 2 -1/2 HP. Anyway, just for what it's worth, that was my experience with a DC motor. There are probably DC motors and controllers that would not have these problems, but if you decide to use one, make sure what you're getting before paying your money.

Go with the VFD even if you don't have a 3-phase motor. 3-phase motors are cheap from some of the surplus sources. Check Surplus Center for example. You'll have more control options besides full-speed fwd to full-speed reverse without blowing breakers and making smoke. The VFD's are cheap and very versitile. You can even run 220 volt motors from a 115 volt wall outlet if you can stay under about 1 hp. They're great!

Blob and Vandis' posts together seeem to clarify the issue. A good DC motor, made for industrial use, costs more than a comparable 3Ø motor. And a DC controller with full torque control and regenerative braking costs more than a VFD.

Of course, I am not a great comparative shopper--perhaps you can find a package somewhere that will prove me wrong. But I've been looking for a while, and that's my conclusion.

Mostly right...

I'd say that a good DC motor with a good controller is the premium setup. Unless you're doing some specialized work that benefits from the programmable featues in the VFD.

And that the AC setup is plenty good enough, and a better value.

I've had both AC/VFD lathes in this size and DC setup in this size. Definitely liked the quality DC setup better.

No offense, but the treadmill motor example just doesn't cut it. Those ratings are like cheap asian import quality spec cards. Try picking up (physically) a U.S. made 1.5 hp DC pm motor, you'd better brace yourself a bit and have a good back. I can pick up an armload of so called "2.5 hp" treadmill motors and dance about the shop with them. No comparison in how they're built or how they operate. Kind of like those motor ratings on the bargain air compressors.

VFD's do offer more features and are cheaper to buy as a system (motor and controller cost) and they do work great, and are plenty good enough for most applications. AC 3 phase motors of similar size and rating tend to be a lot cheaper than quality DC motors.

If cost is not a factor, and you don't need the programming features found in a VFD, DC is a better lathe drive.

Just my 2 cents, though.

The best, smoothest running most powerful SBL 10L I've ever run, had a Baldor 180 volt 2 hp DC motor with a Baldor BCWD140 drive, it came from South Bend set up that way, and it was fantastic.

A 13" lathe (Sharpe) w/ 3hp AC motor and a VFD comes in a respectable second. Was a $12,000. machine, in new condition.

But, there was a clear difference, and the South Bend w/ DC was better in the running charcteristics that matter to me, IMO.

I tried to improve on the factory settings on the Sharpe, via programming, but they were optimum as supplied by the factory.

I came down with the Flu and just got back to reading my post. Wow a whole lot of info. Going to set back and read all the replys.
Thanks to everybody Chet
I will be getting back on which way I deciede to go VFD or DC.

While working everyday as an industrial electrician, I see both AC and DC motor drives in service. The AC VFD are more compact and reliable and easier to install than DC drives.
The VFD are also more effiecient. I just bought a TECO N3 VFD and was pleased to see the performance it provides. The VFD also run cooler to in most applications. A word of warning, when slowing down any motor, remember that the cooling fan on the motor is also turning slower. Some applications like threading for a long period of time might require an extra fan to help cool the motor !!!

Jess10ee said:

I

No offense, but the treadmill motor example just doesn't cut it. Those ratings are like cheap asian import quality spec cards.

Click to expand...

Actually, my example does cut it. That was the whole point. Like you, I have a quality DC motor and drive on another piece of machinery, and it performs very well. What I was trying to say is that if you go cheap, as with a treadmill motor, regardless of the rating, you're not gonna be happy.

I am going to go first class all the way. VFD or DC. NO China or make do from something that was not meant to be used on good machinery. I am only going to do it once. And I want to thank all of you for your input.
THANKS Chet

coyotechet said:

I have a SBL heavy 10L (1976) and want to put an adjustable speed motor system into it.
Have some questions about VFD or DC motor with adjustable speed and reversing.

1. What is the average cost to set up a VFD system if you have the 3-phase motor?

2. Would the VFD have a full range speeds and be totally reversible?

3. Is the VFD as quiet as DC motor at all speeds?

4. Why do most people choose VFD over DC?

5. If given a choice with no cost differents, which would you, choose VFD or DC and why?

Thanks Chet

Click to expand...

1-I have installed a VFD on my 1 hp motor of my Millrite milling machine and the cost for doing it including the VFD unit (TECO), elec cable, and all the other electrical items that I needed cost a bit less than $250.00. In my case I had to run wire from 220 V cable to the other side of my shop and buy new 220v receps and so forth. I paid about $150. for the TECO.

2-Yes it will have a full range of speeds, but running a very low motor rpm while making heavy cuts will require changes to the belt/pulleys. I decided not to add VFD to my 3-phase 10L and bought a new single phase rubber base mounted motor that cost a bit more than $200. I did this because of personal preference not technical.

Although the late 10L's can run at 1400 rpm or so spindle speed, because of balance issues through out the drive train and headstock, 1000 rpm is about where you will end up maxing your speed. For example a loose bull gear to headstock cone pulley connection via the bull gear pin will cause issues at high speed. I know this because I cured some of my vibration issues by making a new larger bull gear pin and drilling a new hole for using it in the headstock cone pulley. I also made a new underdrive shaft, installed new bearings of same and trued the 10inch diameter two step drive pulley of the underdrive---all of which reduced vibration at high speeds. When I run at very high headstock spindle speeds, I usuall use a collet in lieu of a 3-jaw or 4-jaw chuck in order to reduce vibration. Vibration to me is anything I can feed by placing my hand on the headstock cover while the lathe is running.

3-No, the VFD will not be as quite as a DC motor at all speeds, but it will not be harsh to the ears. There is some motor noise that is generated by the VFD, but it can be reduced by the VFD program changes. I have a DC motor driving my 10L carriage feed screw and it is very quite. I reduced nearly all the noise form my Millrite motor by the VFD program.

4- Usually cost/benefit issues

5- In your case (10L and assuming an underdrive cabinet model as were produced in 1976) the selection would be which will run the most quite as the cabinet will amplify any noises from the motor and drive train. Both DC and AC motors can be mounted in rubber so the issue is likely moot.

Regards;
Steve

I have a late model 10l. With only 120 volts to my shop, I went with a baldor dc motor and drive. I picked up the 90 volt motor on ebay , about $60 and the drive was not much more. I liked it so much I picked up a duplicate setup for another project. I do not know much about the ac set ups. I am sold on dc power. Cheers,Alan