VFD QUESTION

[Glenn Brooks]

My oldest, big iron 3Phase lathe has noisy and worn gears in the low speed ranges (50, 100, 150, 240 RPM). Today the 150 RPM gear just developed a new, louder chipmunk running around inside the gearbox, whilst I was taking a heavy cut on some 3" round stock. The upper gears (400, 600, 900, 1280 RPM) are hardly used, hence sound great.

So could I install a VFD and operate the lathe at 600, 900, or 1280 RPM, but use the VFD to slow down the motor to take HSS cuts at 100 or 150 RPM?

I know there is some kind of formula to determine speed range for 3Phase motors with VFD's, but don't know really anything about VFD operation...

Thanks
Glenn

[Harold_V]

You may not develop the required torque, and the motor is likely to run hot, due to lower fan speed and heavier current draw.

Harold

[Glenn Brooks]

Harold, thanks. Just reading about these effects. Looks like a no go so far...

Glenn

[BadDog]

It seems there is a common misconception regarding successful use of a VFD to replace various mechanical drives. Even going with upgraded drives including sensorless vector drives and servos generally don't replace the need for mechanical torque multiplication. Fine tuning to fill in between widely spaced (or inconvenient to change) speeds, sure. Getting a bit more top end range, or even a little slower just for speed is suitable. But when folks try to eliminate the mechanical drive or back gear on a lathe with a VFD, that's a lost cause before it ever starts, unless you are one of the "I just do it for fun so I don't mind taking 20 x 0.050 slow feed cuts just to take 1" off a 3" OD piece of stock" crowd. Just because you can slow it down to the desired speed doesn't mean it will be practical. And then you also have to worry about over heating, adding auxiliary fans just to help alleviate the problem, and still suffer penalties that really can't be addressed without increasing the motor size substantially. When my Rockwell 11" Reeves drive was in need of a fair bit of work, I considered the possibility of converting to VFD (I was a bit enamored with them myself at the time). But using rough calculations it appeared I would need to convert my 1 HP 3PH motor to a minimum of 5 HP to even come close to breaking even, and I was still considerably short on torque at the lowest speeds. But still, some people are perfectly happy with the compromises and seem largely unaware the sacrifices were even made, so real world results vary. But I know I frequently stalled the 11" Rockwell, and have managed on several occasions to significantly load my 7.5 HP 17x60, so loosing the torque multiplication is a non-starter for me.

But then, that's a lathe. Band saws don't face the same challenges in operation that lathes do, and lower speeds are more a function of keeping the teeth from over heating in the cut than fighting ever more leverage and mass (larger diameter work pieces) working against it, though heavier cuts in harder/tougher materials with larger teeth/chips do go down that road. So somewhat like a mill, that application is likely a lot more forgiving than a lathe, though overheating is still a challenge to be addressed. Whether it will work for you in this application only you can determine.

[Glenn Brooks]

Well said Baddog! I've never used a VFD and only recently started reading about the downside you mention. I think I've given up the idea and will instead look at rebuilding the headstock, or pass it along to someone who might be interested in restoring a fine old piece of equipment.

Glenn

[ctwo]

So somewhat like a mill, that application is likely a lot more forgiving than a lathe, though overheating is still a challenge to be addressed. Whether it will work for you in this application only you can determine.

I have thought about eliminating the variable drive on my bridgeport for a fixed belt drive - set up to give me about 6k at the top end. I would still have use of back gear and could run the motor 50% to 150%. Those are just rough ballpark waestimates...

[stephenc]

i have a vfd on my little 10x22 grizzly lathe . while any loss of power at lower motor speeds isnt much of a concern on my little machine i tend to use the variable speed mainly as a way to fill in the gaps of the existing speed ranges .
but on larger work i can definitely notice a power loss if i run the motor under 30 hertz .

[WJH]

Lower fan speed and higher current are valid concerns with a VFD, but a Vectored drive will give you the torque at lower rpms.
Haven't used a VFD on a lathe, but on my Bridgeport, I do NOT care that my head is a belt adjust type. Keeps things simpler and rarely do I need to change the belt position. I also have found that to have a nice variable speed range, increasing the max HZ to 120 works well. A 1850 rpm motor spinning at 3600 rpm at 120hz isn't much of a concern. A 3600 rpm motor going at 7200 rpm might be.

[Hopefuldave]

Re low fan speed, I added a Large fan to my Holbrook's open-frame motor, controlled through a contactor by the VFD: the ABB I use has programmable relays, by setting it to open ABOVE a configurable 'supervisory frequency' (ABB parlance) the fan kicks in if below 35Hz or stationary to cool the motor, no temperature problems so far, even down at 5Hz! The fan' a 10" industrial axial-flow pulled from a big (500KW) UPS at work, had done 3 years of annual tests (so about 4 hours runtime, gotta love Preventative Maintenance) and is mounted using a cut-down chemical drum...

Dave H. (the other one)

[SteveHGraham]

My drill press has a VFD, but I didn't bother taking off the step pulleys. There was no reason to. They weren't hurting anything, and I would have had to find single pulleys to replace them.

It worked out fine. I almost never have to change pulleys, I get the convenience and control of a VFD, and I can move the belt for low-end torque if I need to.

I don't know if putting step pulleys and a VFD on Glenn's lathe is a good idea, but it might be a better option than replacing the gears. I don't know what his bearings can take, but it's possible he could get speeds a lot higher than 1280 RPM without harming the machine, and he might also do well on the low end. More speed would help with small work. I looked up the cheap bearings on my drill press, and they're good to 14000.

Now that I think about it, I should crank up my drill's speed when I'm drilling small holes in PCB board and see what it will do. I'll bet I can beat the factory top speed of 4250 RPM pretty badly. I don't know what the limiting factor was when they made the machine, but my guess is that they didn't want a 10"-wide pulley, and they didn't expect anyone to use the machine with tiny carbide bits.

As for heat, a motor can't tell the difference between its own fan and an external one.

[Magicniner]

Well said Baddog! I've never used a VFD and only recently started reading about the downside you mention.

Calling it a downside is the same as calling the inability of a car to fly a "Downside", if you read the manual and specifications you find it's not a design feature.

Provided the belts/bearings/gears in the machine you are playing with can handle it most motors are easily capable of running at 200% + of rated speed and with a good vector drive also developing good torque and power at 50% of rated speed and below, this can give you a massive continuous speed range often covering a large part of day to day work without swapping the physical drive ratios. That's just one big upside

- Nick

[BadDog]

It is a "down side" if you expect a VFD to completely replace a full range mechanical drive (or expect a car to fly). Unfortunately, while most have a grasp of reality that prevents them from expecting cars to fly, it seems that a LOT of new hobbyist see (reasonable if not complete) descriptions of VFD applications and make the jump to "I could just replace the entire noisy/worn drive with a VFD!". And it doesn't help that in doing "Google Research" you can find a fair few posts on boards and web pages describing just that with seeming success. Even if it mentions the trade off, many don't really seem to recognize the significance, particularly when compared to loss of torque they are also sacrificing. I know that even though I was aware of the issues, I didn't really realize the full impact until I did a guesstimate rundown of the numbers for my Rockwell 11.

So unlike expecting cars to fly, it's quite easy for many to think that there is no downside to replacing a mechanical drive with VFD...