Variable Speed Motor in Lathe

I have a lathe that I bought used several years ago. The variable speed mechanism has been removed and just replaced with motor and VFD. When I use the Lathe in high speed with the VFD turned down, of course the HP decreases and I cannot take much of a depth of cut without bogging down the machine. Is there a work around this problem where I can control the speed of the motor without loss of power. I have seen Variable speed motors and not sure if those are different than just a regular AC motor with VFD attached.

So you have no gears, belts etc.?

Do you have the model of motor, drive, and lathe?

Check the drive is set to constant-torque or vector mode (not fan/pump, 'eco', or variable-torque), high overload, and the parameters are set correctly.

Why not put the machine into low gear?

Loss of torque at low speed is an issue inherent to induction motors on VFDs. A DC motor would deliver more torque on the low end but would yield poorer speed regulation and could potentially overload the drive train in the lathe. It is better to utilize the cone pulleys/gear head to good effect than to try to hog off material in top gear at 10Hz. That would be like towing a boat trailer out of the launch in overdrive at 200 engine RPMs.

Pretty much all I can effectively work in is Low Gear, but can only get about 150 RPM out of that, which has worked somewhat. I have another toolroom lathe to handle anything small Im doing but I was making a roller and shaft out of 13 inches of 3 inch 4140 and that was just too big for the toolroom lathe. The lathe in question is a Clausing Colchester VS12. British made I believe and almost impossible to get parts for. It had a cable mechanism for variable speed, it's gone, the cable and mechanism. I suppose I could change the motor pull size to get more speed in the low end range but was wondering if there were other motor options. I will have to look when I make it back to the shop as to what motor is currently on the machine.

You might be able to get more RPMs in low gear by increasing the drive's maximum output frequency. It's typically safe for the motor and drive to take this up to around 90-120Hz, but the rest of the drivetrain might not like it.

Certainly sounds like the drive is not set up properly. No idea what kind of drive you got. This is not a HP issue - this is a torque issue. The drive needs to be in sensorless vector mode (assuming your drive has this mode - if not its not worth owning) and auto tuned. Of course proper motor parameters need to be programmed in the drive.

I have a South Bend 16" lathe. I leave the belt permanently on the 75% speed pulley and overspeed the motor a little. 3 hp motor. The other day I was using a 3/8 die to cut threads in some 3/8 mild steel. Not a huge cut by any means compared to larger lathes, but I had the speed pot set to minimum rpm and it did not even blink. Perfect speed regulation.

I kept breaking 1/4 inch taps in my CNC with a floating tap holder in low gear. I bought the machine new. 3 hp spindle. In high gear at minimum speed I could stop the spindle with my pinky. Found the drive setup for v/hz mode. Changed to sensorless vector. It was better, but still could stop the spindle. Tuned the drive and no way can I stop the spindle at minimum speed. Now I can tap with leaving it in high gear and the floating head barely moves.

How is the motor attached? If you can, change pulley ratios that you have more rpm in low gear or less rpm in high gear.

Reminds me of my old F150 that liked to hunt for a gear between fourth and fifth when driving at the speed limit.


I will say that even a drive without any of the fancy modes or tuning should give near full torque at any speed, but you will have full slip at full torque regardless of speed, so you may need to increase the volts per hertz at low speeds to account for this, but be careful. Consider that a 3450 rpm motor has 150 rpm of slip at full load. Now set it to 360 rpm and you will only get 210 rpm at full load. The motor may appear to be stalling out but is totally within its current limits, assuming it has an external fan for cooling.
(Not to say that the features of the VFD shouldn't be used fully, but you can certainly work without it).

Normal variable speed motors are universal motors or a variety of capacitor start/run motors. If you are concerned about torque, neither of those will be your friend. DC motors are awesome but require good drives to work to their full capacity, which aren't as cheaply available as VFDs. To regulate speed on a DC motor you just need IR compensation in the drive, whereas AC motors need some combination of voltage and frequency adjustment to keep them at a certain speed.

Alternatively, you may be able to switch between a 2 pole or 4 pole motor or vice versa if you have no control of the drive ratio.

One factor to consider with running motors at full torque down to low speeds is that the cooling fan RPMs drop with the speed of the motor. Heating could become an issue. In industrial applications where motors are expected to run at high torque across their full range of speeds, independent auxiliary blowers are installed.

GeorgiaDoc said:

I have a lathe that I bought used several years ago. The variable speed mechanism has been removed and just replaced with motor and VFD. When I use the Lathe in high speed with the VFD turned down, of course the HP decreases and I cannot take much of a depth of cut without bogging down the machine. Is there a work around this problem where I can control the speed of the motor without loss of power. I have seen Variable speed motors and not sure if those are different than just a regular AC motor with VFD attached.

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Solving the problem "all electric" - AC OR DC- will probably cost you at least $1,000 MORE than .... finding a way to add-back another mechanical ratio or three.

A pragmatic "rule of thumb" is that decently stable, smooth, and well-regulated under-load stepless electrically variable speed wants twice the motor HP as mechanical ratios need, same tasking, otherwise.

That's not a relative "efficiency" thing. Just that either of VFD or DC drive fall off their 'sweet spot" as they move away from dataplate optimal RPM.

Sooo.. I mean simple step-pulleys, AND retaining what you already have as finer-grain variable within the ranges.

Stepless-variable Reeves Vari-Drives O/E can still be had, new, but a bigger motor and stouter VFD should be MUCH cheaper.

2CW.

And .....all three lathes, two of three drillpresses, both mills, and the shaper ALL variable speed. Four with DC drive, one strange AC series-wound motor (Electro-Mecano DP)

... then all-mechanical variables with one PIV-Werner-Reimers, three Reeves vari-belted or equivalant.

..side-issue, but the Precise grinders are variable, too - off Variacs - and the Panto-engraver same again ELSE air-pressure, depending on what spindle is mounted.

No longer ANY VFD's, nor will there be.

And I'm the better for it!

YMMV

thermite said:

Four with DC drive, one strange AC series-wound motor (Electro-Mecano DP)

... then all-mechanical variables with one PIV-Werner-Reimers, three Reeves vari-belted or equivalant.

..side-issue, but the Precise grinders are variable, too - off Variacs - and the Panto-engraver same again ELSE air-pressure, depending on what spindle is mounted.

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Rocking the old school speed control. I dig that. You don't see too many DC drives anymore. Stupid simple, dead reliable and loads of torque all the way down 'till they're on their knees just about to stall.

Just a Sparky said:

Rocking the old school speed control. I dig that. You don't see too many DC drives anymore. Stupid simple, dead reliable and loads of torque all the way down 'till they're on their knees just about to stall.

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LOL!

"If it ain't broke.."

Spoiled in my youth. Fifty-inch converted Niles cone-head. Fifty HP Dee Cee motor. 10 or 11 inch wide Morse "silent" chain drive.

Whom ever wudda thunk that running that old Dinosaurian would make some weak soul out in Walla Walla Washington go clear TF out of his mind? Well. OK. It was an awfully dam' short TRIP!



See the Reliance "white paper" on that "just about to stall". Their lab research proved 4 to 9 TIMES "nameplate" torque AT locked-rotor.... if but only for 90 seconds 'til commutator segment "wedging".

Dinosaur Current cannot "slip".

Move the load
OR trip a supply-side protective device or limit-point.
ELSE die trying.

Simple as that.

Each 10EE has a spare "nominal" 230 VDC motor. Armature run @ up to 275 VDC, 115 VDC Field bumped to as high a 140 VDC as well.

Motor is rated around 12 FLA @ 230 VDC ,115 VDC Field. SSD drives are set to allow 90 seconds @ 275 VDC & 24 A before throwing a flag on the play. The SSD sees that as a roughly 9 HP limit, energy-wise.

Hog with a 10EE? Nossir. No need. Got a larger 7 HP HBX-360 if need be.
All that is there simply for more stable regulation under load.

Ergo I'm good with the old "3 HP" large-frame. The 5 HP ones were not servo-smooth "Type T".

Got lots more Reliance RPM III in 180 VDC windings, 1 1/2 HP to 5 HP... and bought-NEW KB drives for them a'settin' on the shelf for the NEXT "projects".

- Second go at modifying the Kasto PHS presently in-work.

- Walker-Turner DP "owed" sumthin' better than step-pulleys.

DC Drives are easy. No capacitors to age-out.

But the MSRP on a Reliance Type T motor? A 5 HP starts at around 11 grand.

So I hoard the "NOS" and pro-rebuilt, not used since DC motors! Virgin wires never yet terminated all but a couple of them.



Meanwhile the Reeves drives ain't all THAT broken, either, and I LIKE the PIV-Werner-Reimers..so...

So, a little research on my machine late last night, still cannot get in back to see the motor plate......but, too small pulley on motor and too large on transfer shaft that then goes to lathe. Cheap VFD, and (this part idk but prob an real issue) the speed control is separate from the VFD, so I’m guessing a just a potentiometer that is separate, gotta move machine to see if it’s wired after VFD but I would guess so. So current plans, change the motor to shaft ratio....get a good VFD and control all from there. The way it’s set up, speed, on/off, and forward/reverse are separate from the VFD. I appreciate all the comments, showed me where to research and what things to look for.


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Just a Sparky said:

Loss of torque at low speed is an issue inherent to induction motors on VFDs....:

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Not correct: motors run in this way are constant torque below nameplate speed, and constant hp above nameplate speed.

He's complaining about lack of power really. Slutions: use a larger hp motor, or use a motor with lower nameplate speed. As mentioned if the drive is not set up properly, it will appear as loss of torque in general. A correctly tuned sensorless vector drive will show good torque down to 10 hz or so.

I found a NOS Baldor Tenv 5 hp 1800 rpm vector duty motor for $450 and can run it from 0-90 hz without loss of torque. At 120hz the torque will still be more than what you have currently. The motor is rated 1000-1 in hz reduction and up to 6000 rpm. Allen Bradley CM202 and Baldor IDNM are the same thing. Dave

GeorgiaDoc said:

So, a little research on my machine late last night, still cannot get in back to see the motor plate......but, too small pulley on motor and too large on transfer shaft that then goes to lathe. Cheap VFD, and (this part idk but prob an real issue) the speed control is separate from the VFD, so I’m guessing a just a potentiometer that is separate, gotta move machine to see if it’s wired after VFD but I would guess so. So current plans, change the motor to shaft ratio....get a good VFD and control all from there. The way it’s set up, speed, on/off, and forward/reverse are separate from the VFD. I appreciate all the comments, showed me where to research and what things to look for.


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Click to expand...

It's very likely that all those controls are simply inputs to the VFD; they can't directly act on the motor.

If you give us some data on the VFD we may be able to suggest some parameters to change before you go buy a new one.

Does it actually warn of overcurrent and throw a fault, or just bog down? If it's the latter, as someone suggested you probably just need to switch to Vector and tune it properly.

So, I have done some investigation and have a few pictures to share of what I am working with. Here is the switches and VFD, I cannot find any manufacturers name or model number anywhere on the BFD. All the power snd motor wires go into the VFD, and it appears that all of the switches all also go into the VFD by one wire. So it looks to me that these are just controllers off the VFD.




As far as the motor itself, it appears to be the original 2hp motor. I am including pictures of the original setup I got from manual and the current setup. I have no idea how this speed control mechanism worked.








There is no warning. When it stops it’s mechanical, doesn’t seem electrical.


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That is most likely a cheap chinese inverter... the lack of a 'brand' is the clue.

The motor is a reasonable size, and proper speed for modest general-purpose work... it won't be a beast.

The belt reduction you have there, is substantial. Unless that VFD was programmed for, and capable of substantial overspeed (like... 210hz or so) you won't have high enough spindle speeds to do smaller diameter parts well.

The original variable-speed drive consisted of two V-belt sheaves that have articulating sides. One was spring-loaded closed, to run the belt way out in diameter, while the other had a mechanical or hydraulic actuator that shoved it's articulating side towards the opposite face. When the faces there were wide, the belt ran very low (small diameter) while the other pulley's spring action would squish them close, running the belt at large diameter.
Cranking the manual pulley in, made it effectively larger, and the spring-faced pulley would open up, resulting in smaller diameter.

Clever idea, used in snowmobile and other types of constant-velocity transmissions... and work fine 'till there's cutting oil smeared all over, then they slip around like... nevermind...

Yes, those controls are just direct extension of the VFD's speed and direction controls.

Your VFD is not set up right, or it's not working properly. Both are equally necessary for proper performance.

I would not be surprised if the VFD is bad... those generics are best known for their success at extracting one's revenue, and replacing it with frustration and dismay.

So, current plan is to replace the VFD and controls first. Was looking at KB Electronics KBAC 29 (1P). Then tackle the pulley and speed issue after I get the electronics proper then decide on ratios that get me from current max speed in low gear to the machine rated max speed in low gear. Then high gear should be correct too. Does that sound like a workable plan?


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You had what amounted to a Reeve's drive so the belt ratio was variable between the two adjustable pulleys.

Your plan to replace just the VFD and using that one will have no better results. You should review what others have recommended and see how manufactures address this issue. The KB VFD drives are a very poor choice for this application at the price point, there are much better VFD's for less. I suggest you look at mid price VFD's such as the Hitachi WJ200, Teco E510, Yaskawa V100, Fuji or just about any other one will be a better fit in this application then the KB. You will end up needing to replace the motor with one with more Hp and one that covers a wider speed range. You will need a VFD sized to the motor size, so get at least a 3 Hp. If you do the math, you have physical 2 speeds, so you need to cover something like 60-400 RPM in low and 350-2000 RPM in high. So you need about a 8-10 fold speed range on the motor and you must belt it down the drive so you are operating the motor at a higher speed range. So operating the motor at something like 20-180 Hz. This requires an inverter/vector type motor, minimum would be 3 Hp, these motor in a 4P configuration have a base speed of ~1750 RPM at 60 Hz, the maximum RPM is typically 6K (these are not your typical motors, Marathon BlackMax or BlueMax, Baldor IDNM, Lincoln, etc.). These motors have flat torque down to a few Hz, but Hp drops linear below the base speed. Standard newish motors the torque starts to fall off below 15 Hz, and Hp drops off above 90 Hz as well as toque dropping significantly, so if you plan to use your current motor at say 10 Hz for low speed, forget it. Only other option is to add another belt ratio which is doable.

Otherwise you will need to increase the motor Hp, use a bigger VFD and more cost.

If you want a different solution AC clutches or PTO clutches can easily handle 2 hp and essentially allow a pulley to freely spin on a shaft or couple it to the shaft. You can have several ratios selectable by a switch. Select more than one for a brake (good control design is recommended)


If you find you absolutely need full power at any speed, a mechanical change in reduction is needed.


Motor->Jack shaft->various sized pulleys->clutches->spindle