VFD/ 3ph motor on lathe – Need countershaft?

[WhoWhatNow]

I picked up an Atlas 12” lathe and plan on tearing it down shortly to clean it up and fix a few issues. One of the things I would like to do is replace the 1 ph motor with a VFD and 3 ph motor. I would like to do this swap for two reasons: the ease of changing speeds and to convert the lathe to an underdrive to better fit into my miniscule shop. This type of Atlas lathe originally came either as a benchtop unit with the motor hanging off the back, or and underdrive model. The underdrive model had a different pulley on the spindle and two sets of countershafts plus the motor under the headstock. If I add a 3 ph motor and VFD with sensorless vector drive do I need to also recreate the additional countershaft setup or can I simply have the motor driving the spindle? Do the additional pulleys add a torque advantage over just driving with the motor? Also, what speed motor should I get? The lathe currently has a replacement Baldor 1 hp 1 ph motor spinning at 1750rpm. Do I simply calculate the top speed I want and put the correct size pulley on the motor? The original top speed of the lathe is listed at ~2200rpm.

 

[Gerald O]

If the driving pulley has a smaller diameter than the driven pulley then there will be a torque gain on the driven shaft with a corresponding reduction in speed. Equal size pulleys = no change. Generally lathes will have one or more stages of speed reduction through pulleys with the torque being multiplied at each stage. Your motor might not have enough torque to rely solely on the variable speed drive for speed reduction.

 

[A_Pmech]

It's helpful to start from the top or the bottom of the speed range. So, let's say you want a top speed of 2,200 RPM. Sensorless vector drives can provide about a 5:1 variation in speed while maintaining a reasonable percentage of motor power. That would provide a bottom speed of about 440 RPM in open belt at some percentage of the motor nameplate power.

If that's insufficient, a back gear or counter shaft would provide lower speeds. A good overlapping range would be a 1:5 back gear, which would provide a lowest speed of 440 / 5 = 88 RPM and a top speed of 2,200 / 5 = 440 RPM.

As far as overheating goes, rsanter is correct. Running at much less than 60 Hz. will heat the motor up. A cheap ODP motor with a fan blowing through the windings will alleviate the issue. If it does fail, it was cheap. Or, one can scrounge an inverter duty motor off the 'bay.

 

[rsanter]

I think you need a countershaft just to put the lathe speeds in the range you can use them.
If you rely on only being able to turn the speed down with the VFD you will be running under 30hz all the time which will overheat the motor.
It is best to be able to move a belt to get into the range you want and then fine tune with the VFD

Bob

 

[WhoWhatNow]

Thanks for the good info. I didn't realize that running the motor at 30hz will cause it to overheat.

It appears that the back gears are a 1:6 so from a speed perspective it may be OK. If I can score a cheap inverter duty motor off eBay I will go that route. If I don't find one by the time I am ready to start this project I will fab up a frame and mount the existing counter shaft under the head stock.

Thanks again for the help.

 

[rsanter]

The motor will overheat because it will not have enough air flow through it to cool it running lower speeds for a extended time.
I have seen where people add separate cooling fans to help counteract this situation

Bob

 

[WhoWhatNow]

The motor will overheat because it will not have enough air flow through it to cool it running lower speeds for a extended time.
I have seen where people add separate cooling fans to help counteract this situation

Bob

So a standard 3 ph motor and a fan blowing through one end? I guess I could get away with a shaft mounted fan similar to what DC treadmill-type motors use?

 

[larry_g]

With a VFD you have the capability to overspeed an motor just as you can slow it down. I'm assuming that you don't have a lot of lathe experience, or know the work you will be doing. You've got a perty lightweight lathe so your not going to be running a lot of big highspeed work on it, consuming a lot of HP. I would suggest you ball park the work your going to be doing and the speed you should be running at for those jobs. Set the drive up so that this is your mid range speed with the motor running at 60hz. Now you should have a stepped pulley to speed up or slow the spindle. From there you can use the VFD to alter the drive from 40hz to 80hz to stay in the safe range of most motors.

Spend some time doing the math on speeds and feeds to get this set up to where it does what YOU need it to do. If in doubt then fudge toward the lower operating speed.

lg
no neat sig line

 

[gorilla]

So a standard 3 ph motor and a fan blowing through one end? I guess I could get away with a shaft mounted fan similar to what DC treadmill-type motors use?

Motors overheat when the frequency is lowered because they don't have enough iron in the core not because the fan turns slower. From what I remember you derate a motor 20% when you change the frequency from 60 hz to 50 hz. I'm sure a auxiliary fan will help keep it cool but air flow isn't the reason for heating up.