Lathe VFD and spindle speed selectors

[llamatrails]

I asked this in another thread, but brought it out on its' own after it languished ...

One point I have not seen addressed has to do with where to place the spindle speed selectors when using a VFD and a pot to fine tune the spindle speed ?

I have my VFD controlled PM-1236T pulleys both on the larger side, and basically find that each spindle speed selector setting gives me the same spindle rpm range with the pot.

Is there an advantage to one spindle speed selector setting over another when using the VFD pot, i.e. just use the lowest for a mechanical advantage?

 

[JimDawson]

Under most conditions running the motor between 30 and 90 Hz is a good practice. So in general, use your gear change levers as the course adjustment and fine tune with the VFD.

 

[Ken226]

Under most conditions running the motor between 30 and 90 Hz is a good practice. So in general, use your gear change levers as the course adjustment and fine tune with the VFD.

This is what I do as well. I got the 3phase/VFD primarily for the smooth power delivery, quiet operation and improved surface finish.

I still use all of my headstock gears for coarse speed range selection, and occasionally use the pot to fine tune. Such as when facing, slowly increasing the speed as I get closer to the center, or when moving to a different diameter portion of the same part/setup.

 

[Beckerkumm]

I run a vector duty motor so I usually pick a gear where I can stay in the 50-90 hz range. That type of motor likes to run over 60 hz and staying there puts full hp and smooths out the rpm on the four pole motor. On my lathe I think I get a better finish in open belt ( don't ask me why ) so I try to thread at lower motor rpm than when turning to avoid the back gear. Fairly easy as I have a 4-1 reduction with a two speed clutch which makes the 8-1 back gear not very necessary yet still maintain a motor speed above 30 hz.

Gear noise is your friend here. When overspeeding a gear range, your machine will let you know if the gears are happy. Straight gears will get clanky if speeded up above their design so stay within the mechanical range when increasing the motor range above 60 hz. Dave

 

[jaek]

I have my VFD controlled PM-1236T pulleys both on the larger side, and basically find that each spindle speed selector setting gives me the same spindle rpm range with the pot.

Are you sure you are looking at the spindle speed and not the motor speed?

There should be no way that you get the same spindle speed range in 1-H as you do in 3-L....

 

[mksj]

The belting speeds on the 1236T/1340GT are ~2:1 in low and 1:1 in high, by flipping the motor pulley you get 1.5:1. Typical speed range for the VFD would be 30-90Hz which gives the same speed range as both belt ratios. You can go to 20Hz for light duty cutting/threading, the chuck will also stop faster. The top end speed I recommend is 100Hz for newish non-inverter rated motors, as motor performance does drop off a bit above 100 Hz and you need to be mindful of the spindle rated top speed. When going below the motor base speed, although motor torque remains mostly constant you loose the mechanical advantage of the mechanical ratio. In reality because the VFD runs in sensorless vector mode and can boost torque, this is usually not an issue with the exception of very low speeds and large diameter work.

 

[llamatrails]

Thanks everyone for your replies. Sounds like the best for the gears and motor would be to use the gears for their intended spindle speed with the VFD to tune up/down around that speed.

 

[Firebrick43]

Thanks everyone for your replies. Sounds like the best for the gears and motor would be to use the gears for their intended spindle speed with the VFD to tune up/down around that speed.

In most instances you can get away from using half of the speeds, say every other one. Unless your in a production environment and are trying to take off the maximum amount of metal per hour, the conveniences of just changing speed in most hobbyist or even tool room type jobs will save you much time and increase enjoyment.