mach3 turn lathe spindle stalls

[jasminder]

friends,
i am having a very old problem i am fed up of. My lathe's spindle stalls when using a parting off tool. I can set any rpm using the s word in mach3 turn But it seems that mach3 is not synchronizing the speed with its output. i think that the freq on the VFD shud increase when load is put on the spindle and mach should regain the rpms it is set for. Am i right?

thanks in advance for your help,
jasminder singh

[Hood]

Your VFD should try and maintain the commanded speed, if you are having problems then it is likely the VFD is not set up properly or the motor is just to small for what you are wanting to do.
What size spindle are we talking?
What revs are you parting at and what % of full motor speed is it?
Hood

[jasminder]

dear hood,
the motor's full speed is 1370 rpm and at spindle it is 1700 rpm.
1 hp motor.
i use parting tool at 800 rpm (it is about 25 hz at my vfd)
max freq at vfd is 50

[Hood]

I think you may have just found out the limitations of using a VFD. I have not used many but I do know that the one thats on my Bridgeport has very little torque when down about that kind of frequency. I have heard some of the newest VFDs are great once set up properly but afraid I have no knowledge of them
Hood

[jasminder]

thanks hood.
May be i shud switch to 2hp motor.
My vfd is already 2hp.

many thanks,
jasminder singh

[Hood]

It would likely be better, I know a lot of commercial machines that used to run using VFDs often had motors 2 or 3 times the size that you would expect on an equivelent machine that used pulleys, this was because the loss of torque as the revs dropped, so they double or tippled the size of the motor to compensate.
Hood

[Overloaded]

If shopping for another motor, be sure to check out Inverter Duty motors. They are very different than general purpose motors which fail miserably with a VFD as Hood describes.
They cost a bit more but perform much better with a VFD.
I use Marathon Black Max and Micro Max Inverter Duty motors.
They have 1000:1 Constant Torque operation from 0 to base speed (TENV), and 20:1 Constant Torque operation from 1/20 speed to base speed (TEFC).
I got lucky and picked up several new ones for $29 ea., suggested retail was $850 ea.
Also bought a couple new from Automation Direct.

Real workhorses.
Good luck,
Russ