- Oct 14, 2017
Hi all. My old LeBLOND lathe has a max RPM of 500. Is it safe to use the VFD to increase? And how high of a frequency is ok? 1 hp 3 phase motor.
Not necessarily. In an induction motor torque is related to slip - the difference in speed between the rotating magnetic field and the actual rotor rotational speed. A good VFD should be able to give near full torque at any speed from standstill to well over rated speed.More rpm = less torque.
Torque falls off above a motor's base speed because the V/Hz ratio cannot be maintained above the motors base speed. So torque falls off in a somewhat non-linear fashion above this point for standard motors. What is not often factored in is that a machine with a fixed speed motor, to achieve the same machine spindle speed needs to be geared/belted up and thus torque is reduced based on the change in motor to spindle ratio. There are some variations on this where you have a dual voltage motor, say 230/460, you wire it for 230 but run the VFD to 460V. BLDC and other motor variants may also be able to maintain full torque within the operating design of the system. Hp falls off in a linear fashion below the base speed, one reason why VFD machines often oversize their motors vs. a comparable geared fixed speed motor machine. VFDs can also boost torque/current to the motor for short periods of time, so you will often see 150% boost for 1 minute. An older motor should be set to 100%. The speed range that a motor can maintain torque below it's base speed varies by motor design. Typical older motors might have a 2:1 constant torque ratio so 30-60Hz, newer inverter rated motors maybe 10:1 so 6-60Hz, and vector motors have a 1000:1 or greater so full torque down to 0 RPM.A good VFD should be able to give near full torque at any speed from standstill to well over rated speed
Wow. Lots of info.
I think I will keep it simple like Cooter mentioned. Might speed it up some but watch the spindle bearings.
Thanks for all tour input.
That's not true of my VFDs. They will run to at least 120 hz. I would not run my motors long at that rate, but I can double the speed according to the Tachometer. I usually do not go above 150 percent. I don't drop below 50 percent either, because the torque drops significantly. But between 30 and 90 hz all run well.A VFD will only decrease the input frequency from 60Hz down to 0Hz. That will only make your lathe run slower.
Edit: I didn't know there were VFD's that push more than 60Hz. I love google sometimes. More rpm = less torque.
That's very interesting as none of my Induction Motors have rotor windings and I have yet to see one that does ;-)going too fast can cause a catastrophic failure of the windings from centripetal force.
Depends on the motor... most induction motors are the “squirrel cage” type where the rotor is constructed with copper bars in a cage shape. Some motors like the ones in your washing machine as well as specialty high speed induction motors will have wrapped wire windings. Regardless I referred to them as windings. If you’d prefer, let’s use the word rotor. The same principles applyThat's very interesting as none of my Induction Motors have rotor windings and I have yet to see one that does ;-)
Induction motor rotors are made in exactly the same way for 4 pole motors as for 2 pole motors and they don't fit different bearings for the same frame size in 2 & 4 pole motors.
Applying some engineering logic this strongly suggests that Inverter Duty 4 pole Induction Motors can be safely run at 200% of rated speed.
And that is indeed the case ;-)