I have a 2”x72” belt grinder with a Brook Crompton bf4n.75-2 motor.
It is connected to a Teco L510 VFD. Running here in the United States on 110v. My question is about turning up the RPMs sometimes when working on projects and hogging out material.
Is this doable? What would be the safest RPM without causing damage to the motor or bearings?
I know with my VFD I can up the frequency to do so and have seen many people do it. Just don’t want to do this if it is going to burn up my motor.
Normally 1800ish RPM is fine but every so often at the start of projects it would be nice to get the belt moving a bit more and removing material.
Best Answer
I believe that NEMA standards require some mechanical overspeed capacity. I may be able to look that up later, but I am guessing it might be 25%. Based on various projects that I have worked on or quoted, I think there is a very good possibility that 50% overspeed will be ok.
The primary concern will be rotor balance and the balance of the external fan. You might consider removing the external fan and replacing it with a separately powered blower. There is a good chance that the bearings will be ok, but you need to be concerned about overhung load.
You probably already know that the torque will drop drastically unless you can increase the voltage as the speed increases. You may be able to get constant power up to 150% speed, but the available torque will drop at a faster rate above that.
NEMA Standard MG1
The following excerpt from the condensed version of NEMA standard MG1 indicates that the motor should be fine at 3600 RPM. Note that the table is for direct coupled applications, not driving the load through a belt.