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.

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    \$\begingroup\$ You acknowledged that this is electrically possible, this question is more about the physics & engineering of doubling the operating RPM of a mechanical system. It isn't really appropriate for this site. \$\endgroup\$
    – Eric Urban
    Commented Jan 9, 2019 at 3:48
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    \$\begingroup\$ Most of the knowledge needed to reliably answer this question comes from the domain of folks who use that sort of tool. I could give you a whole laundry list of things that might go wrong, but I don't know what would go wrong. OTOH, I'm sure there's plenty of shops out there where people have over-speeded their belt sanders by a factor of two and have either lived to tell the tale, or are survivors of the event from several feet away. I'd ask on a woodworking group (or machining group, if you're using it in a metal shop). Note that a factor of two speed-up is a lot. \$\endgroup\$
    – TimWescott
    Commented Jan 9, 2019 at 3:50
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    \$\begingroup\$ If the motor has sufficient torque, you could use a drive wheel of larger diameter. \$\endgroup\$
    – John Canon
    Commented Jan 9, 2019 at 6:41
  • \$\begingroup\$ Thank you everyone for the time and thoughts! I might try a larger diameter drive wheel and see what I get. Thank you again! \$\endgroup\$ Commented Jan 10, 2019 at 13:18

2 Answers 2


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.

enter image description here

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    \$\begingroup\$ +1 about the fan, that would be my greatest concern, too. \$\endgroup\$
    – Janka
    Commented Jan 9, 2019 at 5:38
  • \$\begingroup\$ It's often instructive to look whether there's a 2 pole (3600rpm) motor available in the same outline, and in this case there is, with an identical outline drawing referenced, so it's likely (though not guaranteed) that the fan and bearings, and probably rotor construction, are common, and will be OK at the higher speed. Cooling necessarily goes up substantially with a higher speed and the same fan, so motor temperatures are unlikely to be a problem if the phase currents are no higher. I'd be more worried about the belt tbh. \$\endgroup\$
    – Phil G
    Commented Jan 9, 2019 at 15:27

Despite the issue of whether or not the motor is designed to run at that speed (in terms of balancing, bearing ratings, cooling etc.) you will have to address the torque issue. If you use the motor at the 230V windings and run the frequency up to 120Hz to get 3600RPM from it, you will not be able to deliver the proper V/Hz ratio that the motor needs to create torque, because you cannot increase the Voltage above the capacity of the VFD. So once you get above 60Hz, the motor runs as a "constant HP" motor, meaning it is 1HP at 1800RPM and it is still 1HP at 3600RPM. But HP is a function of torque and speed, so by only increasing the speed, you are DECREASING the torque. In theory your motor will have 1/2 of the torque at 120Hz than it does at 60Hz. In reality is ends up being less than that because you will have significantly increased losses in the motor.

So what will matter is what you intend to DO with this machine at that higher speed. For something like a belt grinder, it's likely that it will stall...


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