I run a 3600 rpm motor at 300 rpm a quite often - just not too long at a time (5 minutes?). If you wish to run less than 10% speed for an hour, put a fan blowing on the motor for cooling. How much cooling will be needed depends on how much load you are putting on motor. At those low rpm the motor is basically running with no load so it's not heating up near so fast as if turning a load.
Ok guys I have my VFD and motor and am starting a new grinder build can someone point me to a place to buy a remote on of switch and a potentiometer knob so i can have it at the grinder and mount the VFD farther away.
Here ya' go. You want a DPDT toggle switch rated at 15 amps with 3 positions, ON-OFF-ON which will give you Forward-OFF-Reverse. The switch will only be carrying mA of power, but the 15 amp is to get the larger switch and not a mini-toggle switch. https://www.ebay.com/itm/401623319313
You want something like this: https://www.ebay.com/itm/232964213932? for the speed control. A 10K "linear" pot, NOT a typical volume control pot which is a logarithmic pot. You might find a 5K pot works better, but ether should work.
You want to mount all 3 of those items in the same box. The ON-OFF-ON switch and pot both carry mAs and 10 VDC or so. The OFF-ON pot will carry the full supply voltage and full current of the VFD. There "should" be a divider inside the box between the two sections.
You "should" (maybe that's "might") be able to find those 3 items at your local Radio Shack.