IS this the 3 speed KMG with pulleys and a TEFC 1ph motor? What HP is the motor? How were you grinding - really bearing down hogging metal that puts a heavy load on motor? OR, light grinding? As mentioned if the fan has came loose and not cooling would cause the motor to overheat. If an open frame motor crud can get inside and need blowing out.
Good luck - and as you mentioned, a VFD and 3 ph motor makes all the difference in the world. I used a 3 speed pulley setup for 2 or 3 yrs and thought that was all I needed. Once I got a 2hp 3ph TEFC motor and VFD (total cost $245) I wondered why I didn't make the move years ago. Yes, it's a NEMA 1 Chinese VFD, but with filters over the air inlets they tend to last for years.