An AC electric motor requires the cooling that is generated at the speed it runs normally. This is achieved by simple cooling fins usually pressed onto the main shaft. If a potentiometer or rheostat is hooked up, "in the hot side of the 110V power wire works easily enough," the time the motor runs at a slower speed needs to be short. Over heating could be checked by physically checking the housing with your hand to feel if it is hot. Normally it will create heat, but if it is to hot to hold your hand on it for more than a few seconds, it is over heating. If you smell a sort of chemical odor emanating from the grinder, the amateur coating is burning and that means it is over heating as well. All of the different size bench grinders I have owned and still use, I have outfitted them with a basic dimmer switch for the interior lights from 24V Diesel application and they still work. Although I have the switch bypassed with another wire with an On-OFF switch for use during normal operation.