[SpEEdyBL]

Keep in mind that where the efficiency @ low loads is poor, the motor is drawing very little current, and therefore isn't making much heat. For example even if it's 50% efficient at a 10 amp load, and 80% efficient at a 50 amp load, then you are still making twice as much heat with the 50 amp load. That said, a good motor shouldn't overheat at low loads.