Inherent-overheating protection

Abstract

INHERENT-OVERHEATING protection is, as the name implies, protection based upon the actual motor temperature; thus, the motor is protected on the basis of total temperature and not temperature rise, as is the case when an external current-limiting device is used. Fundamentally, in order to realize true inherent-overheating protection, the temperature of the thermal element should follow at all times the motor-winding temperature, thus limiting the winding to a definite temperature, regardless of conditions of line voltage, motor load, ambient temperature, or ventilation. A common location for the thermostat is in the motor end shield, where it is in as close heat-receiving relationship to the stator windings as practicable. An auxiliary heater is provided in the thermostat to compensate for the difference in temperature between the stator copper and the thermostat mounting and to provide fast tripping under locked-rotor conditions. The auxiliary heater maintains the disk at a temperature somewhat above its ambient temperature; thus, the tripping characteristics of the disk-type thermostat are affected in a predetermined way by the ambient temperature of the thermal element.