Motor Overload Protection for Motor Actuated Valves

Normal load and abnormal load conditions for valve actuators are discussed and load characteristics are illustrated for typical valves. Overload protection for short time duty rated motors is discussed when twice normal loads occur and when locked rotor occurs. The required performance of the overload device is outlined with emphasis on protection during locked rotor. The condition of locked rotor temperature rise is analyzed and tests conducted to compare with calculated values. Different types of overload protection devices are described and the limited protection provided by the internal temperature sensor under the conditions of locked rotor is pointed out. A systematic approach to external overload relay selection is illustrated