An investigation of magnesium rotors in motor operated valve actuators

Failure mechanisms of induction motors in the valve actuators of motor operated valves are investigated. It is found that motors which contain cast magnesium rotors are susceptible to cracking and melting after carrying locked rotor current for as little as 61 s. Safety-related valve actuator motors may be thermally protected near this value to ensure operability of valve actuators. Corrosion is also discussed as a failure mechanism. An inspection procedure which allows a simple examination to be made to determine whether degradation may have occurred in in-service motors is provided