Magnesium Rotor
Magnesium Rotor is a type of material used in some motors. The operating experience has shown that a magnesium rotor has a failure mechanism that should be monitored or placed.
Magnesium has a lower density then aluminum, on some of these huge rotors that density made a pretty big difference. You will mostly see it on Limitorque models SMB-2, SMB-3, SMB-4, SMB-5. The properties of the magnesium allowed for a greater torque output from the motor. Also, when you shut off the motor on an MOV, there is a lot of inertia, which causes the disk to seat harder into the valve which makes it a bit more difficult to control
OE
- BWROG-TP-09-005, Inspection of Motor Operated Valve Limitorque AC Motors with Magnesium Rotors
- Information Notice 2006-26, Failure of Magnesium Rotors in Motor-Operated Valve Actuators
- Information Notice 2008-20, Failures of Motor Operated Valve Actuator Motors with Magnesium Alloy Rotors
Limitorque models SMB-2, SMB-3, SMB-4, SMB-5
Monitoring
Magnesium rotors are susceptible to degradation over time, especially if there is high humidity, radiation, or temperature in an area. They have an enamel-like coating over them to protect the magnesium. Sometimes that coating can wear quicker especially when the rotor heats up, expands, and cracks the coating, exposing the magnesium underneath
Borescope PM is used to periodically check if the rotor is degraded and needs replacement.