Design Considerations For A 66,000 HP Motor Driving An Air Compressor.

LecturePg. 81-92The design of a 66,000 hp electric motor to drive an air compressor presented significant design challenges. Using conventional assumptions in assessing the bearing support and foundation stiffness yielded a scenario where it would be impractical to achieve a 20 percent separation margin between operating speed and the lateral critical speeds of the motor. Structural stiffnesses well above conventional values were required in order to achieve the specified separation margin. Finite element studies were performed on the bearing pedestals and motor base. As a result of the studies, it was deemed necessary to perform a modal finite element study on the complete motor assembly. Eventually, it was decided that a modal analysis would be performed on the entire drive train (motor, gear, compressor, and foundation). These studies confirmed that the structural stiffness needed was achievable. Factory and field tests substantiated the analysis. Another challenge in this design was the ability of the motor to accelerate a drive train with a polar inertia in excess of 355,000 lbᵐ -ft^2. The rotor construction is such that there is not a discrete amortisseur. On startup, currents are induced in the rotor pole face, which lead to significant heating. Surface temperatures in excess of 400 ᵒC (750 ᵒF) were calculated. Plastic deformation of the rotor pole face will occur at this temperature due to large thermal strains. Consistency of the shaft forging mechanical properties is paramount if shaft bending is to be avoided. A stringent forging specification was written, which limited the number of forging suppliers who could meet the requirements. Finite element studies were performed to calculate the stresses and plastic strains. High temperature mechanical testing was performed on the forging material to confirm that the stress-strain hysteresis loop would close after repeated cycling. Furthermore, care needed to be taken to ensure that the rotor winding and insulation system was protected from damaging temperatures. Once again, factory and field test confirmed the analytical work