Rotordynamic Analysis of a Two-Pole Synchronous Motor with Sleeve and Pressure Dam Bearings

A two-pole synchronous motor was recently rewound for the von Karman Gas Dynamics facility at Arnold Engineering Development Complex, Arnold Air Force Base, Tennessee. After installing the rewound rotor, unexpected vibration amplitudes were recorded during motor checkouts. To resolve this issue, an investigation was initiated to investigate the causes of the vibration issues. The investigation discovered that the original design used sleeve bearings rather than pressure dam bearings. A study was formed to determine the effect of changing the pressure dam bearings back to sleeve bearings. Because only one spare bearing shell existed, the bearing with the highest vibration amplitudes was chosen to be switched. A lateral rotordynamic analysis was performed to determine the impact of this switch, prior to performing the bearing swap.

The rotordynamic model predicted that the rotor was operating near the second critical speed. Regardless of the bearing change, the second critical speed was not impacted. However, the determination was made to change the bearing to the sleeve bearing due to predicted lower vibration amplitudes. From the motor checkout runs, the model prediction was verified and the sleeve bearing was kept. Vibration amplitudes have been reduced, but issues still remain with the rotor operation near the second critical speed. Further analysis is required to successfully shift the rotor operation away from the second critical speed.