High vibration was detected during startup of three 2000-hp motors driving critical service compressors. A system natural frequency was close to the running speed. Extensive field tests were conducted; rotor dynamics were studied; and a simple finite element model was made to include the rotor, bearing oil film, motor frame, supporting steel structure, and concrete foundation. Results from the finite element model correlated well with field tests in the areas of natural frequencies, mode shapes, frequency shifts caused by changes in test conditions, transfer functions, and sensitivity to rotor unbalance. Design and procurement strategies to prevent or control future similar problems are proposed.

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