Torsional Vibration

Paper No. 119

High vibration levels were occurring on the integrally geared compressor causing the unit to trip. The compressor manufacturer verified the vibration readings, replaced the bearings, and found evidence of backlashing on the gear teeth. There is a variable frequency drive (VFD) that is primarily utilized for electrical soft-starting but remains engaged at full operating speed of the induction motor (3583 RPM, 60 Hz).

A strain gage telemetry system was installed on the motor shaft and used to measure alternating torque of more than ±120% of the full load torque (FLT), which is an abnormal condition for centrifugal machinery. The predominant frequency of the torque fluctuation was the first torsional natural frequency (TNF) of the system. The excitation source was the VFD when operating at any speed above the TNF even though there was sufficient separation margin (SM) from torsional resonance.

The medium voltage drive was found to be in Volts/Hz (scalar) mode, which is typical for equipment not requiring precise speed or torque control. Correlation was made between the torque fluctuation and sidebands in the electrical current at the motor junction box, also indicating the VFD was the source of the excitation. Several attempts were made to tune the VFD parameters with no significant improvement.

The VFD was changed from scalar mode to sensorless vector mode, which is better able to control the torque and magnetizing portions of the current. The motor had to be decoupled from the compressor and operated solo so that the drive parameters could be tuned. Upon restart of the compressor unit, the alternating torque was dramatically reduced to an acceptable level. If this torsional vibration problem had not been detected and corrected, the high alternating torque levels could have damaged the motor shaft, coupling components, and gearing in the compressor.

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