Torsional Vibration

Motor Cooling Fan Failures Solved with Modal and Finite Element Analyses

Paper No. 111

By Dr. Jae-Yeol Park, Dong-Ju Lee, Yong-Jae Jang, and Troy Feese

Torsional Vibration Symposium 2022 – Salzburg, Austria

Various problems occurred on two new synchronous motor – reciprocating compressor units. Torsional testing indicated that the first torsional natural frequency (TNF) of the system was coincident with the 8th compressor order and responsible for a compressor crankshaft failure covered in reference [1]. This paper focuses on failures of the motor external cooling fan and the solution method used to improve the system. At the worst compressor load condition, torsional oscillation of the motor fan had elevated levels at 8× running speed. Modal testing of the motor fan identified a second torsional mode that could be excited by the 12th compressor order. An increase of the compressor flywheel inertia was recommended to detune the first TNF but was predicted to cause insufficient separation margin between the second TNF and the 60 Hz electrical line frequency (11× running speed). An inertia ring was sized using finite element analysis (FEA) and added to the motor external cooling fan to increase the separation margin. Follow-up torsional tests of the modified system confirmed adequate separation margins for the first and second TNFs. The speed fluctuations measured at the modified external cooling fan were significantly reduced and acceptable throughout the entire compressor load range.

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