Case StudyTorsional instability during starting of an induction motor has caused excessive torque oscillation and eventual failure of gearbox pinion teeth of a cooling tower fan train. A torsional instability appeared due to a steep slope of the induction motor starting torque curve, combined with a torsionally soft driveshaft and a large moment of inertia of the driven fan. In this paper, mechanisms of torsional instability occurrence during startup of an induction motor in a cooling tower fan train are discussed, as well as details of the investigation outcomes of troubleshooting activities, including field measurements. The methods of modeling and analysis are also described. To examine the mechanism of torsional instability occurrence, in addition to transient torsional analysis, the concept of torsional stability analysis is introduced. Practical engineering guidelines to avoid this type of problem, such as a motor selection guideline, are also addressed
