Abstract
This paper reports the results of an activity of failure analysis performed on a 370 MVA generator rotor that was scrapped after about 133 000 operating hours and about 400 start-ups, when the on line monitoring system revealed a progressive increase in vibrations amplitude, due to the presence of a deep crack. It was actually detected by automated ultrasonic equipment using advanced signal processing. Failure analysis and laboratory tests demonstrated that the crack started from a forging defect located at the root of a longitudinal slot, due to the high stress range induced by oil film instability problems. During the following years the crack grew mainly during transients and not during operation.
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