The use of air-gap magnetic flux probes has proven effective in the detection of generator rotor winding shorted turns and has helped to improve the quality of predictive maintenance decisions concerning when or if to perform rotor rework. Analysis of air-gap flux probe data can pinpoint the number and location (pole and coil) of shorted turns while the generator is running at normal operating loads. This allows the identification of shorted turns which may not be detectable when the unit is offline since the operating centrifugal and thermal forces are not in effect at standstill. A recent case history shows the development of a turn short in a large two-pole generator rotor that was signaled by a generator core monitor (GCM) alarm. The GCM provided a 5–10 minute alarm that was immediately followed by a significant increase in rotor bearing vibration. Testing of the unit showed that the higher vibration levels were thermally sensitive. Flux probe testing showed the development of an unambiguous turn short in one of the #3 coils.

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