Vibrodynamical behavior of bulb units, due to specific design and operating conditions, depend very much on the relative position of the rotor and stator as well as the bearings. Due to the horizontal shaft layout and the fact that the unit is submerged, rotor sag and construction thermal conditions have a large impact on the machine dynamic condition.
On two bulb units at the same plant, the condition was monitored for several years, with the purpose of investigating the correlation of excessive events (instabilities) to operating and start-up conditions. The monitoring system included rotor vibrations, air gap and flux, rotor pole temperatures, stator winding temperatures, axial displacements and process operating quantities (e.g. active and reactive power) as well as electrical values.
Especially prominent were the problems on unit B where cracks on the rotor rim appeared that could have been related to vibrations in combination with manufacturing defects. The monitoring system’s comprehensive database enabled cross correlation of different values to assist in identifying the root cause of this instability, which appeared to happen quite unexpectedly. Detailed analysis of the data recorded by the monitoring system showed that the described vibrational instabilities were the result of the rotor thermal deformations and rotor’s thermal imbalance likely related to the attachment between the rotor poles and the rotor rim.
Key words: Vibrations, generator, bulb unit, monitoring, deformations, imbalance