Rotor shape vs. rotor field pole shorted turns: Impact on rotor induced vibrations on hydrogenerators

In the past few years, quite a few cases have arisen where shorted turns were detected or suspected on a pole of a hydrogenerator. In many instances, the shorted turns are suspected because of sudden excessive vibrations near the rotor, which are observed when the magnetic field suddenly appears in the air gap. In other instances, scheduled tests during outages, such as voltage drop tests, are performed and results indicate shorted turns on one or many poles. Many engineers believe that the presence of shorted turns on hydrogenerators cause excessive vibrations because the shorted turns create a weaker magnetic field at a specific pole when compared to the other poles. Therefore; it is believed that a corresponding vibration at the shaft is induced; which shows up at the upper guide bearing and/or at the combined guide bearing below the rotor. This paper will show that this is not the case, unless the conditions are quite severe, as in a case with an extremely high number of shorted turns, located on many specifically located poles. We will discuss the causes of vibration emanating from the rotor, from the perspective of rotor magnetic flux as well as rotor mechanical anomalies. We will compare the principle of vibrations induced on non-salient 2-pole machines vs machines with large numbers of salient poles, explaining the confusion between the two concepts and why many believe the same issue exists on hydrogenerators. Finally, we will explain why rotor shape is a greater contributing factor to rotor induced vibrations than actual rotor shorted turns.