Quadrature Axis Force and Generator Core Vibration

In earlier papers1 2 the authors described investigations into the behaviour of stator end windings and stator cores. Details were given of the results of vibration measurements at the external circumference of the stator core laminations2. It was postulated that the vibration levels observed at rated current conditions were principally determined by the electromagnetic forces generated by the interaction of the currents in the stator and rotor conductors. This paper gives core vibration results from a generator running on test under no-load conditions, both at rated voltage on open circuit and at rated current on three phase sustained short circuit. The measured levels of vibration are compared with those determined by calculation. Because the magnitude of the exciting force is influenced by the length of the air gap, which in turn controls the short circuit ratio, a study is made of the way in which the choice of short circuit ratio can determine the vibration level. Also considered is operation under load conditions and, since the power factor at which the generator is operating determines the phase relationship of the two principal components of force, a study is made of the effect of power factor. Finally some anomalous behaviour observed on a large generator is discussed together with the possible effect of core vibration on the losses in, and temperature rise of, the stator core end laminations.