Hydrogen-cooled two-pole turbogenerators are equipped with one single-stage radial fan for cooling gas ventilation. The impellers with outer diameters of up to 1.5 m operate with 3000 rpm or 3600 rpm. High cycle fatigue cracks have occurred at two radial fan impellers operated at 3000 rpm.
An extensive root cause analysis was performed to identify the cause of these cracks. Impeller vibration measurements in the generator unit finally allowed the successful identification of the root cause. The very strong impeller vibrations were caused by an aeroelastic self-excitation mechanism. By modification of the stationary impeller surrounding the aeroelastic self-excitation was eliminated.