The radial impeller of the main fan of hydrogen cooled generator has been noted to sustain fatigue failure. Experimental data pointed out to the impeller selfexcitation as the cause of the failure. According to the mechanism and the mathematical model suggested in the paper the impeller selfexcitation is due to the feedback between the natural impeller vibrations and the acoustic field in the side room adjacent to the impeller. The impeller vibrations induce the pressure oscillations in the side room that in turn influence the impeller. Under special conditions of the fan dynamic behaviour and the acoustic properties of the fan side room this feedback leads to the selfexcitation. The suggested mechanism and the model fit all the experimental data. Their validity has been also later confirmed by the maintaince experience of other similar machines.