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A spectral analysis of the electrical power draw of the two 5.2 MW motors driving a large grinding mill is performed to determine whether they are the source of a severe 55.8 Hz vibration. The applicable electrical and mechanical data was measured during October 2006 and recorded with a sample rate of 5 kHz over a few minutes. It is shown that the motors do not supply the mechanical system with any average power to cause the vibration. Rather, the motors provide insignificant damping for the vibration and circulate minimal energy associated with the vibration via their common 11 kV bus bars. The results correlate with a Simulink simulation of two motors fed off a common supply with out-of-phase torque disturbances at their two shafts.