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Motor current signal analysis has been an effective way for many years to monitor electrical machines. However, little research work has been reported in using this technique for monitoring their downstream equipment. The research investigating the characteristics of electrical current signals measured from an induction motor for monitoring faults from a downstream gear transmission system by a novel modulation signal (MS) bispectrum method. Both analytic analysis and experimental study are conducted based on a motor drive system and have found that the increase of bispectral peaks can be the basis of mechanical fault diagnosis. Particularly, a fault on a gear will causes a larger increase of the bispectral peak at both the gear shaft frequency and accompany with a smaller increase in the relating shaft frequency. However, a shaft misalignment only leads to a bispectrum peak increase at the shaft bifrequency along.