This paper presents a novel scheme to predict and detect different mechanical faults in a variable speed motor drive (e.g. eccentricity, defective bearings, faulty gearbox), by monitoring components in the rectifier input current. The scheme is based on the principle that a torque disturbance in the machine (produced by the mechanical fault) propagates from the machine through the PWM inverter, the DC link LC filter, and through the diode bridge rectifier. This paper provides a theoretical analysis of how the disturbance propagates through the system such that the disturbance components in the rectifier input current can be predicted for a specific fault at a specific motor speed to enhance condition monitoring. Experimental results are presented for a 4kW servo drive with a programmed load disturbance. An agreement is observed between the theoretically predicted components and those measured from the experimental system.