Reliable detection of rotor faults under the influence of low frequency load torque oscillations for applications with speed reduction couplings

Low frequency torque oscillations in the load can induce frequency components in the vicinity of that of rotor faults resulting in false alarms when applying motor current signature analysis (MCSA). False rotor fault indications due to load oscillations are most common in applications that employ speed reduction couplings for high torque, low speed operation. Recently, ideas for separating rotor faults and load oscillations have been proposed in the literature; however, the case where two components overlap at the same frequency has not been investigated. Several cases where rotor fault- and load oscillation-induced components are identical have been observed in the field by the authors with commercial MCSA equipment. It is shown in this paper that overlap between the two components can produce a false positive or false negative indication because they can add or cancel depending on the relative phase between the components. Alternative options for reliable rotor fault testing are evaluated and verified in this paper for cases where the two components overlap and produce false indications.