Separating load torque oscillation and rotor fault effects in stator current-based motor condition monitoring

Stator current spectral analysis techniques are usually used to detect rotational faults in induction motors. However, position-varying load torque oscillation at multiples of the rotational speed has exactly the same effect. Harmonics resulting from the load torque oscillation often obscure or even overwhelm the detection of rotor faults since the magnitudes of a load torque oscillation induced harmonics are usually much larger. This paper develops a simplified method to detect rotor faults effectively in the presence of a load torque oscillation. The interaction between the negative sequence harmonics produced by rotor faults and the positive sequence harmonics generated by a load torque oscillation creates a rotor fault indicator in a new characteristic fault frequency reference frame. A significant advantage of the proposed scheme is that it does not need any information about motor parameters or knowledge of the flux-oriented synchronous reference frame. Simulation and experimental results demonstrate the efficacy of the proposed scheme. The sensitivity of the rotor fault indicator on the relative position between the rotor faults and a load torque oscillation is also investigated through repeated simulation trials.