Detection of broken rotor bar of induction motors by spectral analysis of the electromagnetic torque using the Luenberger observer

Rotor asymmetries lead to perturbations of airgap flux patterns in induction machines. These perturbations in flux components affect the electromagnetic torque, as well as stator currents and voltages. The supervision of these signals enables the detection of underlying mechanical asymmetries. The effect of mechanical imperfections on stator signals can be negligible in certain cases. Thus, this makes the detection of minor rotor imperfections a difficult task when the load is weak. In this paper we study the detection of rotor imperfections by spectral analysis of the electromagnetic torque. This torque is computed by rotor flux observers based on two induction motor models : fourth-order and second-order model. A comparison of spectral analysis methods applied to the torque shows the significance of the method we describe, which makes use of mechanical velocity, in order to detect low levels of asymmetry in the rotor cage at low level load. Finally, it is to be noted that this method is applied to real signals with and without imperfections.