Detection of an End-Ring Fault in Asynchronous Machines by Spectrum Analysis of the Observed Electromagnetic Torque

For asynchronous machines, 20% of the failures are due to the rotor [1] and particularly the end-ring fault of cage motors. Studies [2,4] have shown that a rotor fault generates flux harmonics which induce in term of harmonics in the stator currents. So, the analysis of the torque which is the product of these two quantities, must give a better information on the faults. In this paper a new technique is presented allowing the diagnostic of an end-ring fault of a cage threephase asynchronous machine. Previous researches by the authors [5j hm,e given the expression of the instantaneous electromagnetic torque with these defects influence. It has been observed actually that the ruptures influence greatly this quadratic quantity. The torque was calculated from the line currents and the e-m-f across flux sensors placed in the stator windings. In this method, the flux sensors are not essential to determine the electromagnetic torque. The i'otor flux can be directly estimated by using an observer. This study presents ho~v a model of the asynchronous machine is used to build the observer. 1. Measurement of the Electromagnetic Torque