ETEP zy Asynchronous Motor Cage Fault Detection Through Electromagnetic Torque Measurement F. Thollon, G. Grellet, A. Jammal Abstract zyxwvutsrqp In this article a new method is presented allowing the diagnostic of bar or ring rupture of a cage of asynchro- nous machine. It is shown thata modelling of the asynchronous machine taking into account each bar and each portion of ring of the cage allows to consider the influence of a rupture upon the expression to the instanta- neous electromagnetic torque. It has been put up an instrumentation permitting the measurement of this tor- que. The spectral analysis of the parameter leads to the diagnostic of a cage rupture by discriminating this fault from otherfaults, particularly mechanical ones. It is also observed that ruptures influence greatly the el- ectromagnetic torque. The peak variation of the significant spectral lines is so important that it allows the zyx de- tection zyxwvutsrqp of afirst rupture without the need of a very sophisticated instrumentation. 1 Introduction Undesired stopping of production due to machines failures is costly and can represent an important part of the initial investment. In certain cases, it is sensible to in- strument the machines during their conception, thus al- lowing a follow-up and on-request maintenance rather than a systematic one. For asynchronous machines zyxwvuts - 20 % of the failures being accountableto the rotor - it has been decided to con- centrate upon the detection of the faults of the rotor and particularly upon the broken bars and rings of cage motors. Recent studies [ 1-31 have shown that a bar rupture generates zyxwvutsr flux harmonics. The amplitude of high-order harmonicsdue to ruptures are superiorto those of the same order harmonicsgeneratedby other faults(mechanicalec- centricity, . . .). This therefore allows to discriminate the faults which are of interest. By another way the ruptures also generate harmonics within the stator currents. Finally it has been shown that ruptures of bars or rings led to a torque decrease in steady-state operation and to sensible modifications in the evolution of the torque during transient state. The influence upon the flux and the stator currents, due to a rupture within the cage, have the consequence to focus the interest upon the instantaneous electromag- netic torque. This study presents two steps: on one hand the mod- elling of a cage asynchronous machine and formulation of the electromagnetic torque, allowing to consider the ruptures, and, on the other hand, putting up a method for measuring the electromagnetic torque. 2 Detailed Modelling of a Cage Asynchronous Machine Numerous paper have dealt with such a problem as zy [4] and [6]. Here, a multi-winding model is considered where each bar and each ring portion is represented as shown in Fig. 1. In order to reduce the complexity of the impedance matrices, but still preserving the entirety of the rotor, the Park Transformation is used to refer the stator windings in a fixed reference with respect to the rotor. The electrical equation of the machine can then be written as: where zyxwv [I?,] and [&,I are the resistances and inductances matrices of the machine, of dimension (N + 3) x (N + 3), N being the number of bars and: zyxw I ET143.1A Fig. 1. Multi-winding model of a cage asynchronous motor [Irn] = &jr, -54r. -,, -,, .--, - rotor mesh currents ETEP Vol. 3, No. 5, September/October 1993 375