SDEMPED 2003 Symposium on Diagnostics for Electric Machines, Power Electronics and Drives Atlanta, GA, USA, 24-26 August 2003 Rotor Cage Fault Detection in Induction Motor using global modulation index on the Instantaneous Power Spectrum G. Didier 1 , Student Member IEEE, H. Razik 1 , Senior Member IEEE, O. Caspary 2 and E. Ternisien 2 1 Groupe de Recherches en Electrotechnique et Electronique de Nancy 2 Centre de Recherche en Automatique de Nancy Universit´ e Henri Poincar´ e - Nancy 1 - BP 239 F – 54506 Vandoeuvre-l` es-Nancy, Cedex, France Tel : +33 3 83 68 41 42 , fax : +33 3 83 68 41 33 e-mail : gaetan.didier@green.uhp-nancy.fr Abstract— Electric motors play a very important role in the safe and efficient running of any industrial plant. Early detection of abnormalities in the motor will help to avoid costly breakdowns. Accordingly, this work presents a technique for the diagnosis of broken rotor bar in induction motor. Stator voltages and currents in an induction motor were measured and employed for computation of the input power of one stator phase. Waveforms of the instantaneous power were subsequently analysed using the Bartlett periodogram. The latter is calculated either with a rectangular window or a Hanning’s window. The evaluation of the global modulation index on the instantaneous power spectrum is used for fault detection. Several rotor cage faults of increasing severity were studied with various load effects. We show some experimental results to prove the efficiency of the employed method. I. I NTRODUCTION The induction motor, especially the asynchronous motor, play an important part in the field of electromechanical energy conversion. It is well-known that the interruption of a manufac- turing process due to a mechanical problem induces a serious financial loss for the firm. We know a variety of faults which can occur in induction machines [1] [2], such as rotor faults (broken bar or end ring) or rotor-stator eccentricity. In fact, if faults are undetected, they may lead to potentially catastrophic failures. The consequences of a faulty rotor are excessive vibrations, poor starting performances, torque fluctuation or higher thermal stress. The breaking of rotor bars can be induced by: • a thermal stress due to thermal overload or unbalance, • magnetic stresses caused by electromagnetic forces, elec- tromagnetic noise and vibrations, • a residual stress due to manufacturing problems, • a dynamic stress arising from shaft torques, centrifugal forces and cyclic stresses, • environmental stresses caused by contamination and abra- sion of rotor material due to chemicals or moisture. Various techniques have been proposed to detect a rotor fault. One of the well-known approaches for the detection of broken rotor bars in an induction machine is based on the monitoring of the stator currents to detect sidebands around the supply frequency [3] - [10]. Another way to detect a rotor fault is the measurement of torque harmonics, speed or external flux [11]. In this paper, we put forward a broken rotor bars fault detection using the power of the sidebands. It is based on the averaging periodograms. The broken bar detection can be connected to the analysis of the global modulation index. We estimate the global modulation index corresponding to the contribution of all detected sidebands. In order to find the frequency and the amplitude of each sideband to estimate its modulation index and, therefore, we apply a non-parametric power spectrum estimation, called averaging periodograms or Bartlett method. This method is applied on the instantaneous power in induction motor. We show that additional information carried by instantaneous power improves the detection of the sidebands. In fact, the instantaneous power method can be interpreted as a modulation operation in the time domain that translates the spectral components specific to the broken rotor bar to a 0-100 Hz frequency well-bounded. [12] [13]. II. THE INSTANTANEOUS POWER SIGNATURE First of all, we consider an ideal three phase supply voltage. The instantaneous power p(t) of a phase is classically given by: p(t)= v s (t)i s (t) (1) where v s (t) is the instantaneous voltage and i s (t) is its line current phase. If those two conditions are expected, the supply voltage is sinusoidal and the speed is constant (no ripple), the instantaneous power can be written as follows: