Ф Abstract — The aim of this paper is the monitoring of a gearbox by using the stator current signature analysis in the driving induction machines. The detection of mechanical faults using the vibration signal analysis has been well studied before. Recently, some works have been performed in order to observe the vibration components of the mechanical part in the stator current spectrum for an induction machine but without any theoretical detail. This paper proposes a theoretical framework based on torque oscillations due to the characteristic torsional vibrations in a gearbox and the source of mesh and rotating frequency components in the stator current is presented. In order to verify the theoretical development, a test-bed based on a 5.5kW three-phase squirrel-cage machine connected to a gearbox has been used. Index Terms — Current measurement, Mechanical systems, Monitoring, Vibration measurement. I. INTRODUCTION HE monitoring of the mechanical abnormalities in the electromechanical systems has been mainly studied using the vibration signals [1],[2]. The vibration signals have numerous disadvantages like signal background noise due to external excitation motion, sensitivity to the installation position and its invasive measurement nature. The stator current signal analysis can represents a good alternative for mechanical monitoring because it can be easily accessible in nearly all the industrial applications. In this way, the air-gap eccentricities as well as the bearing faults have been studied in many publications using the machine current signature analysis (MCSA). It is shown that the static and dynamic air- gap eccentricity happens simultaneously and it is related to the amplitude modulation of the stator current [3], [4]. In [5] for the bearing faults diagnosis, the authors have associated the fault vibration frequencies to the stator current signal spectrum by an advanced theoretical analysis. In [6] the authors have analytical and experimentally demonstrated that the load torque oscillations make the stator current phase modulated, showing the feasibility of the MCSA monitoring of mechanical perturbations. The gearbox monitoring using MCSA has been rarely proposed while gearboxes are widely used in the industrial applications. The publications related to gearbox monitoring using MCSA in induction machine-based electromechanical systems are presented in [7]-[9]. In [7], [8], Ф This work is supported by the Regional Council of Picardie, Amiens, France. Shahin Hedayati Kia, Humberto Henao and Gérard-André Capolino are with the Department of Electrical Engineering, University of Picardie “Jules Verne,” 80039 Amiens Cedex 1, France (e-mails: Shahin.Hedayati.Kia@u-picardie.fr; Humberto.Henao@ieee.org; Gerard.Capolino@ieee.org). the authors studied a multistage gearbox for a mechanical fault case showing that the rotating as well as the mesh frequency components can be detected in the stator current signal. Moreover, it is observed that the mesh frequency amplitudes are always less than the other mechanical frequency components as the pinions and wheels rotation frequencies. In [8], a method based on the discrete wavelet coefficients is proposed in order to remove the noise in the MCSA for a better location of mesh frequencies in mechanical fault detection. In fact, for one tooth cut condition more vibration and consequently more amplitude related to mesh frequency can be observed in the stator current signature rather than two teeth or more cut fault. Nevertheless, these last results have been shown without any proposed explicit theoretical development for the analyzed frequency components. In this paper, a theoretical framework based on the observation of the torque spectrum is developed for an induction machine connected to a gearbox without any faults for monitoring purpose. It is shown that due to the torsional vibration induced by the transmission error in the input and output wheels and the stiffness variation of the gear teeth contact, the gearbox adds the rotation and mesh frequency components into the torque signature. This effect makes the stator current multi-component phase modulated. The stator current frequency-domain proposed analysis, gives the gearbox related frequency components for the mechanical monitoring. In order to verify the theoretical development, a test-bed including a gearbox connected to a 5.5kW three-phase induction machine is used in steady state condition. II. EFFECT OF A GEARBOX ON TORQUE OSCILLATIONS The effect of torsional vibration for a two mass-spring in the stator current signal is studied in [10] showing that for a two different joint inertia mass, the source of torque oscillation is related to a single torsional vibration frequency. For a one- stage gearbox system, the sources of vibration are principally due to the periodic changes in tooth stiffness and the gear transmission errors. Particularly, the gear transmission errors are related to the tooth profile, eccentricity of wheels, installation defects and defects appearing during transmission. These sources of vibration produce the input and output rotation and the mesh and mesh-related frequencies in the vibration and torque signals [11], [12]. Fig. 1 shows a normalized vibration signal spectrum in the [0Hz, 100Hz] and [600Hz, 800Hz] frequency bandwidths for a three-phase 55kW induction machine connected to a gearbox with N r1 =57, N r2 =15 (N r1 and N r2 are respectively the input and output teeth numbers) and f r1 =12.5Hz, f r2 =47.5Hz (f r1 and f r2 are Gearbox Monitoring Using Induction Machine Stator Current Analysis Shahin Hedayati Kia, Humberto Hénao, Senior Member, IEEE, Gérard-André Capolino, Fellow, IEEE T 1-4244-1062-2/07/$25.00 ©2007 IEEE 149