Load torque effect on diagnosis techniques consistency for detection of mechanical unbalance

Because of initial manufacture and ageing phenomenon, rotating machines present an inherent mechanical disequilibrium. When a small mechanical unbalance fault affects the driving system part ensured by an induction machine, an additional undesirable torque will be superposed to the torque imposed by the load. This supplementary torque is a time varying signal according to rotor position. This time torque variation will be reflected on stator and rotor currents amplitudes by modulation behavior at rotational frequency. Stator current spectrum will contain sideband harmonics that are characteristic of mechanical unbalance defect. Moreover flux beaming around machine body, which results of rotor and stator currents variations, will be affected. In addition, the total inertia of the revolving part is also modified and creates a non uniform distribution of mass which, when the system rotates, generates a centrifugal force tending to move the rotor axis in radial direction. A suitable transducer allows the measurement of vibration signal that can be a useful amount to detect the apparition of the considered defect. This paper studies the effectiveness limits of stator current spectrum technique compared to classical vibration method for monitoring motor mechanical unbalance when it drives a real load presenting intrinsic mechanical unbalance. Experimental results show that axial stray flux analysis can be an alternative solution to cover effectiveness limitation of stator current analysis method.