Stator current analysis of a squirrel cage motor running under mechanical unbalance condition

Mechanical unbalance fault is one of the most common reasons of excessive machine vibration. The control of such a type of defect makes it possible to avoid the damage and to guarantee machines functioning with the maximum effectiveness. This fault leads essentially to load torque oscillation around its average value at rotational frequency. Theoretical development based on magneto-motive forces approach shows the modulation effect of mechanical unbalance on rotor current expressions. Taking account rotor winding function distribution, supplementary induced currents in rotor meshes lead to an amplitude modulation of stator current around its supply frequency and its principal slot harmonic, at rotational frequency. Because a real load is not ideal and presents an intrinsic rotor asymmetry involving an additional time varying torque according to its rotor position, the stator motor current spectral harmonics produced by the load will overlap the harmonics caused by the main fault (rotor unbalance) since induction machine becomes sufficiently loaded. This paper examines detection of such machine defect by analysis of stator current spectrum for unloaded and loaded squirrel cage motor.