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Broken rotor bars and eccentricity are common faults in squirrel cage induction motors. These two faults usually occur simultaneously. This paper will deal with this kind of mixed fault. It is well known that the characteristic frequency of broken bars in the stator line current of a squirrel cage induction motor is very near to that of the fundamental component. In the spectrum of the stator line current, the characteristic components related to broken rotor bars are always submerged by the fundamental one, and it is difficult to detect the broken bar fault at an early stage. In our work, instantaneous power of the motor is used as the quantity to be monitored. Theoretical analysis indicates that the spectrum of ac level of the spectrum of the instantaneous power is clear from any component at the fundamental supply frequency, and the fault characteristics can be highlighted, which is effective toward the separation of mixed faults and the quantification of the fault extent. Experimental results have demonstrated the effectiveness of the proposed technique.
Date of Publication: Sept. 2004