Detection of broken bars in the cage rotor on an induction machine

Techniques for the detection of broken bars in the cage rotor of a 30 hp four-pole induction machine with a deep-bar cage rotor are described. Stator yoke, tooth tip and external search coils, and thermocouples were installed for test purposes. A shaft torque transducer was installed in line with a DC-load machine. The cross section of the machine was modeled with finite elements, and the field distribution and mechanical performance were computed using a nonlinear complex steady-state technique. Broken bars were shown to produce high localized airgap fields and to degrade mechanical performance. The field perturbation associated with broken bars, which are deliberately disconnected from the endrings by machining, produces low-frequency components and harmonics in the search coil-induced voltages and give rise to an oscillatory torque that produces noise and mechanical vibration. Experimental results show that analysis of the voltage induced in an external search coil is adequate to detect the presence of broken bars.<
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