The detection of broken bars in the cage rotor of an induction machine

Techniques are described for the detection of broken bars in the cage rotor of an induction machine. A 30 hp, four pole induction machine with deep bar cage rotor was procured. A 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, deliberately disconnected from the endrings by machining, produces low-frequency components and harmonics in the search-coil-induced voltages, and gives rise to an oscillatory torque which 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.<>