Evaluation of the amplitudes of high-order fault related components in double bar faults

This paper addresses the problem of diagnosing cage motors with double bar breakages involving non-consecutive rotor bars. As it is stated in previous works, the conventional diagnosis approaches fail when diagnosing this fault for certain relative positions of the broken bars. This work is focused on the study of the high order harmonics of the airgap-field produced by the fault; simple mathematical expressions are deduced which enable to evaluate the influence of the relative position of the broken bars on the amplitude of the fault related components (Left Sideband Harmonic and other high order fault components) of the stator current. These expressions demonstrate to be useful for confirming the diagnostic in uncertain cases; they are validated by finite element simulations and laboratory tests, using different diagnosis methodologies based both on steady-state and on transient analysis.