Identification of the broken bar fault in induction motors with rotor air ducts through the torque spectrum

Large induction motors contain axial air ducts in their rotor body for cooling purposes. It has been noted lately that, the presence of the rotor air ducts affects the rotating magnetic field by introducing asymmetries. Those asymmetries produce higher harmonic content in the induction motor electromagnetic variables, same like the broken bar fault. So, the diagnostic procedure of a healthy motor with axial rotor air ducts could lead to a false broken bar fault alarm, which leads to unnecessary inspection cost, as well a significant cost because of the production process interruption for the motor inspection. In this paper, it is shown that the higher harmonic index of the electromagnetic torque can reliably be used to distinguish the healthy from the broken bar case reliably in induction motors with rotor axial air ducts.