Evaluation of the influence of rotor magnetic anisotropy on condition monitoring of 2 pole induction motors

Non grain-oriented silicon steel laminations used in electric machines show a small degree of non-ideal magnetic anisotropy, which is introduced during the rolling process. If the rotor laminations are punched and stacked in a uniform direction, magnetic asymmetry is present in the rotor core. In this paper, it is shown that rotor core anisotropy can produce twice slip-frequency, 2sf_s, modulation that can be misinterpreted as rotor faults in 2 pole induction motors when performing motor current signature analysis (MCSA). Unnecessary motor inspection due to false rotor fault alarms is a common on-going problem in the field, and it is shown for the first time in this work that rotor anisotropy is a root cause of false MCSA alarms in 2 pole induction motors. The influence of rotor core anisotropy is analyzed in detail, and it is shown that 2sf_s components produced by the rotor fault and rotor anisotropy interact, making reliable fault detection difficult with MCSA. It is also shown that off-line testing is currently the only available means of detecting the fault, and on-line monitoring of the space harmonics-induced current components is proposed as a viable solution for providing reliable rotor fault detection for 2 pole motors with rotor anisotropy. The analysis and conclusions presented in this paper are verified through testing on custom built lab motors and on 3.3 kV motors.