Evaluation of the Influence of Rotor Magnetic Anisotropy on Condition Monitoring of Two-Pole Induction Motors

If nongrain-oriented rotor laminations are punched and stacked in a uniform direction, magnetic asymmetry is present in the rotor core due to the nonideal magnetic anisotropy. In this paper, it is shown that the influence of rotor core anisotropy can be misinterpreted as rotor faults in two-pole induction motors when performing motor current signature analysis. A detailed analysis of the influence of rotor core anisotropy shows that components produced by the rotor fault and rotor anisotropy interact, making reliable fault detection difficult. It is also shown that offline testing is currently the only available means of detecting the fault, and online monitoring of the space harmonics-induced current components is proposed as a viable solution for providing reliable rotor fault detection for two-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.