Research on the Current Eigenfrequencies for Motor with Bearing Fault and Mixed Eccentricity

Abstract

The current feature for motor carried with natural mixed eccentricity and bearing fault is merely focused even though it is very important for motor online diagnosis. Considering both of the air-gap length change and torque vibration caused by bearing fault, the stator current expression were deduced for mixed eccentric motor. The new current characteristic frequencies for motor with bearing fault and mixed eccentricity were obtained. Finally, a series of simulation studies were conducted, the results show that the new eigenfrequencies are real and useful for online diagnosis of those motor in service. Keywords-motor; eccentricity; bearing; fault I INTRODUCTION Induction motor bearing faults can make the air-gap length change and cause torque vibration. Both the air-gap length change and torque vibration lead to the modulation to MMF(Magnetomotive Force), the work (1-2) analyze the characteristic frequency (|fs±kfc|) related to the bearing fault in stator current spectrum; while the eccentric for motor is studied by analyzing the magnetic field in work (3-4), the eccentric characteristic frequency (| fs±kfr|) can be found in stator current spectrum as well. However, the related studies on bearing fault are based on the ideal motor, namely, regardless of eccentric. It is well known that motor eccentric is inevitable because of the manufacturing and installation process. Hence, the current characteristic frequency of mixed eccentric motor with bearing fault is worth to be researched, and is helpful for online diagnosis of motor bearing fault. II MOTOR ECCENTRICITY Eccentricity is caused by motor manufacturing and installation process and it is inevitable. Rotor eccentricity in induction motors takes two forms, namely, static eccentric (the rotor is displaced from the stator bore centre but is still turning upon its own axis) and dynamic eccentric (the rotor is still turning upon the stator bore centre but not on its own centre). According to the work (5), the air-gap length for the motor with mixed eccentricity can be expressed as follows