Model and simulation tests of a squirrel - cage induction motor with oscillation of the air gap

Damage of bearings is the most common cause of failures of squirrel-cage induction motors. A faulty bearing results in additional motor vibrations. They are reflected in the harmonic content of stator currents. In certain operating conditions the current signal is the sole source of information on the state of the motor. The paper presents a mathematical model of the motor, allowing for rotor eccentricities. Calculated spectra of the stator currents are presented for the cases of operation with rotor vibrations mapping with some approximation the vibrations caused by faulty bearings. The paper presents also results of laboratory simulations for a motor put into vibrations of adjustable frequency. Vibrations of the motor housing simulate the rotor oscillations caused by a damaged bearing. The results of laboratory simulations permitted to define the relations linking vibrations of the air gap with the components in the stator current spectrum.