Effects of equalizing currents on electromagnetic forces of whirling cage rotor

The electromagnetic force acts between the rotor and stator of an induction motor, when the rotor is performing cylindrical circular whirling motion with respect to the stator. The nonsymmetric flux distribution due to the eccentricity induces circulating currents in the rotor cage and parallel branches of the stator winding. These currents tend to equalize the flux distribution, and by doing this, reduce the electromagnetic force and change the direction of the force from the direction of the shortest air gap. Impulse method is utilized in the finite element analysis to calculate the frequency response of the force and the eccentricity harmonics of the flux density in the air gap and the circulating currents. The frequency responses were calculated for the test motors with different stator connections and with and without rotor cage. The frequency response function of the force is calculated for two induction motors with different geometry, one with open and another one with closed rotor slots. The results show that the equalizing currents in both parallel branches in the stator winding and rotor cage damp strongly the amplitude of the force. The damping effects depend on the whirling frequency, loading condition and the geometry of the machine.