Combined effects of static and dynamic eccentricity on airgap flux waves and the application of current monitoring to detect dynamic eccentricity in 3-phase induction motors

Simple theory is put forward to show that when both static and dynamic eccentricity are present then side band currents with frequencies f_s ± f_r are induced into the stator windings of three-phase induction motors. This is demonstrated experimentally and it is shown that the degree of static and dynamic eccentricity and also the motor load has a great influence on the magnitude of these side-band currents. Further computational work is necessary to be able to predict these side band currents with the aim of quantifying the individual static and dynamic eccentricity components by comparison of the measured side-band magnitudes with predictions at several load points. It is shown that whilst static eccentricity causes an increase in the rotating force vibration usually associated with dynamic eccentricity, it is a very small increase. Hence, this technique may be used in conjunction with side-band current monitoring to assess any increase in rotor eccentricity in large induction machines where offline time of the machine has to be kept to a minimum and internal physical examination is difficult. In addition, the airgap permeance expression is an approximation; further attention to the analysis may yield more harmonic current components suitable for condition monitoring