Modulation of transient reactances of induction machines caused by different types of eccentricity

Abstract

System reliability is getting more and more important in modern industrial drives. Currently the most frequent source of a drive breakdown is a defect of the bearings, which at the same time is most difficult to detect especially under inverter operation. The exploitation of the transient current slope was shown to give an accurate means to identify the machines transient reactances and thus also to detect possible asymmetries in the airgap. In this paper the influence of different types of eccentricity on the transient current slope of the machine excited by voltage pulses is investigated. Special attention is paid to the influence of combined static and dynamic eccentricity as this combination, due to the non linear behavior of the lamination gives rise to specific modulation that is clearly detectable. The investigation is done at machines with two and four pole pairs. The signal obtained corresponds to the inverse transient reactance. It contains different components of inherent asymmetries in the machine as well as in the case of eccentricity additional modulations that can be extracted. For a machine with two poles static eccentricity leads to a change in the signal offset, whereas dynamic eccentricity rotating with the rotor leads to a modulation with twice the mechanical frequency of the rotor. Mixed eccentricity finally leads to change in signal offset as well as a modulation corresponding to a fundamental wave with respect to the mechanical frequency. For a machine with four poles it is advantageous to use the signals obtained from single phase excitation separately.