Detection of airgap asymmetry in induction machines with different pole pair number using the current step response

In the past years the incident of fault conditions in modern drives has increased. One reason for that loss of reliability is the fast switching of the power electronics applied that puts additional stress to all components of the machine. Besides the stator isolation breakdown and the rotor bar defect the degradation of the bearings has been given increased attention recently from both industry and academia. Bearing defect is the most frequent source of a drive breakdown and at the same time this type of defect is the most difficult to detect. Monitoring methods based on frequency analysis like fourier or wavelet transform are facing problems with the additional harmonics introduced by the inverter, the load, as well as the high dynamic control of the machine itself that have to be separated from the harmonics induced by the bearing defect. In this paper an online method based on the time step response of the machine current is investigated and the influence of the pole pair number on the resulting fault indication signal is discussed. In a machine with one pole pair, eccentricity leads to very distinct changes in the offset values of the resulting signal caused by a change in the phase values of the leakage inductances. This effect diminishes with increasing pole pair number making it necessary to identify other indicators. This other indicators are found in the signal components linked to the slotting of the machine. Their magnitude is increased by a factor 3-4 making it suitable to determine the airgap eccentricity of the machine. Measurement results on machines with different pole pair numbers specially manufactured to accurately change and measure the airgap are presented and evaluation methods to detect defect bearings at different types of machines are suggested.