Eccentricity in Induction Machines—A Useful Tool for Assessing Its Level

Janusz Petryna,Maciej Sułowicz,Arkadiusz Duda

doi:10.3390/en14071976

Abstract

In the condition monitoring of induction machines operating in various industry sectors, the assessment of eccentricity is as important as the assessment of the condition of windings, bearings, mechanical vibrations or noise. The reasons for the eccentricity can be various; for example, rotor imbalance, damage or wear of the bearings, improper alignment of the rotor and the load machine and finally, assembly errors after overhaul. Disregard of this phenomenon during routine tests may result in the development of vibrations transmitted to the stator windings, faster wear of the bearings and even, in extreme cases, rubbing of the rotor against the stator surface and damage to the windings and local overheating of the machine core. On the basis of years of experience in the diagnosis of large induction machines operating in various industries, the article deals with the problem of developing reliable indicators for assessing the levels of commonly accepted types of eccentricity. Starting from field calculations and analyzing various cases of eccentricity, the methodology for determining the indicators for evaluation from the stator current spectrum is shown. The changes in the values of these indices for various cases of simultaneous occurrence of static and dynamic eccentricity are shown. The calculation results were verified in the laboratory. Also shown are three interesting cases from diagnostic practice in the evaluation of high-power machines in the industry. It has been shown that the proposed indicators are useful and enable an accurate diagnosis of levels of eccentricity.

Highlights

Reliable diagnostics of induction machines, in addition to recognizing the continuity and symmetry of windings, the condition of the iron packet and the mechanical condition, cannot do without assessing the level of eccentricity [1,2,3,4,5,6,7,8]
The concept of introducing numerical indicators for the description of the degree of eccentricity is justified by the authors due to positive experiences with the similar description of the winding condition of an induction motor rotor by means of Rotor Fault Index (RFI) [9,10]. They are calculated on the basis of so-called slip harmonic amplitudes in the stator current or axial flux spectrum
In the case of testing the motor with dynamic eccentricity, a rotor was mounted on eccentric bearings, while for the motor with static eccentricity, the rotor was mounted on specially grooved bearing plates

Summary

Introduction

Reliable diagnostics of induction machines, in addition to recognizing the continuity and symmetry of windings, the condition of the iron packet and the mechanical condition, cannot do without assessing the level of eccentricity [1,2,3,4,5,6,7,8]. The reason for the increase in vibration and noise may be just eccentricity, which affects the operation and condition of the entire machine, including vibration of the coils in the slots, faster wear or even damage to the bearings, and can even cause the worst problem of rotor to stator rubbing with all its consequences It is worth introducing, as in the case of vibrations, a numerical description of the phenomenon that will allow tracking its development over time, as well as the possibility of comparing different machines according to one single criterion. The concept of introducing numerical indicators for the description of the degree of eccentricity is justified by the authors due to positive experiences with the similar description of the winding condition of an induction motor rotor by means of Rotor Fault Index (RFI) [9,10] They are calculated on the basis of so-called slip harmonic amplitudes in the stator current or axial flux spectrum. In a complete machine condition analysis, data on changes in eccentricity from measurement to measurement are as important as data on cage health, vibration, noise and temperature

Results

Discussion

Conclusion