Raphael Romary

Study of Rotor Faults in Induction Motors Using External Magnetic Field Analysis

This paper presents a new signature for detection of rotor faults in induction motors, such as eccentricity and broken rotor bars, that uses the external magnetic field analysis. The proposed method is based on the variations of axial flux density in the presence of these faults. The low frequency part of the magnetic field spectrum is particularly analyzed. The analysis is realized through a machine modeling based on permeance circuit under eccentricity fault and also by machine modeling based on coupled magnetic circuit theory under broken rotor bars fault. Analytical relations which describe the machine operation under broken bars fault highlight the influence of speed variation to modify the low frequency components of the external magnetic field. The theoretical results have been validated by experimental measurements. In particular, an inverse stator cage induction machine have been used to measure the bar currents under healthy and faulty cases.

Stator-Interlaminar-Fault Detection Using an External-Flux-Density Sensor

This paper presents the results of a research work devoted to the detection of short circuits between stator laminations of an electrical machine using external magnetic field. The theoretical developments lead one to display the influence of various phenomena on this magnetic field in a wide frequency range. It is shown that surface currents due to burrs at the external surface of the machine have an important contribution compared to the increase of eddy currents in the short-circuit volume. Finally, experimental measurements confirm this theory, and an online method of detection for large generators is proposed.

Electrical machines fault diagnosis by stray flux analysis

The stray flux in the vicinity of an electrical machine is inherent to its running. It is tied to the magnetic state of the machine and therefore can be affected by the presence of a fault in the machine. Accurate modeling of the external magnetic field is quite complicated and simple models are required in order to analyze the effect of an internal fault on the stray flux. This paper displays the possibilities offers by the stray flux for fault detection. A simple model of the external magnetic field is presented and a study in healthy and faulty conditions is developed. Two kind of fault in induction machine are considered: stator inter-turn short-circuit and rotor broken bar. Experimental results are in agreement with the theory, they highlight the differentiation between the radial flux and the axial flux.

Eccentricity and broken rotor bars faults - Effects on the external axial field

This paper shows a new signature for detection of rotor bar faults in induction motors trough external magnetic field analysis. The proposed method is based on the variations of the axial flux density in the presence of rotor faults, particularly regarding the low frequency part of the frequency spectrum. The analysis of the external magnetic field variation in low frequency is realized through a machine modeling with permeance circuit which brings into play the permeances relating to the circulating fluxes on the machine extremities, in the presence of eccentricity. Analytical relations of the machine operating in the presence of a broken bar highlight the influence of speed variation to modify the low frequency components of the external magnetic field.

Offline and Online Methods for Stator Core Fault Detection in Large Generators

Fault diagnosis techniques play a crucial role in large turbogenerators. Due to the fact that it can cause catastrophic damage to the machine in a very short time, the development of new and more efficient methods represents a huge challenge for the researchers. Following this, the aim of this paper is to present two real-scale experimental devices manufactured to study interlaminar short-circuit faults in the stator of large generators. The first one is based on a stack of laminations in which faulty sheets can be inserted. In the second device, a special attention is given to the development of a partial exciter of a real 125-MW turbogenerator. Experimental online and offline results are presented and show that the measurement of flux density at the level of the external key bar can provide information about a short circuit between laminations.

An online universal diagnosis procedure using two external flux sensors applied to the AC electrical rotating machines.

This paper presents an original non-invasive procedure for the diagnosis of electromagnetic devices, as well as AC electrical rotating machines using two external flux coil sensors that measure the external magnetic field in the machines’ vicinity. The diagnosis exploits the signal delivered by the two sensors placed in particular positions. Contrary to classical methods using only one sensor, the presented method does not require any knowledge of a presumed machine’s healthy former state. On the other hand, the loading operating is not a disturbing factor but it is used to the fault discrimination. In order to present this procedure, an internal stator inter-turn short-circuit fault is considered as well.

Finite element investigation of the short-circuit fault in the stator winding of induction motors and harmonics of the neighboring magnetic field

Based on a time domain finite element analysis of the electromagnetic field, the paper studies effects of the short-circuit fault in the stator winding of an induction motor and the influence of this fault on the magnetic field outside the motor. The detection of the short-circuit fault through the magnetic field in the motor neighboring is based on the comparison of harmonics of the output voltage of coil sensors in the healthy and faulty motor states. The influence of the motor frame on the efficiency of fault detection is studied.

Finite Element Analysis and Experimental Study of the Near-Magnetic Field for Detection of Rotor Faults in Induction Motors

This paper deals with the flnite element analysis and experimental study concerning the in∞uence of the broken bar and rotor dynamic eccentricity faults on the magnetic fleld outside a squirrel-cage induction motor. The spatial distribution of the magnetic fleld, the time variation of the magnetic ∞ux density at a point outside the machine and the time variation of the electromotive force delivered by a coil sensor are evaluated based on the flnite element models of the healthy and faulty states of the motor. The increase of amplitude from the healthy to the faulty states of some low frequency harmonics measured in the near-magnetic fleld is emphasized. For broken bar fault, the increase of the amplitudes of speciflc harmonics of the coil sensor electromotive force, with frequency lower than 25Hz, is experimentally conflrmed.

Finite element diagnosis of rotor faults in induction motors based on low frequency harmonics of the near-magnetic field

The influence of different faults of squirrel-cage rotor in induction motors on the near-magnetic field are studied in this paper. The 2D finite element analysis of electromagnetic field is used to investigate the detection of broken rotor bars and rotor eccentricity faults. This analysis is focused on the low frequency harmonics of the near-magnetic field, which are much modified under faulty conditions and less attenuated by the motor frame.

Real scale experimental devices for stator core fault analysis

That paper presents two real scale experimental devices manufactured to study the stator interlaminar short circuit fault in large generators. The first one is based on a stack of lamination in which faulty sheets can be inserted. The second one is a partial exciter of a real 125MW turbo generator. These devices are useful to validate experimental procedures for detection of stator core faults.