Remus Pusca

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.

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.

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.

An improvement of a diagnosis procedure for AC machines using two external flux sensors based on a fusion process with belief functions

In this paper, a method for diagnosis of AC machines using the spectrum of the near magnetic field is presented. The method is associated to a fusion process based on belief functions which analyze the measurements. In previous works, it has been shown that it is possible to detect the inter-turns short circuit in the stator windings of electrical machines using a noninvasive method. It is based on the analysis of the variation of sensitive harmonics when the load varies, and eliminates the main drawback presented by other diagnostic methods which use the comparison with a healthy state assumed known. Several measurements around the machine are necessary to increase the probability of the fault detection because the fault position relatively to the sensor can strongly influence the results. So in this paper it is proposed to exploit conjointly the whole measurements in order to obtain a more robust and reliable diagnostic and to increase the probability of detecting the fault. The merging of the different estimations being realized through the belief functions framework, this approach is tested on real measurements. Experimental tests are performed on a special rewound induction machine in order to validate the theoretical approach.

On the Use of a Dimmer for a Robust Frequency Control of a Self-Excited Three-Phase Induction Wind Generator

This paper concerns a three-phase self-excited induction generator used for autonomous power generation. It presents a robust control strategy which makes it possible to maintain the frequency quasi constant during the voltage regulation without any -control loop on this variable. This strategy, which also prevents the machine disengagement, uses as power converter a simple dimmer. The obtained theoretical and/or numerical results are validated on a laboratory test bench that allows the analysis of this control law effectiveness.

Finite element analysis of electromagnetic and mechanical effects of rotor faults in induction motors

This paper analyzes the consequences of the eccentricity and one broken bar faults in the squirrel-cage induction motors related the rotor speed time variation of the radial unbalanced force on the rotor armature, supported by the motor bearings. This study is based on finite element models, which allows analyzing the independent or the combined influence of each fault through the specific signature of faults in the magnetic field outside the motor. The detection of the incipient faults based on these signatures is suggested in order to anticipate the driving system shutdown.

Advances in diagnosis of electrical machines through external magnetic field

Promising parameter in the analysis and diagnosis of electrical machines is the external magnetic field that can be measured in the vicinity of the machine. This variable provides an image of the internal magnetic state of the machine and it is sensitive to very small imbalances in the machine. Therefore, information about the presence of an anomaly can be found in the analysis of this field. The magnetic field is measured close to the machine by specific wound flux sensor and is analyzed considering the evolution of its spectral content in presence of stator or rotor electrical faults. The paper gives an overview of measurement methods, fault signatures, and analysis tendency in the diagnosis of rotating electrical machines using the measurement of the external magnetic field.