F. Pedrayes

Unambiguous Detection of Broken Bars in Asynchronous Motors by Means of a Flux Measurement-Based Procedure

Different techniques have been developed to detect rotor asymmetries in asynchronous motors. Although the reliability and the amount of information these techniques provide about a machines state are indubitable, they still have a serious limitation: failure detection when the motor is driving a variable load torque. In this case, the motor phase currents are modulated by torque oscillations, and the information they contain about the integrity of bars and end rings is altered. This paper reports a new diagnostic method, based on the measurement of the magnetic flux linked by one stator tooth, which allows perfect simple discrimination between the actual presence of rotor asymmetries and the spurious effects caused by the oscillations in the load torque off the driven machine even when it is part of an adjustable speed drive and the motor is operating under a variable supply frequency.

A New Online Method Based on Leakage Flux Analysis for the Early Detection and Location of Insulating Failures in Power Transformers: Application to Remote Condition Monitoring

Power transformers figure to be amongst the most costly pieces of equipment used in electrical systems. A major research effort has therefore focused on detecting failures of their insulating systems prior to unexpected machine outage. Although several industrial methods exist for the online and offline monitoring of power transformers, all of them are expensive and complex, and require the use of specific electronic instrumentation. For these reasons, this paper will present online analysis of transformer leakage flux as an efficient alternative procedure for assessing machine integrity and detecting the presence of insulating failures during their earliest stages. A 12-kVA 400-V/400-V power transformer was specifically manufactured for the study. A finite-element model of the machine was designed to obtain the transient distribution of leakage flux lines in the machines transversal section under normal operating conditions and when shorted turns are intentionally produced. Very cheap and simple sensors, based on air-core coils, were built in order to measure the leakage flux of the transformer, and nondestructive tests were also applied to the machine in order to analyze pre and post failure voltages induced in the coils. Results point to the ability to detect very early stages of failure, as well as locating the position of the shorted turn in the transformer windings.

A New Portable, Self-Powered, and Wireless Instrument for the Early Detection of Broken Rotor Bars in Induction Motors

The detection of rotor asymmetries in squirrel-cage induction motors has been an important research topic during the last two decades that has given rise to the development of various diagnostic techniques. Despite these techniques being reliable and providing much information about a machines state, they still lack the capability to detect a failure when the motor is driving a machine, producing oscillations in the load torque. If this happens, then the motor phase currents are modulated by torque oscillations and the information that they contain about the integrity of the bars and endrings is no longer correct. Currently, the authors are not aware of a simple and reliable method that exists for the industrial diagnosis of rotor asymmetries in working cage motors under arbitrary load conditions. This paper describes a new diagnostic method and a new portable electronic instrument that are based on the measurement of the magnetic flux linked by one stator tooth, which allows for an optimal simple discrimination between the actual presence of rotor asymmetries and the spurious effects that are caused by the oscillations in the load torque of the driven machine.

Analysis of the fatigue causes on the rotor bars of squirrel cage asynchronous motors: experimental analysis and modelling of medium voltage motors

The rotor bars of squirrel cage asynchronous motors are subject to mechanical forces and electrical and thermal transient processes. During the transient operation of the machine, especially during its start up, the high values of current flowing through the cage produce small deformations in bar and end rings. These deformations, combined with the expansions and contractions caused by transient thermal processes and the presence of unavoidable manufacturing defects, lead to rotor bar fatigue. The fatigue starts with small cracks in the junction between rotor bar and endrings and ends with the complete breaking of the bars. In this paper, the causes of fatigue in squirrel cage rotors are studied. By means of several finite element models of low and medium voltage machines the forces applied on the rotor during motor start up are calculated. These forces are used as loads on mechanical models of the rotor bars. By applying the finite element method the stress distribution caused by the forces on the bars is calculated. Once this stress distribution is known fatigue analysis is also performed. The results obtained are then validated by means of destructive tests on real specimens and by the analysis of models including intentional manufacturing defects.

Application of a Dynamic Model based on a Network of Magnetically Coupled Reluctances to Rotor Fault Diagnosis in Induction Motors

The growing application of asynchronous motors in industrial processes that require high security and reliability levels has led to the development of multiple methods for early fault detection. The design and verification of these methods imply the use of complex mathematical models that allow the study of the influence produced by the machine operating conditions over the diagnosis procedure. The present paper describes a model for asynchronous motors based on a network of magnetically coupled reluctances. The aim of this model is its application to the study of the typical failures of this type of machine i. e. rotor asymmetries, air gap eccentricity, operation with an open phase etc. The dynamic properties of the model allow the simulation of the spatial evolution of all the motor variables, without neglecting complex phenomena such as magnetic saturation. Time domain analysis of air gap torque, as well as the calculation of current harmonic components is also possible. This initial study is aimed to check the accuracy and computation economy of the model. To do this, the model will be used to analyse a new diagnosis method for rotor fault detection. Simulations of healthy and faulty motors will be presented and the evolution of the machines main electrical and mechanical variables will be obtained.

Finite Element Model for the Study of Inter-Turn Short Circuits in Induction Motors

This paper presents a finite element model for the study of the behavior of induction motors under inter-turn short circuit conditions. The main advantages of this model are its simplicity and the possibility of simulating different types of short circuits, involving different number of shorted turns located in different positions. This is achieved by means of the combination of a peculiar coil definition in the bi-dimensional finite element model with an external circuit coupled with it. The proposed model has been applied to a low-voltage random-wound induction motor, especially rewound for this study in order to allow the authors to introduce inter-turn short-circuits in any specific place of the windings. Experimental tests were performed on this machine in healthy and short-circuit conditions and, in both cases, measurements of supply currents and electromagnetic torque were taken. The spectral analysis of these magnitudes showed a good agreement between simulation and experimental results, validating the model for this kind of study.

Detection of stator winding insulation failures: On-line and off-line tests

Insulation faults in electrical rotating machines are a matter of great industrial interest since the economic losses associated with these events are usually equal or even higher than the cost of the machine itself. For this reason, the development of monitoring and diagnostic techniques to detect this failure during its earliest stages has been a subject of great interest. On the other hand, the accuracy of the quality controls applied to the manufacturing of insulation systems is an essential issue to reduce fault rate and increase machine life. The aim of this paper is to present a comprehensive review of the scientific literature in this field, including not only research papers but also existing standards for off-line tests. A critical review of their diagnosis capability, especially on low voltage machines, is essential to clarify the reliability of these techniques as well as the convenience of its generalized industrial use.

A new electronic instrument for the early detection of broken rotor bars in asynchronous motors working under arbitrary load conditions

Different techniques have been developed for the early detection of rotor asymmetries in squirrel cage asynchronous motors over the last two decades. Although their reliability and the amount of information they provide about the a machinepsilas state is indubitable, they still have a serious limitation: failure detection when the motor is driving a machine capable of producing oscillations in the load torque. In this case, the motor phase currents are modulated by torque oscillations and the information they contain about the integrity of bars and endrings is hidden or altered. In fact, it can be affirmed that almost all diagnosis methods based on spectral analysis of external electrical variables are affected by this phenomenon. In order to overcome this drawback, attempts have been made to find new procedures leading to new indicators to discriminate between the influence of the load and the rotor failure. Although certain complex systems applied to specific motors for on-line diagnosis have been successfully designed, as far as the authors know, no simple and reliable method exists for industrial diagnosis of rotor asymmetries in working cage motors under arbitrary load conditions. This paper describes a new diagnosis method and a new electronic instrument, based on the measurement of the magnetic flux linked by one stator tooth, which allows perfect, simple discrimination between the actual presence of rotor asymmetries and the spurious effects caused by the oscillations in the load torque of the driven machine.

Low cost method for on-line remote monitoring of power transformers and induction motors

Resumen — The application of Finite Element Method (FEM) to the analysis of the magnetic flux in power transformers and asynchronous motors has allowed the design of two new diagnosis procedures for the early detection of failures. These new methods are by its simplicity and low cost very suitable tools for remote monitoring of power transformers and motors. In the case of transformer diagnosis the installation of search coils on the machine windings for the measurement of leakage flux allows the detection of interturn shortcircuits and winding deformation. For motor condition monitoring the measurement of the magnetic flux linked by a stator tooth permits the detection of broken bars even in the case of oscillating mechanical loads. This is an important advance with respect to the conventional motor current signature analysis techniques (MCSA), which are not reliable when the driven machine provokes mechanical oscillations in the rotor shaft. The results of simulations have been validated by means of experimental measurements on actual motors, and the utility of both methods has been demonstrated.