M.F. Cabanas

Influence of Nonconsecutive Bar Breakages in Motor Current Signature Analysis for the Diagnosis of Rotor Faults in Induction Motors

Studies of rotor asymmetries in squirrel-cage induction motors have traditionally focused on analyses of the effects of the breakage of adjacent bars on the magnetic field and current spectrum. However, major motor manufacturers have reported cases where damaged bars are randomly distributed around the rotor perimeter of large HV machines. In some of these cases, the motors were being monitored under maintenance programs based on motor current signature analysis (MCSA), and the degree of degradation found in the rotor was much greater than that predicted by analysis of their current spectra. For this reason, a complete study was carried out, comprising a theoretical analysis, as well as simulation and tests, to investigate the influence that the number and location of faulty bars has on the traditional MCSA diagnosis procedure. From the theoretical analysis, based on the application of the fault-current approach and space-vector theory, a very simple method is deduced, which enables the left sideband amplitude to be calculated for any double bar breakage, per unit of the sideband amplitude corresponding to a single breakage. The proposed methodology is generalized for the estimation of the sideband amplitude in the case of multiple bar breakages and validated by simulation using a finite-element-based model, as well as by laboratory tests.

Improving the Dynamics of Virtual-Flux-Based Control of Three-Phase Active Rectifiers

Virtual flux (VF)-oriented control (VFOC) and VF-based direct power control (DPC) (VF-DPC) have been developed to improve voltage-oriented control and DPC of three-phase active rectifiers. The VF space vector is utilized in transformations between stationary and rotating coordinates in VFOC and in obtaining instantaneous power in VF-DPC. The VF space vector is calculated by integrating the grid voltage space vector. This integration is usually performed using a first-order low-pass (LP) (FOLP) filter, which counteracts the saturation and dc-drift problems associated with pure integrators. However, the dynamics of FOLP filters can be enhanced to a great extent. This paper presents a new, simple, and fast integration algorithm for VF-based control methods. Simulations and experimental tests on a VF-DPC-based system showed that the proposed algorithm leads to rapid recoveries after grid voltage sags occur. Moreover, the performance of VF-DPC under nonideal grids is discussed.

Study of an induction motor working under stator winding inter-turn short circuit condition

Research has proven that several methods are able to detect shorted turns in the stator winding of operating motors, even in cases in which few turns are involved. One important aspect of these methods is its ability to be applied to condition-based maintenance systems; moreover it has the sensitivity to predict how far the machine is from the end of its useful life. Any information that contributes to establishing an alarm level to predict the remaining life of the motor would be very useful. This paper presents the results of a study carried out on a motor working under a turn to turn short condition. In this condition, interesting results about the behaviour of the motor are obtained and a comparison between the sensitivity of some diagnostic methods to detect shorted turns is made.

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.

Coupling Inductor Fault Detection and Estimation in Three-Phase Adjustable-Speed Drives With Direct Power Control-Based Active Front-End Rectifiers

Coupling inductors are the most commonly utilized filtering option in grid-connected direct power control (DPC)-based active front-end rectifiers integrated into three-phase adjustable-speed drives (ASDs). Inductor faults, e.g., common interturn short circuits, can drive the whole system to failure. In this paper, a novel strategy is proposed for the early detection of incipient faults in the coupling inductors of the aforementioned ASDs. The proposed method enables a fast identification of the defective phase. The strategy also provides an accurate estimation of the inductance value of the coupling inductors that improves the performance of the DPC method. Simulations and experimental tests demonstrate the validity of this detection and estimation strategy.

Diagnosis of Electrical Distribution Network Short Circuits Based on Voltage Park's Vector

This paper proposes a short-circuit diagnosis tool for electrical low-voltage (LV) distribution networks. The tool relies on analyzing Parks vector representation of the LV phase voltage in the sag produced by a short circuit. Parks vector representation is considered to be an identification mark for the type of short circuit that the installation is exposed to and the phases that it affects. In addition, it shows whether the fault is located in the medium voltage or LV network. The characteristics of the vector representation corresponding to the different types of short circuits are analyzed in this paper. Finally, the relationship between Parks vector representation and the location of the defect is also studied. The results can be used as part of an expert system for the automatic classification of disturbances.

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.

A new on-line method 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 machines 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 end rings 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, 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.