Induction motor asymmetrical faults detection using advanced signalprocessing techniques

Preventive maintenance of electric drive systems with induction motors involves monitoring of their operation for detection of abnormal electrical and mechanical conditions that indicate, or may lead to, a failure of the system. Intensive research effort has been for sometime focused on the motor current signature analysis. This technique utilizes the results of spectral analysis of the stator current. Reliable interpretation of the spectra is difficult, since distortions of the current waveform caused by the abnormalities in the drive system are usually minute. In the present investigation, the frequency signature of some asymmetrical motor faults are well identified using the fast Fourier transform (FFT), leading to a better interpretation of the motor current spectra. Laboratory experiments indicate that the motor current signature FFT-based analysis, with the proposed approach, is still a reliable tool for induction motor asymmetrical faults detection     

Characterization of coil faults in an axial flux variable reluctance PM motor

Variable-reluctance (VR) and switch-reluctance (SR) motors have been proposed for use in applications requiring a degree of fault tolerance. A range of topologies, of brushless SR and VR permanent-magnet (PM) motors are not susceptible to some types of faults, such as phase-to-phase shorts, and can often continue to function in the presence of other faults. In particular, coil-winding faults in a single stator coil may have relatively little effect on motor performance but may affect overall motor reliability, availability, and longevity. It is important to distinguish between and characterize various winding faults for maintenance and diagnostic purposes. These fault characterization and analysis results are a necessary first step in the process of motor fault detection and diagnosis for this motor topology. This paper examines rotor velocity damping due to stator winding turn-to-turn short faults in a fault-tolerant axial flux VR PM motor. In this type of motor, turn-to-turn shorts, due to insulation failures, have similar I-V characteristics as coil faults resulting from other problems, such as faulty maintenance or damage due to impact. In order to investigate the effects of these coil faults, a prototype axial flux VR PM motor was constructed. The motor was equipped with experimental fault simulation stator windings capable of simulating these and other types of stator winding faults. This paper focuses on two common types of winding faults and their effects on rotor velocity in this type of motor.     

Influence of Saturation Level on the Effect of Broken Bars in Induction Motors Using Fundamental Electromagnetic Laws and Finite Element Simulations

This study is dedicated to faulty induction motors. These motors are often used in industrial applications and the monitoring of their condition becomes important for production maximization. This paper gives precise understanding of the effect of the broken bar on the stator currents, a quantity more and more used for fault detection and quantification. This study uses 2-D finite element (FE) models for detailed field analysis and considers different saturation levels (voltage levels). Fundamental electromagnetic laws are used to interpret FE results. It is shown that the presence of local saturation tends to lower the effect of a broken bar on the stator currents. This effect increases with the voltage level and must be taken into consideration for precise quantification of the fault. Furthermore, on a local point of view, the presence of a broken bar leads to highly saturated regions in the neighborhood of the broken bar that will influence the progression of the fault.      

Instantaneous power factor signature analysis for efficient fault diagnosis in inverter fed three phased induction motors

Majority of the electricity used in the industry is consumed by induction motors. Early detection of abnormal cases that occur at the electrical and mechanical parts of these motors is very important for the safe operation of industrial facilities and the decrease of economic losses. In this presented study, the detection of parallel, angular and mixed misalignment fault in inverter fed induction motors has been carried out via the harmonic analysis of the instantaneous power factor signal. Results obtained from experimental studies carried out under different speed and loads indicated that the detection of misalignment fault can be carried out successfully. The presented method can be used effectively without any additional cost since inverter fed motors have both voltage and current sensors. Also online monitoring of induction motors with the suggested method not only improves the motor's performance and longevity but also its efficiency. The main innovative parts of this study is that instantaneous power factor signal was used at the first time for detection of misalignment faults.     

Pattern recognition-a technique for induction machines rotor brokenbar detection

A pattern recognition technique based on Bayes minimum error classifier is developed to detect broken rotor bar faults in induction motors at the steady state. The proposed algorithm uses only stator currents as input without the need for any other variables. Initially, rotor speed is estimated from the stator currents, then appropriate features are extracted. The produced feature vector is normalized and fed to the trained classifier to see if the motor is healthy or has broken bar faults. Only the number of poles and rotor slots are needed as pre-knowledge information. A theoretical approach together with experimental results derived from a 3 hp AC induction motor show the strength of the proposed method. In order to cover many different motor load conditions, data are obtained from 10% to 130% of the rated load for both a healthy induction motor and an induction motor with a rotor having 4 broken bars     

What stator current processing-based technique to use for induction motor rotor faults diagnosis?

In recent years, marked improvement has been achieved in the design and manufacture of stator winding. However, motors driven by solid-state inverters undergo severe voltage stresses due to rapid switch-on and switch-off of semiconductor switches. Also, induction motors are required to operate in highly corrosive and dusty environments. Requirements such as these have spurred the development of vastly improved insulation material and treatment processes. But cage rotor design has undergone little change. As a result, rotor failures now account for a larger percentage of total induction motor failures. Broken cage bars and bearing deterioration are now the main cause of rotor failures. Moreover, with advances in digital technology over the last years, adequate data processing capability is now available on cost-effective hardware platforms, to monitor motors for a variety of abnormalities on a real time basis in addition to the normal motor protection functions. Such multifunction monitors are now starting to displace the multiplicity of electromechanical devices commonly applied for many years. For such reasons, this paper is devoted to a comparison of signal processing-based techniques for the detection of broken bars and bearing deterioration in induction motors. Features of these techniques which are relevant to fault detection are presented. These features are then analyzed and compared to deduce the most appropriate technique for induction motor rotor fault detection.     

Online rotor mixed fault diagnosis way based on spectrum analysis of instantaneous power in squirrel cage induction motors

Broken rotor bars and eccentricity are common faults in squirrel cage induction motors. These two faults usually occur simultaneously. This paper will deal with this kind of mixed fault. It is well known that the characteristic frequency of broken bars in the stator line current of a squirrel cage induction motor is very near to that of the fundamental component. In the spectrum of the stator line current, the characteristic components related to broken rotor bars are always submerged by the fundamental one, and it is difficult to detect the broken bar fault at an early stage. In our work, instantaneous power of the motor is used as the quantity to be monitored. Theoretical analysis indicates that the spectrum of ac level of the spectrum of the instantaneous power is clear from any component at the fundamental supply frequency, and the fault characteristics can be highlighted, which is effective toward the separation of mixed faults and the quantification of the fault extent. Experimental results have demonstrated the effectiveness of the proposed technique.     

Induction motor stator faults diagnosis by a current Concordia pattern-based fuzzy decision system

This paper deals with the problem of detection and diagnosis of induction motor faults. Using the fuzzy logic strategy, a better understanding of heuristics underlying the motor faults detection and diagnosis process can be achieved. The proposed fuzzy approach is based on the stator current Concordia patterns. Induction motor stator currents are measured, recorded, and used for Concordia patterns computation under different operating conditions, particularly for different load levels. Experimental results are presented in terms of accuracy in the detection of motor faults and knowledge extraction feasibility. The preliminary results show that the proposed fuzzy approach can be used for accurate stator fault diagnosis if the input data are processed in an advantageous way, which is the case of the Concordia patterns.     

Multiple Reference Frames Theory: A New Method for the Diagnosis of Stator Faults in Three-Phase Induction Motors

This paper proposes the use of the multiple reference frames theory for the diagnosis of stator faults in three-phase induction motors. The development of a simplified mathematical motor model allowed the establishment of the equivalent circuits of the motor, in$d!!-!!q!!-!!0$axes, in the presence of stator interturn short circuits. The use of the stationary reference frame, clockwise and counterclockwise synchronous reference frames, allows the extraction and manipulation of the information contained in the motor supply currents in a way that the effects introduced by the fault are easily isolated and measured. A severity factor is defined and the simulation and experimental results presented demonstrate its independence in relation to the working conditions of the motor, such as the load level and unbalances in the voltage supply system. Although the technique is here introduced for the diagnosis of stator faults, it is possible to extend its use for the diagnosis of other asymmetries such as broken rotor bars and air-gap eccentricity.     

Detection of stator short circuits in VSI-fed brushless DC motors using wavelet transform

The paper presents methodologies to detect and locate short-circuit faults on the stator winding of VSI-fed PM brushless dc motors. Normal performance characteristics of the motor are obtained through a discrete-time lumped-parameter network model. The model is modified to accommodate short-circuit faults in order to simulate faulty operation. Fault signatures are extracted from the waveforms of electromagnetic torque and phase-voltage summation using wavelet transform. Three independent detection techniques are introduced. Experimental measurements agree acceptably with simulation results, and validate the proposed methods. This work sets forth the fundamentals of an automatic fault detector and locator, which can be used in a fault-tolerant drive.     

Real-Time Implementation of Wavelet Packet Transform-Based Diagnosis and Protection of Three-Phase Induction Motors

This paper presents a real-time implementation of an online protection technique for induction motor fault detection and diagnosis. The protection system utilizes a wavelet packet transform (WPT)-based algorithm for detecting and diagnosing various disturbances occurring in three-phase induction motors. The criterion for the detection is the comparison of the coefficients of the WPT of line currents using a mother wavelet at the second level of resolution with a threshold determined experimentally during the healthy condition of the motor. The algorithm is implemented in real-time using the Texas Instrument TMS320C31 32-b floating-point digital signal processor with the help of object-oriented programming. The proposed technique is tested on two three-phase induction motors. The online test results give a response signal at the instant or within one cycle of disturbance in all cases of investigated faults. In addition, the algorithm is also tested during no load and full load operating conditions of the motor.     

Detection of broken bars in induction motors using an extendedKalman filter for rotor resistance sensorless estimation

This paper deals with broken bars detection in induction motors. The hypothesis on which detection is based is that the apparent rotor resistance of an induction motor will increase when a rotor bar breaks. To detect broken bars, measurements of stator voltages and currents are processed by an extended Kalman filter for the speed and rotor resistance simultaneous estimation. In particular, rotor resistance is estimated and compared with its nominal value to detect broken bars. In the proposed extended Kalman filter approach, the state covariance matrix is adequacy weighted leading to a better states estimation dynamic. Its main advantage is the correct rotor resistance estimation even for an unloaded induction motor. As part of this estimation process, it is necessary to compensate for the thermal variation in the rotor resistance. Computer simulations, carried out for a 4 kW four-pole squirrel cage induction motor, provide an encouraging validation of the proposed sensorless broken bars detection technique     

An analysis of bearingless AC motors

Several types of AC bearingless motors are proposed. These bearingless motors have conventional four-pole stator windings and additional two-pole windings, whose currents produce radial forces acting on the rotor. General expressions of the machine inductances and radial forces are derived for the cylindrical rotor and salient-pole motors. No-load characteristics of laboratory squirrel-cage induction and reluctance-type synchronous bearingless motors are provided. The test motors were successfully driven by the control circuits     

Transient performance of series connected three phase slip-ringinduction motors

The possibility of operating a three phase slip-ring induction motor as a series connected motor is explored. The merits and drawbacks of this mode of operation are discussed. For this purpose, the necessary mathematical models along with the corresponding computer simulation are developed and experimental verification of the theoretical results is also carried out. It has been found in general that operation of slip-ring induction motors as series connected motors is possible providing that the rotor winding is connected in a sequence opposite to that of the stator winding. This adds flexibility to the modes of operation of these motors. Moreover synchronous operation is also possible, providing that the series connected motor is driven initially at twice its synchronous speed and then connected to the supply     

Modeling and simulation of saturated induction motors in phasequantities

This paper proposes a new model for saturated induction motors. The saturation effects are incorporated in the magnetizing inductance and the stator mutual inductances, taking into account the nonuniform distribution of magnetic saturation within the motor core. The proposed model can be used to analyze the manner in which the induction motors interact with the supply network or power source, since it can predict the motor current/voltage harmonics produced by magnetic saturation. Experimental tests show that the proposed model represents with reasonable accuracy (8%) the motor saturation effects at nominal stator voltage as well as for overvoltage operation     

Mechanical fault detection in a medium-sized induction motor using stator current monitoring

This paper presents the results of an experimental study of the detection of mechanical faults in an induction motor. As is reasonably well known, by means of analysis of combinations of permeance and magneto-motive force (MMF) harmonics, it is possible to predict the frequency of air gap flux density harmonics which occur as a result of certain irregularities in an induction motor. In turn, analysis of flux density harmonics allows the prediction of induced voltages and currents in the stator windings. Reviewing this theory, equations which may aid in the identification of mechanical faults are presented. These equations include both those which indicate eccentric conditions and those which have been suggested to help identify bearing faults. The development of test facility to create eccentricity faults and bearing fault conditions is described. This test facility allows rapid access to the motor bearings, allowing an investigation into the ability to detect faulted bearing conditions using stator current monitoring. Experimental test results are presented, indicating that it may be possible to detect bearing degradation using relatively simple and inexpensive equipment.     

Determination of radial-forces in relation to noise and vibrationproblems of squirrel-cage induction motors

The radial electromagnetic forces in induction motors play an important role in the production of audible noise and vibrations. The magnetic flux pulsations at the iron surfaces produce these radial forces, which act on the stator and rotor structures. An analysis for the calculation of the various field harmonics and radial forces in squirrel cage induction motors is presented in this paper. To verify the validity of the analysis, a squirrel cage induction motor is analyzed. Theoretical and experimental results are presented with a view to determine the actual role played by the air-gap harmonic fields on the radial forces. Also, the effects of loading on the radial forces and the ensuing vibrations are closely examined     

Multiple signature processing-based fault detection schemes for broken rotor bar in induction motors

The existence of broken rotor bars in induction motors can be detected by monitoring any abnormality of the spectrum amplitudes at certain frequencies in the motor current spectrum. It has been shown that these broken rotor bar-specific frequencies are settled around the fundamental stator current frequency and are termed lower and upper sideband components. Broken rotor bar fault detection schemes should depend on multiple signatures in order to overcome or reduce the effect of any misinterpretation of the signatures that are obscured by factors such as measurement noises and different load conditions. Multiple discriminant analysis (MDA) provides an appropriate environment to develop such fault detection schemes because of its multi-input processing capabilities. The focus of this paper is to provide a new fault detection methodology for broken rotor bar fault detection and diagnostics in terms of its multiple signature processing feature and the motor operation partitioning concept to improve the overall detection performance. This paper describes two fault detection schemes within this methodology, and demonstrates that multiple signature processing is more efficient than single signature processing. The first scheme, which will be named the "monolith scheme," is based on a single large-scale MDA unit representing the complete operating load torque region of the motor, while the second scheme, which will be named the "partition scheme," consists of many small-scale MDA units, each unit representing a particular load torque operating region.     

Speed Control System for Brushless Cascade Induction Motors in Control Range of Slips s1>1 and S2>1

This paper describes principles together with analytical and experimental studies of a speed control system for brushless induction motors of cascade connection in the control range of the slips s1> 1 and s2>1 which was devised by one of the authors. Steady state analyses of the performances of the motor control system are discussed, and results of the tests are presented. It is found that the motor can be controlled widely and comparatively efficiently by controlling the converters which are connected electrically to one of the stator windings of the two motors.