Athanasios N. Safacas

Asynchronous Machine Rotor Fault Diagnosis Technique Using Complex Wavelets

This paper introduces a novel approach for the detection of rotor faults in asynchronous machines, based on wavelet analysis of the stator phase current. To be more specific, the measured stator phase current is filtered through a complex wavelet. Theoretical analysis validates that the spectrum of the modulus of the result of the filtering is free from the fundamental supply frequency component, and the fault characteristics can be highlighted. This is advantageous, especially if the induction machine operates at low slip values, where the characteristic frequency components of the rotor fault are very close to the fundamental frequency component. At the same time, by matching the wavelet function to the frequencies of the faulty components, a narrow bandpass filter at the frequency region of the fault characteristic spectral components is obtained. Furthermore, in the context of this paper, features extracted using the proposed technique are used as input to a support vector machine classifier that is employed for the detection of the rotor fault. Simulation and experimental results demonstrate the effectiveness of the proposed technique.

A new stator resistance tuning method for stator-flux-oriented vector-controlled induction motor drive

Field-oriented-controlled induction motor drives have been widely used over the last several years. Conventional direct stator-flux-oriented control schemes have the disadvantage of poor performance in the low-speed operating area when the stator flux is calculated using the voltage model, due to the stator resistance uncertainties and variations. In this paper, a new closed-loop stator-flux estimation method for a stator-flux-oriented vector-controlled induction motor drive is presented in which the stator resistance value is updated during operation. This method is based on a simple algorithm capable of running in a low-cost microcontroller, which is derived from the dynamic model of the induction machine. The effects of stator resistance detuning, especially in the low-speed operating region, are investigated and simulation results are shown. The motor drive system as well as the control logic and the resistance estimator are simulated and characteristic simulation results are derived. In addition, the proposed control scheme is experimentally implemented and some characteristic experimental results are shown. The simulation as well as the experimental results reveal that the proposed method is able to obtain precise flux and torque control, even for very low operating frequencies.

Detection of Induction Motor Faults in Inverter Drives Using Inverter Input Current Analysis

Broken rotor bar/end-ring and eccentricity are common faults in squirrel-cage induction motors and have been thoroughly investigated in the case of ac grid supply. In this paper, these types of faults are studied in the case of inverter-driven motors. More specifically, a novel investigation in inverter input current spectrum is made in order to reliably detect the fault-indicative frequency components. These harmonics give the ability for fault detection and classification, even under low load and low motor severity fault condition in steady-state or transient operation. The proposed diagnostic method has been particularly designed for open-loop controlled variable frequency induction motor drives, which is a usual case in low-cost industrial drives where precise speed control of the motor is not required. Moreover, an alternative technique is also presented in case the stator current is measured.

An improved sensorless vector-control method for an induction motor drive

In the present paper, a new improved sensorless vector-control method for an induction motor drive is presented. The proposed method is based on an improved closed-loop stator-flux estimator, based on the dynamic model of the asynchronous motor, which achieves precise stator-flux estimation over a wide area of operation. This new stator-flux estimator ensures stability of the overall control scheme in a very-wide-speed operation area, as it will be shown in this paper. The rotor-speed-estimation method is based on an observer based on the model reference adaptive systems (MRAS) theory. The control scheme is based on a stator-flux-oriented direct vector-control method, where both flux and speed controllers are optimal tuned. In addition, implementation of the proposed method is based on a simplified algorithm capable of running in a low-cost microcontroller, which is discussed in detail. Also, the motor-drive system, including the stator-flux estimator, the speed estimator, and the control logic are simulated and some characteristic simulation results are presented. These results reveal that the proposed method is able to obtain precise flux and speed control over a wide operation area, including very low operating frequencies.

Open-Circuit Fault Diagnosis for Matrix Converter Drives and Remedial Operation Using Carrier-Based Modulation Methods

A novel monitoring system, designed to detect open-circuit (OC) faults that occur in the matrix converter (MC) topology, is proposed in this work. In this monitoring system, a new diagnosis method is implemented which is based on the discrete wavelet transform analysis of the measured output current waveform. In order to ensure the effectiveness of the proposed method and its resistivity to erroneous fault detections, a fuzzy expert system is used in the designed monitoring system. The main advantages of the proposed method are that the implementation cost is minimized because no extra sensors are used and that no information from the control algorithm about the modulation parameters or the applied pulse sequence is required, reducing its implementation complexity and facilitating a more modular design. Additionally, it can be easily adapted to modified matrix topologies. A simple and robust method for the localization of the open-circuited transistor(s) within the identified faulty leg is also proposed. The proposed techniques are validated by simulation and experimental tests. The remedial operation of MC drives after the occurrence of an OC fault by using a redundant leg is also studied. The use of carrier-based modulation methods for this operation is experimentally validated, and related issues are discussed.

Automatic Pattern Identification Based on the Complex Empirical Mode Decomposition of the Startup Current for the Diagnosis of Rotor Asymmetries in Asynchronous Machines

This paper presents an advanced signal processing method applied to the diagnosis of rotor asymmetries in asynchronous machines. The approach is based on the application of complex empirical mode decomposition to the measured start-up current and on the subsequent extraction of a specific complex intrinsic mode function. Unlike other approaches, the method includes a pattern recognition stage that makes possible the automatic identification of the signature caused by the fault. This automatic detection is achieved by using a reliable methodology based on hidden Markov models. Both experimental data and a hybrid simulation-experimental approach demonstrate the effectiveness of the proposed methodology.

Detection of Backlash Phenomena Appearing in a Single Cement Kiln Drive Using the Current and the Electromagnetic Torque Signature

Fault diagnosis in electromechanical drives has been widely investigated over the last decades, and many diagnostic methods have been proposed based on the reported faults. Additionally to already published works which have dealt with gear faults inside low reduction ratio gearboxes, this paper aims to present a simple and effective method for the early diagnosis of evolving faults in a high reduction ratio gear transmission system used in cement kiln drives. The under study system basically consists of a three-phase induction motor mechanically connected to a gearbox. The output shaft of the gearbox drives a gear pinion which, in turn, rotates a girth gear rim surrounding the cement kiln. The identification of mechanical vibrations due to backlash phenomena appearing between the pinion gear and the girth gear rim of the kiln is realized using the motor current signature analysis and processing the motor electromagnetic torque. The proposed diagnostic method is presented, analyzing the experimental results from an under-scale laboratory simulating system.

A study of the effects of the stator slots wedges material on the behavior of an induction machine

The opening of the slots in the electrical motors distorts the waveform of the field in the air-gap and produces high order harmonics, which causes increased losses, torque pulsations and reduction of the efficiency of the machine. In this paper the effect of the closure of the slots of an asynchronous motor using wedges to reduce the flux pulsation is performed. Wedges of different materials have been used and their influence on the performance characteristics of the machine is investigated. The electromagnetic torque, the currents and losses in the machine have been calculated and discussed using the 2D finite element model of the motor.

Deployment of an Adaptable Sensorless Commutation Technique on BLDC Motor Drives Exploiting Zero Sequence Voltage

This paper introduces an improved signal processing technique of zero sequence voltage (ZSV) with the scope of position sensorless commutation of a BLDC motor drive system. Rotor position information is extracted in the proposed technique from ZSV. This signal includes all time intervals where back electromagnetic force (EMF) zero crossings (ZCs) occur, from which the commutation instants are determined. This technique exploits maximally flat group delay of Bessel low-pass filters for switching noise suppression. This attribute leads to a speed-independent time displacement of ZSV. The fundamental frequency of ZSV is determined through a median-filtering algorithm, which is required for a dynamic rate limiter that eliminates erroneous ZCs. In sequence, a pulse train is generated by the detected ZCs of the emerged signal and properly shifted to determine the correct commutation instants. The proposed technique ensures a wide operational speed range for the drive system. A theoretical analysis of the drive system is presented, considering a BLDC motor with nonideal back-EMF. Experimental results presented in this paper validate the effectiveness and robustness of the proposed position sensorless commutation technique under various operational conditions.

Dynamic and Vibration Analysis of a Multimotor DC Drive System With Elastic Shafts Driving a Tissue Paper Machine

In this paper, the influence of the stiffness and damping of real elastic shafts on the behavior of a multidrive system driving a tissue paper machine is investigated. The drive system consists of four DC motors, where two of them drive the Yankee drying cylinder and the other two drive the presses section. Six-pulse thyristor bridge power converters that are connected to the same power transformer supply the motors. The inertias of the motors and the driven machines, in conjunction with the shaft stiffness, form mechanical resonators. Thus, resonance phenomena may occur, which may lead to undesirable results. This particular case results to two three-mass and one five-mass systems. This paper presents an analysis of the resonance frequencies of two sections of the machine during startup and machine production, considering the shaft stiffness (shaft diameter, length, and thickness). The elastic shafts eliminate the ripple of the motor electromagnetic torque, which is transferred to the driven machine. The behavior of elastic shafts with different stiffness in the transmission system is studied. Moreover, the steady state and dynamic behavior of the system considering the influence of the elastic shafts and vibration analysis were investigated using simulation. A model algorithm for system simulation was developed using the MATLAB/SIMULINK software. Characteristic simulation results are presented and are expressed in terms of speed and torque response. Considering the resonance frequency analysis and simulation results, mechanical design guidelines can be given for the most significant drive components in order to avoid torsional oscillation resonance phenomena and minimize torsional oscillations of the drive system.