Y. Gritli

Advanced Diagnosis of Electrical Faults in Wound-Rotor Induction Machines

The aim of this paper is to present a diagnosis methodology for the detection of electrical faults in three-phase wound-rotor induction machines (WRIMs). In the considered application, the rotor windings are supplied by a static converter for the control of active and reactive power flows exchanged between the machine and the electrical grid. The proposed diagnosis approach is based on the use of wavelet analysis improved by a preprocessing of the rotor-voltage commands under time-varying conditions. Thus, the time evolution of fault components can be effectively analyzed. This paper proves also the importance of the fault components computed from rotor voltages in comparison to those coming from rotor currents under closed-loop operation. A periodical quantification of the fault, issued from the wavelet analysis, has been introduced for accurate stator- or rotor-fault detection. Simulation and experimental results show the validity of the proposed method, leading to an effective diagnosis procedure for both stator and rotor electrical faults in WRIMs.

Detection and Localization of Stator Resistance Dissymmetry Based on Multiple Reference Frame Controllers in Multiphase Induction Motor Drives

Multiphase drives are receiving increasing attention by the research community in high-power applications. In this paper, the behavior of multiphase induction machines with an odd number of phases is investigated under the assumption that the resistances of the stator winding are unbalanced owing to poor connections. A high-resistance connection can cause overheating and supply voltage unbalance, which may reduce the efficiency and increase the fire hazard. The main contribution of this paper is a control scheme that can detect the stator resistance unbalance, that can localize the faulty phase, and, at the same time, that can keep the drive behavior unchanged, both in transient and steady-state operating conditions. The control scheme is based on an analytical model that shows the effect of the unbalance on the current components related to the high-order spatial harmonics of the air-gap magnetic field. The theoretical analysis and the feasibility of the control scheme are confirmed by experimental tests.

Investigation of motor current signature and vibration analysis for diagnosing rotor broken bars in double cage induction motors

This paper investigates the detectability of rotor broken bars in double cage induction motors using current signature and vibration analysis techniques. Double cage induction motors are commonly used for applications where successive loaded starts-up are mandatory. Experimental results were performed under healthy and faulty cases, and for different load conditions using each technique. Rotor broken bars fault detection based on sideband current components may fails due to the presence of inter bar currents that reduce the degree of rotor asymmetry, yielding to a decrease of the magnitude of these spectral components. But inter bar currents produce core vibrations in the axial direction, which can be detected using vibration analysis, in order to overcomes the limits of the classical (MCSA) in this condition.

Advanced Diagnosis of Outer Cage Damage in Double-Squirrel-Cage Induction Motors Under Time-Varying Conditions Based on Wavelet Analysis

It is known that classical fast-Fourier-transform-based steady-state spectrum analysis, such as motor current signature analysis, may fail to detect outer cage damage in double-squirrel-cage induction motors. This is because the magnitude of the rotor fault frequency components (RFFCs) in the current spectrum of faulty motors is small, due to the low-magnitude current circulation in the outer cage under a steady-state operation. The probability of misdetection is higher in time-varying load applications, such as conveyor belts, pulverizers, etc., for which double-cage motors are frequently employed. In case of load variation, the small RFFCs are spread in a bandwidth proportional to the speed variation, which makes them even more difficult to detect. A diagnosis method based on discrete wavelet transform and optimized for sensitive detection under transient operating conditions is proposed in this paper. An experimental study on a custom-built fabricated Cu double-cage-rotor induction motor shows that the proposed method can provide improved detection of outer cage faults particularly used in time-varying load applications.

Experimental investigation of fault-tolerant control strategies for quad-inverter converters

Fault-tolerant control strategies for quad-inverter based multiphase-multilevel converters are proposed and experimentally verified in this paper. Explicitly, the conversion scheme consists of four standard 2-level three-phase voltage source inverters (VSIs), able to supply a dual three-phase induction motor in open-end stator winding configuration (asymmetric six-phase machine), quadrupling the utility power of a single VSI within given voltage and current ratings. The developed modulation scheme has the capability to generate multilevel output voltage waveforms in healthy conditions, equivalent to the one of a 3-level VSI, and to share the total motor power among the four dc sources in each switching cycle. This sharing potentiality is investigated under post-fault operating conditions, when one VSI completely insulated due to a severe failure on it. In such circumstances, the quad-inverter system can perform with reduced power rating by a proper modulation of the remaining three healthy VSIs. The complete multi-phase-multilevel conversion system with the proposed control algorithm under healthy and post-fault operating conditions has been verified by experimental implementation in open-loop control aspect using two dsp TMS320-F2812 processors with two three-phase passive loads in open-end configuration.

Advanced diagnosis of broken bar fault in induction machines by using Discrete Wavelet Transform under time-varying condition

The diagnosis of induction machine faults is commonly carried out by means of Motor Current Signature Analysis (MCSA), i.e., by classical spectrum analysis of the input currents. Specifically in case of broken bars, the amplitude of the left sideband component of a phase current is monitored in order to sense its signature. However MCSA has some drawbacks that are still under investigation. The main concern is that an efficient frequency transformation cannot be made under speed-varying condition, since slip and speed vary and so does the left sideband frequency. In this paper, an advanced use of the Discrete Wavelet Transform (DWT) is proposed to overcome the limitation of the classical approaches based on Fourier Analysis (FA). Experimental and simulation results show the validity of the developed approach, leading to an effective diagnosis method for broken bars in induction machines.

Double frequency sliding and wavelet analysis for rotor fault diagnosis in induction motors under time-varying operating condition

The detection of rotor faults in squirrel cage motors has long been an important but difficult matter in the area of electrical machine diagnosis. Under time-varying condition, the typical rotor fault frequency components (RFFCs), which appear in the phase current spectrum of faulted motors, are spread in a bandwidth proportional to the speed variation and are difficult to detect accurately. Thereby a new diagnosis method based on the combined use of bi-frequency sliding (BFS) and discrete wavelet transform (DWT) is proposed here for stator phase current analysis. Simulation and experimental results are reported to validate the effectiveness of the proposed approach under critical speed varying conditions.

Doubly Fed Induction Machine stator fault diagnosis under time-varying conditions based on frequency sliding and wavelet analysis

The paper introduces a monitoring and diagnostic technique for the detection of incipient stator electrical faults in Doubly Fed Induction Machine (DFIM) for wind power systems. Operating in aggressive environments, the detection of anomalies at an incipient stage is crucial to decide about the operating continuity of the machines. Discrete Wavelet Transform (DWT) is used to detect stator faults under time varying-condition in two mainly different contexts: Transient-Speed conditions and Fault-Varying conditions. A frequency sliding (FS) with High Multiresolution Analysis (HMRA) approach is proposed for improving the ability of DWT in extracting the most relevant stator fault frequency component dynamically over time thereby. A dynamic mean power calculation at different resolution levels was introduced as a diagnostic index to quantify the fault extent. Simulation and experimental results show the effectiveness of the proposed approach in discriminating stator fault severities leading to an effective diagnostic procedure for stator faults in DFIM.

Fault-tolerant operating analysis of a quad-inverter multiphase multilevel AC motor drive

This paper investigates a new fault-tolerant strategy of a multi-phase multi-level ac motor drive. The proposed approach is based on four conventional 2-level three-phase voltage source inverters (VSIs) supplying the open-end windings of a dual three-phase motor (asymmetric six-phase machine), quadrupling the power capability of a single VSI with given voltage and current ratings. The developed fault-tolerant control algorithm is able to generate multi-level voltage waveforms, equivalent to the ones of a 3-level inverter, and to share the total motor power among the four dc sources within each switching period. The investigated ac motor drive has been numerically implemented, under healthy and for three degraded modes of the system, are given to prove the effectiveness of the whole strategy.

Online Detection of High-Resistance Connections in Multiphase Induction Machines

High-resistance connections in electrical machines cause unbalances in the stator resistances, reduce the efficiency, and increase the fire hazard. In this paper, the problem of detection of high-resistance connections is investigated for multiphase induction machines with an odd number of phases. The main contribution of this paper is a control scheme that can determine the stator resistance unbalance of all phases, under transient and steady-state operating conditions. The theoretical analysis and the feasibility of the control scheme are confirmed by experimental tests.