Time-Frequency Analysis.

This paper presents a new time–frequency (TF) analysis method called matching synchrosqueezing wavelet transform (MSWT) to signals with fast varying instantaneous frequency (IF). The original synchrosqueezing wavelet transform (SWT) can effectively improve the readability of TF representation (TFR) of signals with slowly varying IF. However, SWT still suffers from TF blurs for signals with fast varying IF. Moreover, the variable operating conditions of the aeroengine always make the vibration a signal with fast varying IF, especially when it comes to significant speed changes, which results in the obscure TFR for aeroengine vibration monitoring. In this paper, the MSWT introduces a chirp rate estimation into a comprehensive IF estimation to match the TF structure of the signals with fast varying IF and thus to achieve a highly concentrated TFR as the standard TF reassignment methods. Most importantly, the MSWT retains the reconstruction benefit like the SWT. The proposed MSWT is validated by both numerical simulation and applications in a bat echolocation signal analysis. Finally, a case study of a dual-rotor turbofan engine is given to illustrate the effectiveness of the proposed method for aeroengine vibration monitoring.

Matching Synchrosqueezing Wavelet Transform and Application to Aeroengine Vibration Monitoring

Preventing induction motors (IMs) from failure and shutdown is important to maintain functionality of many critical loads in industry and commerce. This paper provides a comprehensive review of fault detection and diagnosis (FDD) methods targeting all the four major types of faults in IMs. Popular FDD methods published up to 2010 are briefly introduced, while the focus of the review is laid on the state-of-the-art FDD techniques after 2010, i.e. in 2011–2015 and some in 2016. Different FDD methods are introduced and classified into four categories depending on their application domains, instead of on fault types like in many other reviews, to better reveal hidden connections and similarities of different FDD methods. Detailed comparisons of the reviewed papers after 2010 are given in tables for fast referring. Finally, a dedicated discussion session is provided, which presents recent developments, trends and remaining difficulties regarding to FDD of IMs, to inspire novel research ideas and new research possibilities.

A review and comparison of fault detection and diagnosis methods for squirrel-cage induction motors: State of the art.

This paper presents an effective method of motor current signature analysis for detecting half- as well as full broken single rotor bar fault of a squirrel-cage induction machine under various loading conditions and speeds. The proposed method is based on spectral preprocessing of the stator current followed by subspace decomposition of the signal autocorrelation matrix to detect relatively low-amplitude fault sidebands. This method is found to be very effective in detecting low-amplitude sinusoids in a signal dominated by high-amplitude fundamental. The extended Kalman filter is used to estimate and track the fundamental component of the stator current. This component is subtracted from the measured stator current at every time step generating a resultant signal with a very low or negligible fundamental component. Subsequently, multiple-signal classification (MUSIC) is applied on the resultant stator current signal. Motor slip is estimated from principle slot harmonic to decide the approximate location of the fault sidebands. For effective fault detection, a hypothesis test is proposed to check the presence of sufficient fault frequency sideband in the current spectrum. This test works better if the lobe in the MUSIC plot due to the fault frequency is not distorted or overlapped by the fundamental component. Therefore, for each data window, the minimum size of the autocorrelation matrix is determined to generate distinct peaks. The proposed method applies to steady-state condition and is found to exhibit superior performance even during the light-load conditions with a half-broken bar.

A Method for Detecting Half-Broken Rotor Bar in Lightly Loaded Induction Motors Using Current

Transient-based methods for fault diagnosis of induction machines (IMs) are attracting a rising interest, due to their reliability and ability to adapt to a wide range of IM’s working conditions. These methods compute the time–frequency (TF) distribution of the stator current, where the patterns of the related fault components can be detected. A significant amount of recent proposals in this field have focused on improving the resolution of the TF distributions, allowing a better discrimination and identification of fault harmonic components. Nevertheless, as the resolution improves, computational requirements (power computing and memory) greatly increase, restricting its implementation in low-cost devices for performing on-line fault diagnosis. To address these drawbacks, in this paper, the use of the short-frequency Fourier transform (SFFT) for fault diagnosis of induction machines working under transient regimes is proposed. The SFFT not only keeps the resolution of traditional techniques, such as the short-time Fourier transform, but also achieves a drastic reduction of computing time and memory resources, making this proposal suitable for on-line fault diagnosis. This method is theoretically introduced and experimentally validated using a laboratory test bench.

/pdf/short-frequency-fourier-transform-for-fault-diagnosis-of-44jvxkeibg.pdf

Short-Frequency Fourier Transform for Fault Diagnosis of Induction Machines Working in Transient Regime

We conducted a 3-D electromagnetic analysis coupled with a 3-D mechanical analysis to analyze end-winding vibrations and deformation in an induction machine caused by steady-state magnetic forces on the end winding. Both the analyses were based on the finite-element method. The electromagnetic analysis was used to calculate magnetic forces. During the mechanical analysis, complex support structures in the end region were simplified. We first updated and validated the mechanical model according to a modal model obtained from a modal test, and afterward analyzed deformation, vibrations, and stresses. According to the analysis, the shape of the rotary dynamic deformation of the end winding caused by dynamic forces is similar to the most excitable mode shape though the natural frequency of that mode is much higher than the excitation frequency. The static deformation caused by static forces tends to expand the coil ends outward. Under both types of deformation, the nose portion of the coil ends experiences larger displacement, but von Mises stresses are larger mainly in the knuckle portion.

End-Winding Vibrations Caused by Steady-State Magnetic Forces in an Induction Machine

A detailed study on the energy content of a principal slot harmonic (PSH) in an induction motor operating at variable slip is carried out. The machine is tested under different faulty conditions, namely, broken rotor bar, mixed eccentricity, and interturn short circuit. The use of a combined time–frequency analysis and particle filtering feature extraction procedure allows tracking the evolution of a PSH under different load profiles and integrating its energy. The proposed fault indicator provides values that are equivalent with the traditional one for stationary operation even in the case of pulsating load. The results show that the energy in the PSH reflects the overall state of the machine under these conditions. Furthermore, procedures to discern the different faults in real applications are proposed.

/pdf/diagnosis-of-induction-motors-under-varying-speed-operation-1ewhav8cwc.pdf

Diagnosis of Induction Motors Under Varying Speed Operation by Principal Slot Harmonic Tracking

Effect of multi-axial stress on iron losses of electrical steel sheets

Fault diagnosis of induction machines operating under variable load conditions is still an unsolved matter. Under those regimes, the application of conventional diagnostic techniques is not suitable, since they are adapted to the analysis of stationary quantities. In this context, modern transient-based methodologies become very appropriate. This paper improves a technique, based on the application of Wigner-Ville distribution as time-frequency decomposition tool, using a particle filtering method as feature extraction procedure, to diagnose and quantify electrical asymmetries in induction machines, such as wound-rotor induction generators used in wind farms. The combination of both tools allows tracking several variable frequency harmonics simultaneously and computing their energy with high accuracy, yielding magnitudes and values similar to those obtained by the application of the fast Fourier transform in stationary operation. The experimental results show the validity of the approach for rapid speed variations, independently of any speed sensor.

/pdf/particle-filter-based-estimation-of-instantaneous-frequency-2sbkhijqhj.pdf

Particle filter-based estimation of instantaneous frequency for the diagnosis of electrical asymmetries in induction machines

This paper proposes an effective method of analyzing the eddy-current loss within the end shield and frame of an electric machine. The standard impedance boundary condition is applied to the conducting surfaces of complete 3-D models, and the models are solved by time-harmonic finite-element analyses. Measurement of the temperature rise is completed as a validation of the proposed method. The effect of the distances between the conducting surfaces and coil ends is studied by computing the eddy-current loss for a series of 3-D models having different distances. In addition, simplified 2-D models are used to study the influence of material nonlinearity. In brief, the proposed method is feasible in computing the eddy-current loss. The eddy-current loss within the regions close to the coil ends is larger than the other regions in the end shield and frame, but the total loss is quite small, compared, for instance, with the copper loss within the stator winding. Furthermore, the farther the end shield and frame are from the coil ends, the smaller the eddy-current loss is. Finally, it is found that the eddy-current loss is larger in the nonlinear case than in the linear case.

/pdf/analysis-of-eddy-current-loss-in-end-shield-and-frame-of-a-3huc5agpv1.pdf

Ari Haavisto

Papers

End-Winding Vibrations Caused by Steady-State Magnetic Forces in an Induction Machine

Diagnosis of Induction Motors Under Varying Speed Operation by Principal Slot Harmonic Tracking

Effect of multi-axial stress on iron losses of electrical steel sheets

Particle filter-based estimation of instantaneous frequency for the diagnosis of electrical asymmetries in induction machines

Analysis of Eddy-Current Loss in End Shield and Frame of a Large Induction Machine