On the broken rotor bar diagnosis using time-frequency analysis:'Is one spectral representation enough for the characterisation of monitored signals?'

© 2019 Institution of Engineering and Technology. All rights reserved. This work enhances the knowledge of the diagnostic potential of the broken bar fault in induction motors. Since a series of studies have been published over the years regarding condition monitoring and fault diagnostics of these machines, it is essential to reach a common ground on why - sometimes - different techniques render different results. In this context, an investigation is provided with regards to the optimal window that should be adopted for the implementation of a proper time-frequency analysis of the monitored signals. On this agenda, this study attempts to set lower and upper bound limits for proper windowing from the digital signal processing point of view. This is done by proposing a formula for the lower limit, which is derived according to the specific frequencies one desires to put under inspection and which are the fault-related signatures. Finally, a discussion on the upper bound is put onwards; results from finite-element simulations are examined with the discussed approach in both the transient regime and the steady state, while experimental results verify the simulations with satisfying accuracy

Tourism Stocks in Times of Crises: an Econometric Investigation of Non-macro Factors

Following the recent terrorist attacks in Paris, the European media emphatically pronounced that billions of euros were wiped from tourism related stocks. This comes at a troublesome time for the tourism industry, in the midst of a global financial crisis, and the unpredictable rise of radical Islamic ideologies, which have caused chaos in the Middle East and Europe. The relationship and vulnerability of the industry to non-macro incidents have been well documented in the literature, mostly in theoretical terms. Nevertheless, the quantifiable impact of such events on tourism-specific stock values, both in terms of returns and volatility, received much less attention. With the use of an econometric methodology, the paper aims to enhance our conceptual capital pertaining to the effects of such possibilities on five hospitality and tourism stock indices. The empirical findings are of interest to stakeholders at all echelons of the spectra of the tourism and financial industries

A New Approach for Broken Rotor Bar Detection in Induction Motors Using Frequency Extraction in Stray Flux Signals

This paper offers a reliable solution to the detection of broken rotor bars in induction machines with a novel methodology, which is based on the fact that the fault-related harmonics will have oscillating amplitudes due to the speed ripple effect. The method consists of two main steps: Initially, a time-frequency transformation is used and the focus is given on the steady-state regime; thereupon, the fault-related frequencies are handled as periodical signals over time and the classical fast Fourier transform is used for the evaluation of their own spectral content. This leads to the discrimination of subcomponents related to the fault and to the evaluation of their amplitudes. The versatility of the proposed method relies on the fact that it reveals the aforementioned signatures to detect the fault, regardless of the spatial location of the broken rotor bars. Extensive finite element simulations on a 1.1 MW induction motor and experimental testing on a 1.1 kW induction motor lead to the conclusion that the method can be generalized on any type of induction motor independently from the size, power, number of poles, and rotor slot numbers

Breakdown Resistance Analysis of Traction Motor Winding Insulation under Thermal Ageing

Stator inter-turn faults are among the most important electric motor failures as they progress fast and lead to catastrophic motor breakdowns. Inter-turn faults are caused due to the windings’ insulation degradation. The main stress which deteriorates the insulation is the thermal one. Proper understanding of how this stress influences the electrical properties of insulation over time may lead to reliable prognosis and estimation of the motor’s remaining useful life. In transport applications where reliability and safety come first it is a critical issue. In this paper, extensive experimental testing and statistical analysis of thin film insulation for traction motor windings has been performed under fixed thermal stress. The results indicate that for high thermal stress the electrical properties of the insulation material present a non-monotonic behavior thus proving the well-known and established Arrhenius law inadequate for modelling this type of problems and estimating the remaining useful life of thin film insulation materials

FEM approach for diagnosis of induction machines' non-adjacent broken rotor bars by short-time Fourier transform spectrogram

Rotor electrical faults are an issue frequently encountered when applying condition monitoring and fault diagnosis on induction machines. The detection via the analysis of the stator current becomes challenging when the rotor cage suffers from multiple breakages at non-adjacent positions. In that case, electromagnetic asymmetries induced by the broken bars can be masked in such a way, that the diagnostic ability is highly likely to be obscured, thus leading to misinterpretation of the monitored signals’ signatures. A new approach is proposed in this work to overcome this problem while the motor is at steady state. In this paper, an industrial 6.6kV, 1.1MW induction motor is simulated with Finite Element Analysis (FEM) and its electromagnetic variables are analysed and studied under healthy state and several faulty conditions. The analysis of the stator current and stray flux waveforms is executed in both the transient and the steady state and aims to diagnose the challenging cases where the rotor breakages are nonconsecutive with regards to their spatial location. The results show the potential of flux analysis to fault severity regardless of the spatial position of the broken bars

Experimental and physics based study of the Schottky Barrier Height inhomogeneity and associated traps affecting 3C-SiC-on-Si Schottky Barrier Diodes

The ability of cubic phase (3C-) Silicon Carbide (SiC) to grow heteroepitaxially on Silicon (Si) substrates (3C-SiC-on-Si) is an enabling feature for cost-effective Wide Bandgap devices and homogeneous integration with Si devices. In this paper, the authors evaluated 3C-SiC-on-Si Schottky Barrier Contacts by fabricating and testing non-freestanding lateral Schottky Barrier Diodes (LSBD). To gain a deep physical insight of the complex carrier transport phenomena that take place in this material, advanced Technology Computer Aided Design (TCAD) models were developed which allowed accurately matching of measurements with simulations. The models incorporate the device geometry, an accurate representation of the bulk material properties, and complex trapping/de-trapping and tunnelling phenomena which appear to affect the device performance. The observed non-uniformities of the Schottky Barrier Height (SBH) were successfully modelled through the incorporation of interfacial traps. The combination of TCAD with fabrication and measurements enabled the identification of trap profiles and pin their influence on the electrical performance of 3C-SiC-on-Si LSBD. The effect of temperature was studied by engaging the identified trap profiles and calculating the occupation distribution of electrons in 3C-SiC at elevated temperature. The investigation constitutes an imperative knowledge step towards the development of devices that take advantage of 3C-SiC material properties