1,721,054 research outputs found
Dynamic behaviour of a four-poles turbo generator with rotor eccentricity
No full textSmooth-poles generators of electrical power
plants are flexible and this causes the eccentricity of the rotor with respect to the stator. The consequent air-gap asymmetrical distribution causes the phenomenon of the unbalanced magnetic pull (UMP). In this paper a non-linear model for the calculation of the UMP in synchronous electric machines is applied to a four-poles turbo-generator of 900 MVA, by taking into account not only the radial component of the electromagnetic stresses but also the tangential one
Detection and modelling of rotor eccentricity in electrical machines - an overview
No full textRotor eccentricity in electrical machines determines the non-uniformity of the air-gap and gives rise to an unbalanced magnetic pull (UMP), which causes vibrations and noise emission, speeds up the bearing wear and can produce a rub between rotor and stator. In the paper, an overview about this subject is presented, by classifying the researches according to the type of machine (induction, synchronous, brushless), the type of eccentricity (static or dynamic) and the type of studied effect (definition of the fault indicators, calculation of the UMP, modelling the dynamic behaviour of the machine)
Unbalanced magnetic pull modelling in a two poles three phase generator
No full textIn rotating electrical machines with eccentric rotor, an imbalance of the electromagnetic forces acting upon rotor and stator surfaces occurs, so that a net radial force, known as Unbalanced Magnetic Pull (UMP), is developed. In case of large size machines, such as turbo-generators supported by oil-film bearings, the approximation of circular orbit normally used in literature for the geometric centre of the rotor is not acceptable. Nevertheless, the authors who have dealt with UMP in big size generators have disregarded that these rotor filtered orbits are elliptical and generally the orbit centre is not concentric with the stator. A more
realistic model is introduced in this work and the actual distribution of the air-gap length during the rotation is determined in analytical terms, by taking into account the effects produced by the actual rotor orbit. The actual UMP is calculated by using the air-gap permeance approach and the simulation of the dynamical behaviour of a 320 MW generator is presented, showing the harmonic content of the UMP and the presence of non-linearities
Calcolo dello sbilanciamento magnetico nelle macchine rotanti elettriche
No full textNelle macchine elettriche rotanti con rotore eccentrico si ha uno sbilanciamento delle forze elettromagnetiche agenti tra rotore e statore che produce una forza radiale netta nota come Unbalanced Magnetic Pull (UMP). Nel caso di macchine di elevata potenza e velocità, quali i turbo-alternatori supportati da cuscinetti lubrificati, non è accettabile l’approssimazione di orbita circolare per il centro geometrico del rotore, normalmente usata in letteratura. La memoria introduce un modello più realistico mediante il quale si determina analiticamente la distribuzione reale del traferro durante la rotazione, tenendo conto degli effetti prodotti dall’orbita del rotore. L’UMP è calcolato con l’approccio della permeanza al traferro e la simulazione del comportamento dinamico di un generatore da 320 MVA mostra il contenuto armonico dell’UMP e la presenza di non-linearità
The effect of the radial and tangential electromagnetic stresses on the vibrations of synchronous machines
Full text linkThe main effect due to rotor eccentricity in electrical machines is an imbalance of the electromagnetic forces acting upon rotor and stator surfaces, with a consequent development of a net radial force, known in the literature as Unbalanced Magnetic Pull. The authors of previous researches have generally calculated this force by means of the expression of the radial component of the Maxwell stress tensor, neglecting the tangential one. In this paper different approaches will be considered in order to evaluate the electromagnetic stresses acting in synchronous machines: the circuit approach, the field approach and a novel approach based on the electromechanical features of the machine. From the Maxwell theory, many authors maintain that the air-gap magnetic field in rotating electrical machines can not be purely radial, otherwise the phenomenon of the torque generation could not be explained. In this paper, the generation of torque is explained by the Lorentz forces, which act tangentially at the air-gap just considering the field as radial. These tangential forces can be associated to a tangential stress, which in turn can be expressed as the Maxwell tangential tensor and therefore correlated to an equivalent tangential component of the magnetic field. A definition of the equivalent tangential flux density and a relationship between the tangential and the radial stresses will be found. Therefore, the values of these parameters will be estimated in case of machines with salient-poles (hydrogenerators) and smooth-poles (turbo-generators). Finally, the effect of rotor eccentricity on open circuit and in load condition will be evaluated, by modifying the expressions of the forces due to the unbalanced magnetic pull obtained by the authors in a previous research. The results point out that, in steady-state conditions, the tangential stress can be actually disregarded for both types of synchronous machines, even if its effect is less negligible for the smooth-poles machines than for the salient-poles ones, due to the constructive characteristics of these machines
On the effects of a shaft crack propagation on the dynamic behavior of a steam turbine
No full textThe symptoms of the propagation of shaft cracks in rotating machines, which can be brought out of the vibration data collected by condition monitoring systems, are often not very evident, especially when the crack depth is not very high. In fact, shaft cracks commonly propagate rather far from the cross-sections where the vibration probes have been mounted. At these measurement planes, the sensitivity of the machine dynamic behavior to the effects caused by a shaft crack can be fairly low. Therefore, it is very important to improve the capabilities of diagnostic techniques aimed at the early detection of shaft cracks. In this paper, the dynamic behavior of the cracked shaft of a HP-IP steam turbine is shown. The results of investigations on the influence of the periodic and non-periodic breathing of the crack on the rotor response, measured in different machine thermal states and at unusual cross-sections of the shaft, are shown and discussed
Modal analysis of turbine blades by means of distributed optical fiber sensors
No full textThis paper investigates the possibility of identifying and monitoring the modal shapes of a turbine blade by means of continuous optical fiber sensors based on Optical Backscatter Reflectometry (OBR). The advantage of this approach would be the possibility of embedding the sensors in future carbon fiber blades, in order to make this modal analysis approach available also for the blade operating conditions, since no modifications in the blade fluid-structure interaction occur. The paper describes the proposed method and provides some experimental results obtained on a 3D printed model of an existing steam turbine blade
An unconventional method for the diagnosis and study of generator rotor thermal bows
No full textThe rotor thermal sensitivity often affects the dynamic behavior of power unit generators. Owing to this phenomenon, increments of field current and other process parameters that are related to it may cause a shaft thermal bow and significant changes in the synchronous vibration. This symptom can also be caused by many other common malfunctions that affect rotating machines. Therefore, diagnostic techniques aimed at identifying the actual fault are very useful for optimizing maintenance activities. The thermal sensitivity of generator rotors can be deemed as a fault because it is commonly caused by a local deterioration of the winding insulation as well as by jamming phenomena between conductors and rotor slots, caused by friction forces due to the different thermal expansions of these components. This paper shows the results obtained applying a diagnostic method, based on multiple linear regression models, which has been developed for the analysis of generator vibrations caused by thermal sensitivity. Nevertheless, nonlinear relationships between vibration and process parameters have also been taken into account. The capabilities of this diagnostic technique have been validated using the analysis of experimental data collected in a power plant. The results of this investigation are shown and discussed in the paper
Optimization of continuous sensor placement for modal analysis: Application to an optical backscatter reflectometry strain sensor
No full textThe methods for experimental modal analysis are consolidated, as are the sensors used to perform this analysis. The characteristics of the most commonly used sensors are to be “discrete” and “independent”, that is, each sensor can measure the magnitude of interest in a single point and can be placed independently from the others on the structure. This kind of layout is widely used, for example, with accelerometers, but it presents a strong complexity if the structure is extended or characterized by high modal density in the frequency range of interest, or if the number of modes to be identified is high. Often the “discrete” sensors also have nonnegligible masses compared to the mass of the structure to be analyzed, thus introducing the well-known “mass effect”. Recently, new dynamic sensors have become available based on optical fibers such as the well-known Fiber Bragg grating sensors or the more recent Optical Backscatter Reflectometry (OBR) sensors. Both are sensors of the “continuous” type, or rather, they are a succession of sensors connected in series inside a single optical fiber. The purpose of this paper is to explore the use of OBR for modal analysis and introduce an original algorithm that allows the OBR sensor to be arranged in an optimized manner, to identify up to a certain number of modes in the most effective way and to respect the constraints imposed by the optical fiber itself, such as fiber length or maximum sensor curvature. The results are illustrated by means of the experimental results obtained on a structure characterized by a simple geometry
Image-Processing-Based Intelligent Defect Diagnosis of Rolling Element Bearings Using Spectrogram Images
Full text link: Due to the excellent image recognition characteristics of convolutional neural networks
(CNN), they have gained significant attention among researchers for image-processing-based defect
diagnosis tasks. The use of deep CNN models for rolling element bearings’ (REBs’) defect diagnosis
may be computationally expensive, and therefore may not be suitable for some applications where
hardware and resources limitations exist. However, instead of using CNN models as end-to-end
image classifiers, they can also be used to extract the deep features from images and those features can
further be used as input to machine learning (ML) models for defect diagnosis tasks. In addition to
extracting deep features using CNN models, there are also other methods for feature extraction from
vibration characteristic images, such as the extraction of handcrafted features using the histogram
of oriented gradients (HOG) and local binary pattern (LBP) descriptors. These features can also be
used as input to classical ML models for image classification tasks. In this study, a performance
comparison between all these image-processing-based defect diagnosis techniques was carried out in
terms of fault detection accuracy and computational expense. Moreover, based upon the detailed
comparison, a hybrid-ensemble method involving decision-level fusion is proposed, which is far less
computationally expensive compared to CNN models while using them as end-to-end classifiers.
The performance of all these models is also compared in the case of minimal training data availability
and for diagnosis under slightly different operating conditions to ascertain their generalizability and
ability to correctly diagnose despite the minimal availability of training data. The performance of the
proposed hybrid-ensemble method remained outstanding for the REBs’ defect diagnosis despite the
minimal of availability training data as well as the slight variation under operating conditions
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