1,721,081 research outputs found
Misure magnetiche per la caratterizzazione di calcestruzzo rinforzato con fibre d'acciaio
No full textDesign and Performance Prediction of Space Vector-Based PMU Algorithms
Full text linkPhasor measurement units (PMUs) are expected to be the basis of modern power networks monitoring systems. They are conceived to allow measuring the phasor, frequency, and rate of change of frequency (ROCOF) of electrical signals in a synchronized way and with unprecedented accuracy. PMUs are intended to apply to three-phase systems and to track signal parameters evolution during network dynamics. For these reasons, the design of the algorithms and, in particular, of the filters that allow rejecting the disturbances while preserving the passband signal content is a paramount concern. In this paper, a novel space vector approach is proposed. It exploits the three-phase nature of the monitored signals together with proper lowpass and differentiation filters. Analytical formulas for performance prediction under almost all the test conditions prescribed by the synchrophasor standard C37.118.1 for PMUs, are introduced. The given expressions are extremely accurate, thus allowing to derive the filter design criteria for phasor, frequency, and ROCOF computation, so that the requirements in terms of estimation errors can be easily translated into filter specifications. The implications of the proposed approach in practical PMU design are illustrated by means of two simple design examples matching P and M compliance classes, respectively, for all the test cases of the standard. The reported performance proves the validity of the proposal
A VI Based Tool for Inverter fed Induction Motor Diagnostic
No full textNowadays virtual instruments (more briefly called VIs) are very useful tools both in research and industrial applications. Their great flexibility, reconfigurability, reliability and ease of use allow an effective implementation of complex measurements architectures. In particular, the capability to manage long measurement processes without staff presence makes the VIs a powerful support for the development of diagnostic techniques which require a continuous monitoring of the system. In this paper a novel Vl-based diagnostic monitoring technique which permits the identification of rotor faults in small inverter-fed induction motors will be presented
Rilievo delle caratteristiche di calcestruzzi fibrorinforzati
No full textIl diffondersi di manufatti realizzati in calcestruzzo fibrorinforzato con fibre d’acciaio (SFRC) ha spinto a studiare e valutare nuovi metodi per la verifica sperimentale delle loro caratteristiche strutturali. In particolare, di grande interesse sono i metodi di tipo non invasivo, utilizzabili anche per la caratterizzazione in situ di elementi già posati. Nel lavoro si presenta una metodologia e un sistema di misura sviluppati dagli autori per il rilievo della concentrazione, dell’orientamento prevalente e delle dimensioni delle fibre presenti negli SFRC. La novità dell’approccio proposto consiste nel fatto che la caratterizzazione del materiale avviene sulla base di misure di grandezze magnetiche, ricavate mediante il semplice accostamento di una sonda alla superficie del materiale. La semplicità dell’apparato di misura, la quantità e la qualità delle informazioni che si possono ricavare e la totale non invasività del metodo, ne rendono molto interessanti le prospettive di sviluppo e impiego.In building construction, the increasing number of steel fiber-reinforced concrete (SFRC) elements has encouraged the development of new methods for the verification of their mechanical properties. In particular, there is a growing interest toward the study of non-invasive methods that can be used in the evaluation of already installed elements. The paper presents a methodology and a measurement system allowing to estimate the concentration, the average orientation and the size of the steel fibers. The novelty of the proposed technique is that these parameters are assessed through magnetic measurements, obtained by means of a probe laid on the surface of the material. The simplicity of the measurement system, the quantity and quality of the information and the non-invasive approach, make this method very promising
Device for monitoring a current of a primary conductor with respect to a predetermined current threshold, and related trip assembly and switching device
No full textA device for monitoring a current in a primary conductor with respect to a predetermined current threshold, comprising: - a magnetic circuit associable to the primary conductor and comprising a fixed part and an element which can rotate about a rotation axis; - at least one spring operatively connected to the rotating element for keeping it in a first position, the spring being elastically deformable along a linear axis; and - sensing means operatively associated to the magnetic circuit. The magnetic circuit is configured in such a way that the rotating element rotates from the first position to a second position when the current in the primary conductor exceeds the predetermined current threshold, so as to at least reduce one or more air gaps between the rotating element and the fixed part and to elongate the spring from a first length to a second length. The sensing means are configured for generating an output electrical signal caused by the rotation of the rotating element from the first position to the second position. The at least one spring is operatively connected to the rotating element in such a way to tilt towards the rotation axis moving above a surface of the rotating element which is transversal to the rotation axis, during the rotation of the rotating element from the first position to the second position
An Improved Model-Based Maximum Power Point Tracker for Photovoltaic Panels
Full text linkIt is well known that in a photovoltaic (PV) plant, the modules are connected to switch-mode power converters to enhance the power output in every environmental condition. This task is performed by the maximum power point tracker (MPPT), which provides a current or voltage reference to the converter. Traditional perturb and observe or incremental conductance algorithms are not efficient in rapidly changing conditions, whereas a model-based (MB) MPPT offers a better dynamic performance. Because it is relatively easy to obtain an accurate model of a single PV panel, thus predicting the maximum power point voltage for given environmental conditions, MB MPPTs seemed to be attractive for employing in module integrated converters. Conventional MB MPPT algorithms, however, usually require an expensive pyranometer to properly operate. In this paper, starting from a new set of equations modeling a PV module, a novel MB MPPT technique, which does not require the direct measurement of the solar radiation, is proposed and experimentally validated
A space vector based approach for synchrophasor measurement
No full textThis paper proposes a simple technique to estimate the positive sequence synchrophasor and the frequency of three-phase voltages or currents in power systems. The approach is based on the space vector transformation using a reference frame which rotates at the rated angular frequency ω0 of the power system. All the frequency components are shifted by ω0, so that the positive sequence phasor appear as a very low frequency term. The frequency shift also decreases the ratio between the frequency of the positive sequence phasor and those of the disturbances, so that they can be rejected by using a simple IIR filter thus requiring low computational effort. Then, the magnitude and phase of the phasor as well as its frequency can be easily estimated through an ordinary least square optimization. A proper tuning of the parameters allows the approach to comply with P-class or M-class requirements of the standard IEEE C37.118.1
Electric welding machine having a power circuit to limit voltage fluctuations induced in the mains
No full textThe present invention concerns an electric welding machine having a power circuit for supplying power to the welding machine from the mains. The power circuit comprises: an AC/DC converter stage (2), designed to receive an AC voltage (V AC ) from said mains at its input and to generate a first DC voltage (V rect ) at its output; an up converter stage (3), designed to receive said first DC voltage (V rec t) at its input and to generate a second DC voltage (V DC ) at its output; an output stage (10) in electric communication with said up converter stage (3), and designed to generate an output voltage (V sal ) suitable for the welding process at its output, and a pair of output terminals (4A, 4B) designed to have a load connected thereto, said output voltage (V sal ) being available at said output terminals.; The invention is characterized in that it comprises control means (6) for controlling said up converter stage (3) as a function of the value of a reference signal (Iref), check means (20) being provided for generating said reference signal (Iref), which are configured to receive at their input at least the value of said second voltage (V DC ) on said energy accumulation means (5), so that said energy accumulation means (5) are able to absorb power changes of said mains as said output voltage (V sal ) changes, due to the power absorbed by the load, to limit a voltage fluctuation induced in said mains
Sull’impiego di materiali cementizi fibrorinforzati ad alte prestazioni: dall’orientamento delle fibre all’ortotropia del comportamento meccanico
No full textLe applicazioni strutturali con materiali cementizi avanzati, quali i compositi cementizi fibrorinforzati ad elevate prestazioni (High Performance Fiber Reinforced Cementitious Materials HPFRCC) necessitano, oltre che di affidabili regole progettuali e metodologie di caratterizzazione sperimentale delle proprietà del materiale, anche di procedure di controllo di qualità egualmente efficaci. Rimane tuttora in discussione la possibilità di ottenere una dispersione uniforme delle fibre: in caso contrario le zone ad esempio prive di rinforzo fibroso si comporterebbero come difetti, innescando meccanismi di crisi strutturale non previsti in progetto e riducendo la capacità portante. In questi ultimi anni si è mostrato come sia possibile sfruttare le superiori prestazioni allo stato fresco dei suddetti materiali sia per ottenere una omogenea dispersione delle fibre ma anche per orientare le stesse lungo la direzione di flusso della miscela fluida durante il getto. L’orientamento delle fibre può così essere progettato in funzione della applicazione strutturale, facendo in modo, attraverso una adeguata progettazione tanto della prestazione allo stato fresco quanto del procedimento di getto, che esso venga a coincidere con la direzione delle tensioni principali di trazione nell’elemento strutturale in opera. La validità di quanto esposto è stata fino ad ora supportata principalmente mediante indagini di tipo distruttivo. Per sfruttare efficacemente quanto sopra detto nelle pratiche realizzazioni strutturali è necessario sia sviluppare tecniche di indagine non distruttive, che consentano di identificare le caratteristiche di dispersione ed orientamento delle fibre, sia correlarne i risultati con il comportamento meccanico del composito. In questo lavoro si è impiegato un metodo non distruttivo basato sulle proprietà magnetiche dei compositi fibrorinforzati per la misura delle caratteristiche di dispersione ed orientamento delle fibre in due lastre (1m x 0.5 m x 25 mm di spessore), realizzate con HPFRCC autocompattanti, con diversi contenuti di fibre e gettate in maniera tale da avere un orientamento preferenziale delle fibre. I risultati ottenuti mediante la metodologia non distruttiva in termini di concentrazione ed orientamento delle fibre nelle diverse zone della lastra, opportunamente convalidati mediante misure distruttive, sono stati quindi correlati al comportamento meccanico in trazione, misurato mediante una nuova metodologia sperimentale “per spacco a doppio incuneamento”. I provini sono stati ottenuti per taglio dalla lastra nel rispetto dello schema utilizzato per le misure non distruttive in maniera tale che il potenziale piano di frattura risultasse sia parallelo sia ortogonale all’orientamento preferenziale delle fibre, quale indotto dalla direzione del flusso di getto. Il presente lavoro rappresenta dunque un punto cardine nella identificazione, anche ai fini degli impieghi progettuali, del comportamento ortotropo dei compositi fibrorinforzati dovuto all’orientamento delle fibre indotto dal flusso di getto
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