5,900 research outputs found
Influence of ac ageing on space charge dynamics in LDPE
No full textPolymeric materials have been widely used as insulation in power industry due to their excellent electrical properties. However, these properties deteriorate in time irreversibly when the material is subjected to electric stress. Although space charge is believed to play an important role in ac ageing, exact mechanisms are poorly understood due to very limited experimental data. In the present work efforts have been made to investigate the influence of ac ageing on space charge dynamics in low-density polyethylene (LDPE). LDPE films with 200mm were aged at 50 kV/mm at 50 Hz for various times at ambient temperature. Space charge dynamics in the samples prior to and after ageing were monitored using the pulsed electroacoustic (PEA) technique under dc electric stress. The results indicate that there is a significant amount of homocharge accumulation in the unaged sample due to charge injection. These injected charges are the captured by the deep traps originated from the interface between crystalline and amorphous regions in LDPE. Ageing under ac condition does not necessarily lead to an increase in amount of charge in the bulk but leads to an increase in mobility of charge carriers. Chemical analysis by infrared spectroscope (FTIR) reveals there are chemical changes taken place in the bulk of the material after ac ageing. It is believed that the chemical changes introduce shallow traps which promote the movement of charge carriers in the bulk. Consequently, the injected charges spread across the sample
A power sharing series power BJT array with isolated low voltage control for AC power control applications
No full textA technique for a continuously variable AC resistance using a series BJT array is presented. This array provides high power dissipation capability and uniform voltage and power distribution across the individual transistors. The array, controlled using a set of optoisolators to maintain the electrical isolation between the control circuits and the power stage, could be used as the basis to develop several useful techniques including a solid state AC regulator with comparable performance to the commonly used ferro-resonant systems; a linear AC electronic load suitable for testing UPS and other power conditioners; and, in other AC power control applications such as switching capacitors in AC resonant circuits
Experimental High-Frequency Parameter Identification of AC Electrical Motors
No full textIn order to predict conducted electromagnetic interference in inverter-motor drive systems, high-frequency (HF) motor models are requested and the involved parameters have to be available. In previous studies, the authors have presented an accurate HF model for induction motors and they have defined the procedures to identify the model parameters. In this paper, these results are extended to several types and sizes of industrial ac motors such as induction, synchronous reluctance (without interior permanent magnets), and brushless motors. The model parameter-identification procedure has been improved, and it is based on a least-squares data fitting applied to the measured magnitude and phase-frequency-response curves of the phase-to-ground and the phase-to-neutral impedances. The aim of this paper is to provide quick indications to select the suitable values of the HF model parameters, with reference to the size and type of the ac motor, to evaluate the HF voltage and current components in inverted-fed ac motor system
Using the AC Drive Motor as a Transducer for Detecting Electrical and Electromechanical Faults
No full textCondition monitoring of AC motors is a subject area that has received extensive research. Whether this monitoring is carried out on a scheduled basis by engineer intervention, or continuously using an on-line unit, the results of this testing enable preventative maintenance work to be a carried out earlier, before any major failure occurs. Monitoring using vibration analysis is the most common and depending on the plant, can be done once or twice a year. This is usually limited to the condition of motor bearings and is not commonly used to detect failures in the motor electromagnetic systems. Monitoring units that use motor current measurements are also available, but these are less widely-used and usually on major plant motors (>250kW for example) that have a large capital outlay to replace.
The industry drivers – as always – are maximum plant and machinery uptime, with the minimal amount of scheduled maintenance. If maintenance is carried out too regularly, costs rise significantly not only due to the maintenance activity itself, but disruption to production schedules. Maintenance schedules that are too infrequent can result in an unacceptable rise in total failures of plant that are unexpected and may cause a significant amount of production disruption and downtime, especially if this occurs during out-of-hours working time. However, industry now faces another big challenge and one that has had a good share of exposure over the last few years. It is of course, the drive to reduce carbon emissions and with it the amount of energy that a plant itself consumes. What has brought this more to the fore recently is the significant rise in energy costs. Whilst product margins have to remain the same, many companies energy costs have seen a two to three-fold increase in energy budgets in the last few years alone. For industry processes that have a significant amount of fan and pump applications, the manufacturers of low-cost AC inverters have saturated the lower-performance market of inverter drives such that any drive can control these type of fan and pump applications, where accurate speed control is not a major driver.
Unfortunately, this can be a step backwards for end-users of plant that use equipment to monitor motor condition via motor current signals. Additionally, vibration analysis that relies upon ‘base-lining’ motor data when the AC motor is running at base speed may not give accurate readings when the motor is under inverter control and running at a different speed.
For manufacturers of AC inverter drives in this low-end market, it can be difficult to sell a product from one manufacturer over another without the unit having a “USP”, or Unique Selling Proposition. Most decisions taken on inverter equipment purchase at this level are usually in favour of the equipment that costs the least to purchase. Credibility of manufacturers based on product history and perceived reliability cuts little ground with an ever cost-conscious industry.
This is where the research into diagnosis of faults on inverter driven motor systems can provide just this USP for manufacturers. If the incorporation of on-line diagnosis for simple inverter applications can be brought to a typical inverter unit at a reasonable cost, the manufacturer who can offer this gains a unique foothold in the marketplace – a drive that can monitor and signal that the motor it is driving is showing signs of early failure.
It will be sensible to limit this research to simple inverter applications as high-end inverter drives that operate equipment such as high-speed printing presses, rotary shears will be more difficult to model and simulate than a fan or pump application.
It is hoped that a typical inverter drive can relay enough detailed information about the load which it is driving to allow this to be used for abnormal motor load conditions as this will provide a platform on which to extend the research beyond this MSc and into the realms of incorporating such technology into a drive manufacturers equipment. If this can be done without major modification to an inverter, then it may be easier to implement in equipment offered by different manufacturers.
It is quite possible that this technology could be licensed under a name that guarantees the performance of the condition monitoring algorithms and reliability from one drive manufacturer to another
Torque behavior of one-phase permanent magnet AC motor
No full textThis paper presents a detailed comparative study of two starting and running methods for a single-phase permanent magnet synchronous motor, equipped with a squirrel-cage rotor. The analysis of the motor performance is realized for a pulse width modulated (PWM) inverter fed motor and for a capacitor-start, capacitor-run motor. The developed approach may be extended to any 1-phase ac motor—induction, synchronous reluctance or synchronous permanent magnet
AC/DC Characterization of a Ti/Au TES with Au/Bi Absorber for X-ray Detection
No full textTransition-edge sensors (TESs) are used as very sensitive thermometers in microcalorimeters aimed at detection of different wavelengths. In particular, for soft X-ray astrophysics, science goals require very high-resolution microcalorimeters which can be achieved with TESs coupled to suitable absorbers. For many applications, there is also need for a high number of pixels which typically requires multiplexing in the readout stage. Frequency-domain multiplexing (FDM) is a common scheme and is the baseline proposed for the ATHENA mission. FDM requires biasing the TES in AC at MHz frequencies. Recently, there has been reported degradation in performances under AC with respect to DC bias. In order to assess the performances of TESs to be used with FDM, it is thus of great interest to compare the performances of the same device both under AC bias and DC bias. This requires two different measurement set-ups with different processes for making the characterization. We report in this work the preliminary results of a single-pixel characterization performed on a TiAu TES under AC and afterwards under DC bias in different facilities. Extraction of dynamical parameters and noise performances are compared in both cases as a first stage for further AC/DC comparison of these devices.Accepted Author ManuscriptImPhys/Optic
Numerical-simulation of ac plasma-arc thermodynamics
No full textA mathematical model and approximate analysis for the energy distribution of an ac plasma arc with a moving boundary is developed. A simplified electrical conductivity function is assumed so that the dynamic behavior of the arc may be determined, independent of the gas type. The model leads to a reduced set of non-linear partial differential equations which governs the quasi-steady ac arc. This system is solved numerically and it is found that convection plays an important role, not only in the temperature distribution, but also in arc disruptions. Moreover, disruptions are found to be influenced by convection only for a limited frequency range. The results of the present studies are applicable to the frequency range of 10-10(2) Hz which includes most industry ac arc frequencies. (C) 1994 Academic Press, Inc
Dans un monde post-national
No full textThe Author in this work deals with the concepts of sovereignty, State and law, in the perspective of globalization and of the so called glocalization. In the light of the end of the modern State, the Author investigates the possibility of law and society of the people.Dans le cadre d’un monde post-national, toutes les formes de vie ne pouvent pas être poursuvies: on doit aspirer uniquement aux forms de vie dont les droits compossibles. C’est au nom de la compossibilité que les revendications avancéees par touts les formes (choix) de vie doivent être jugées, qu’il s’agisse de revedications partagéees, de reventications respecées, de revendications tolérées ou de revendications prohibées
Adiectis passim im margine Responsorum ac Constitutionum capitibus, ex quibus Author suam Epitomen compilauit ...
No full textA Bernardo à Rey, I. V. D. fideliter reddita, ac in Latinam linguam conuersa ; Adiectis passim im margine Responsorum ac Constitutionum capitibus, ex quibus Author suam Epitomen compilauit ...DruckermarkeImpressum im Kolophon: Lvgdvni, Excudebat Ioannes Pvllon, Alias De Trin, Impensis honesti uiri Pauli MirallietiAus dem Vorbesitz des Klosters Rheinau Exemplar der ZB Züric
Meso Mechanical Analysis of AC Mixture Response
No full textOngoing research into performance modeling of Asphalt Concrete (AC) mixtures using meso mechanics approaches is being undertaken at Delft University of Technology (TUD). The approach has already been successfully employed for evaluating the long term performance of porous asphalt concrete. The work presented in this paper extends the use for predicting the uni-axial response of AC mixtures. The approach demands the meso-scale geometry, the loading and the mechanical behavior of the component materials for the AC mixture to be specified. For this purpose, laboratory tests were first carried out on a bituminous mortar. Then, rectangular AC mixture specimens were prepared in the laboratory. The four sides of the AC specimens were scanned to capture the two dimensional (2D) meso-scale geometry of the mixture. The scanned images were then processed and translated into meso-scale finite element models. Viscoelastic model parameters for the mortar were determined from experimental data. The FE models were then used to simulate the frequency domain response of the AC mixture in a uni-axial mode. Similarly, uni-axial tests were carried out in the laboratory to obtain the frequency domain response of the AC mixture. FE simulated and experimental results were then compared. Good agreements in results have been obtained. The paper thoroughly discusses the obtained results and its implications for developing mixture design tools.Structural EngineeringCivil Engineering and Geoscience
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