6,293 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
Deployment of spatial attention towards locations in memory representations: an EEG study
No full textRecalling information from visual short-term memory (VSTM) involves the same neural mechanisms as attending to an actually perceived scene. In particular, retrieval from VSTM has been associated with orienting of visual attention towards a location within a spatially-organized memory representation. However, an open question concerns whether spatial attention is also recruited during VSTM retrieval even when performing the task does not require access to spatial coordinates of items in the memorized scene. The present study combined a visual search task with a modified, delayed central probe protocol, together with EEG analysis, to answer this question. We found a temporal contralateral negativity (TCN) elicited by a centrally presented go-signal which was spatially uninformative and featurally unrelated to the search target and informed participants only about a response key that they had to press to indicate a prepared target-present vs. -absent decision. This lateralization during VSTM retrieval (TCN) provides strong evidence of a shift of attention towards the target location in the memory representation, which occurred despite the fact that the present task required no spatial (or featural) information from the search to be encoded, maintained, and retrieved to produce the correct response and that the go-signal did not itself specify any information relating to the location and defining feature of the target
Experimental High-Frequency Parameter Identification of AC Electrical Motors
Get PDFIn 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
Ácaros-fitoseídeos (Acari: Phytoseiidae) associados a clones de seringueira em Rio Branco, AC.
Get PDFO estudo foi conduzido de outubro de 2018 a janeiro de 2019, tendo por objetivo elaborar uma lista preliminar de espécies de ácaros-fitoseídeos, associados a clones de seringueiras (CPAAC 18, RRIM 600, FX 3899, FDR 5788 e PB 312), localizados no jardim clonal da Embrapa Acre, Rio Branco, AC. Semanalmente eram escolhidas, aleatoriamente, quatro plantas por clone e de cada uma delas coletadas oito folhas, no terço inferior da copa. As amostras eram acondicionadas em sacos de papel identificados e transportadas até o Laboratório de Entomologia da Embrapa Acre. Sob estereomicroscópio, os ácaros encontrados nas faces adaxial e abaxial de cada folha foram capturados com um estilete, montados em lâminas de microscopia em meio de Hoyer e identificados com auxílio de literatura especializada. Foram coletados 164 ácaros-fitoseídeos, pertencentes a oito gêneros e nove espécies (Amblyseius aerialis, Amblydromalus akiri, Propioseiopsis neotropicus, Euseius concordis, Iphiseiodes zuluagai, Typhlodromus transvaalensis, Phytoscutus sexpilis, Typhlodromips sp. e Iphiseiodes sp.). As espécies T. transvaalensis, P. sexpilis e P. neotropicus configuram-se como primeiros registros para o estado do Acre associados a Hevea brasiliensis. A espécie A. aerialis foi a mais abundante (35% do total) e melhor distribuída, não ocorrendo apenas no clone CPPAC 18. O objetivo deste trabalho foi realizar um levantamento de ácaros-predadores da família Phytoseiidae, associados a cinco clones de seringueira no estado do AcreEditores técnicos: Virgínia de Souza Álvares; Fabiano Marçal Estanislau
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
Dissociating linguistic processes in the left inferior frontal cortex with transcranial magnetic stimulation
No full textIs the left inferior frontal cortex (LIFC) a single functional region, or can it be subdivided into distinct areas that contribute differently to word processing? Here we used transcranial magnetic stimulation (TMS) to investigate anterior and posterior LIFC when the meaning and sound of words were being processed. Relative to no stimulation, TMS of the anterior LIFC selectively increased response latencies when participants focused on the meaning of simultaneously presented words (i.e., synonym judgments) but not when they focused on the sound pattern of the words (i.e., homophone judgments). In contrast, the opposite dissociation was observed in the posterior LIFC, where stimulation selectively interfered with the phonological but not the semantic task. This double dissociation shows functionally distinct subdivisions of the LIFC that can be understood in terms of separable corticocortical connections linking the anterior LIFC to temporal pole regions associated with semantic memory and the posterior LIFC to temporoparietal regions involved in auditory speech processing
Language-related field potentials in the anterior-medial temporal lobe: II. Effects of word type and semantic priming.
No full textField potentials were recorded from intracranial electrodes in humans to study the role of the anterior medial temporal lobe (AMTL) in language-related processing. Subjects viewed lists of words in which orthography and word type varied, or in which words were primed by semantic associates. Large negative field potentials were elicited within the AMTL by isolated words. The amplitude and intracranial distribution of these AMTL field potentials were consistent with those in our previous study in which anomalous sentence-ending words were used as stimuli (McCarthy et al., in press). The neocortex, in the region of the collateral sulcus and anterior fusiform gyrus, was identified as the likely neural generator of this field potential. The AMTL field potential was diminished by semantic priming, and was larger for words with semantic content than for words serving grammatical function. Orthographically illegal nonwords did not elicit this field potential. The N400 scalp event-related potential (ERP) has been shown to respond in the same manner to these task manipulations (Nobre and McCarthy, 1994), and, thus, the AMTL field potential was proposed to contribute to the generation of N400. The possible roles in language processing reflected by the AMTL field potential were considered
Long-term Memories Bias Sensitivity and Target Selection in Complex Scenes
Get PDFIn everyday situations, we often rely on our memories to find what we are looking for in our cluttered environment. Recently, we developed a new experimental paradigm to investigate how long-term memory (LTM) can guide attention and showed how the pre-exposure to a complex scene in which a target location had been learned facilitated the detection of the transient appearance of the target at the remembered location [Summerfield, J. J., Rao, A., Garside, N., & Nobre, A. C. Biasing perception by spatial long-term memory. The Journal of Neuroscience, 31, 14952–14960, 2011; Summerfield, J. J., Lepsien, J., Gitelman, D. R., Mesulam, M. M., & Nobre, A. C. Orienting attention based on long-term memory experience. Neuron, 49, 905–916, 2006]. This study extends these findings by investigating whether and how LTM can enhance perceptual sensitivity to identify targets occurring within their complex scene context. Behavioral measures showed superior perceptual sensitivity (d′) for targets located in remembered spatial contexts. We used the N2pc ERP to test whether LTM modulated the process of selecting the target from its scene context. Surprisingly, in contrast to effects of visual spatial cues or implicit contextual cueing, LTM for target locations significantly attenuated the N2pc potential. We propose that the mechanism by which these explicitly available LTMs facilitate perceptual identification of targets may differ from mechanisms triggered by other types of top–down sources of information
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