1,720,996 research outputs found
The impact of impulsive voltage waveforms on the electrical insulation of actuators for more electrical aircraft (MEA)
No full textTo reduce the size and weight of aircraft actuators, wide band-gap semiconductor devices (SiC and GaN) are the most promising solution. These devices have switching times well below those of silicon devices. Very short rise times may enhance the electrical stress within the insulation, triggering partial discharges (PDs). The inception of PDs at carrier frequencies of several tens of kHz can lead to the premature failure of the actuator insulation, sometimes in the matter of minutes. In this paper, we show the results of PD measurements at different frequencies and pressures, and discuss the influence of both frequency and pressure on the inception of PDs. Our measurements show a steady decrease of RPDIV with the frequency up to 100 kHz, and a linear dependence of RPDIV on pressure. These findings can be useful to design reliable actuators
The Problem of Altitude When Qualifying the Insulating System of Actuators for More Electrical Aircraft
No full textThis work concerns the optimization of the sizing of the insulating system of an actuator realized for the aeronautical sector in the framework of the MEA (More Electric Aircraft) and its qualification. Furthermore, the absence of a real standard that takes into account the most stressful conditions at high altitude is discussed. The objective of this article is to provide an easy to use model that allows a conservative choice of the characteristics of the insulation system, given the environmental conditions and the type of power supply installed. Moreover, the calculation methodology on which the tool is based can be a starting point for defining qualification criteria for electric motors used in the aeronautical field
The Impact of Test Voltage Waveform in Determining the Repetitive Partial Discharge Inception Voltage of Type i Turn/Turn Insulation Used in Inverter-Fed Induction Motors
No full textThe reliability of induction motors stators realized using Type I insulation systems is ensured only when the partial discharge inception voltage (PDIV) is larger than the operating voltage. The concept of Repetitive PDIV (RPDIV) has been proposed as a replacement of PDIV for tests performed under surges or repetitive square waves. RPDIV estimates exceed the PDIV level. The difference might depend on the surge repetition frequency, possibly providing contradicting results using different high voltage sources. Thus, the industry is concerned about the use of RPDIV in place of PDIV and the possible implications for the quality of the stators. Also, little has been discussed about the randomness of measurements and how to treat data using appropriate statistical tools. In this paper, we shall show results of PDIV and RPDIV tests performed on twisted pairs subjected to AC sinusoidal voltages, surges from a commercial surge generator, and unipolar square waves from a high voltage MOSFET. Tests are performed on ten twisted pairs to derive the distribution of RPDIV estimates and compare the differences using statistical tools. Different types of wire are considered to carry out a comparative analysis
PWM impacts on the Reliability of DC/DC Converters with High-Frequency Transformer
No full textIn the framework of the More Electric Aircraft, high efficiency and high reliability are the main concerns. Different standard exists regarding voltage and frequency of the electrical power distribution system, but DC systems are attracting an increased interest. In this context, the voltage conversion is normally implemented by means of a high-frequency isolated DC/DC power converters. However, the high-frequency waveforms generated by the power electronics may trigger partial discharge phenomena in the windings of the transformer, decreasing the reliability. This paper attempts to model the lifetime of a Dual Active Bridge (DAB) converter for aerospace applications
Impact of thermal overload on the insulation aging in short duty cycle motors for aerospace
No full textElectric motors for aerospace applications are often characterised by short-time duty rather than continuous-duty operations. Some of the main requirements for these machines are high torque density and reliability. Considerable torque density improvement can be achieved by supplying the windings with current greater than the rated value and thus thermally overloading the machine for short periods of time. However, if the insulation is overheated, the machine lifetime is shortened, and reliability issues can arise. This paper experimentally investigates the influence of short-time thermal overload on the insulation lifetime of low voltage, random wound electrical machines. The analysis is performed on round enamelled magnet wires coils aged by accelerated thermal cycles, and the results are statistically processed by the two-parameter Weibull distribution. The experimental data is also used for building a lifetime estimation model, which is employed for predicting the lifetime of a motor embedded into an electromechanical actuator for landing gear extension/retraction
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Partial Discharges in Electrical Machines for the More Electrical Aircraft. Part III: Preventing Partial Discharges
Full text linkIn this paper, the results obtained from lab tests on twisted pairs subjected to different voltage waveforms and atmospheric conditions are used to propose how to modify the IEC Std. 60034-18-41. The goal is to make the standard suitable for the More Electrical Aircraft (MEA). The results show that it is initially necessary to screen out materials through simple tests. The enhancement factors for temperature can be modified to consider reduced pressures and temperatures using a simple model. The aging enhancement factor can be reduced considering the reduced sensitivity of the partial discharge inception voltage (PDIV) at low pressures on the enamel thickness. Eventually, reference will be made to the drive discussed in Part I of this series to draw conclusions about the likelihood of partial discharge inception in a random wound stator and how to reduce it by modifying either the inverter or the stator insulation. Reference to a random wound motor is made throughout the paper
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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