1,721,029 research outputs found
Band-Constrained Technique for Direct Torque Control of Indusction Motor
No full textIn the paper, a novel technique for the direct torque
control (DTC) of an induction motor is proposed, which overcomes
the trouble of high torque ripple afflicting the conventional DTC
technique. With the novel technique, the inverter voltage vector
selected from the switching table is applied for the time interval
needed by the torque to reach the upper (or the lower) limit of the
band, where the time interval is calculated from a suitable modeling
of the torque dynamics. By this approach, the control system
emulates the operation of a torque hysteresis controller of analog
type since the application time of the inverter voltage vector is dictated
by the allowed torque excursion and not by the sampling period.
It is shown by experimental results that the technique yields a
considerable reduction of the torque ripple. A further and ultimate
reduction is obtained by compensating for the delay inherent in the
discrete-time operation of the control system. The outcome is that
the torque ripple of the motor is constrained within the hysteresis
band of the torque controller, for a band of customary value. An
ancillary merit of the technique is the almost full elimination of the
average torque error inherent in the conventional technique. If the
hysteresis band is shrunk, the torque ripple is bound to swing out
the band limits. Under this circumstance, an extension of the technique
is developed, which helps keep the torque ripple at minimum.
To assess the characteristics of the proposed DTC technique, the
following quantities: average torque error, rms value of the torque
ripple, and inverter switching frequency are measured for different
stator flux angular speeds and hysteresis bands of the torque and
flux controllers. As a comparison, the same quantities are given for
the conventional DTC technique
An Assessment of the Inverter Switching Characteristics in DTC Induction Motor Driver
No full textThe switching characteristics of an inverter feeding
an induction motor controlled with the direct torque control (DTC)
technique are assessed in steady state. At first, the application share
of the inverter voltage vectors for the stator flux covering half a sextant
is defined and predicted. The prediction indicates that, under
operation at fixed inverter dc link voltage and stator flux magnitude,
the application share depends only on the supply frequency
of the motor and, to a small extent, on the load. Afterwards, the
inverter transitions and the corresponding phase commutations
within a stator flux sextant are analyzed. The outcome of the analysis
permits to compute the commutations of the inverter phases in
one turn of the stator flux and, from them, the inverter switching
frequency is obtained. Its value is influenced by the sampling interval
and the control delay arising from the microprocessor implementation
of DTC. For given sampling interval and control delay,
it is shown that the inverter switching frequency depends on the
same quantities as the application share of the inverter voltage vectors.
Acomparison with the switching characteristics of an inverter
controlled with the space vector modulation technique is carried
out. At last, the paper discusses the sensitivity of the switching frequency
of an inverter for DTC to the following quantities: inverter
dc link voltage, sampling interval and control delay. Throughout
the paper simulation and experimental results are given to confirm
the theoretical findings
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
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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