1,720,961 research outputs found
The doubly-fed induction generator as part of the electrical machines curriculum
No full textThe doubly-fed induction machine has long been reputed as a curiosity with few practical applications. Consequently, very often there was not enough space to treat this machine in undergraduate electrical machines curricula in the past. But times changed and today this machine is often an integral part of wind turbines as electric generator. This way, getting familiar with the relevant operating principles becomes a desirable task for undergraduate students. This paper proposes a circuit-based theoretical approach which may serve as a template for an undergraduate lecture. To this end, firstly the basic characteristics of the wind turbine are introduced. Afterward, the operating principles of the doubly-fed induction machine are explained with reference to the single-phase equivalent circuit. A method based on the superposition principle is proposed for circuit solving, exploiting the concept of 'virtual' synchronous speed. An example given in the paper further illustrates the principle of operation and shed light on the power flows
Optimal Selection of Rotor Bar Number in Multiphase Cage Induction Motors
Full text linkRules for the selection of rotor bar numbers which minimize current and torque ripples are derived in this paper for a general symmetrical multiphase cage induction machine with prime phase number and integral slot winding. Analytically obtained expressions for optimal rotor bar number selection are validated by means of totally independent simulations, one based on a parameterized winding function (PWF) model of the induction machine and the other employing time-stepping finite element analysis (TSFEA). As a case study, five-phase four-pole cage induction motors with forty stator slots and different number of rotor bars are comparatively analyzed. Results obtained from the PWF model are in excellent accordance with those independently obtained by TSFEA and both confirm the correctness of the proposed selection criteria. The practical motivation of the study is that an incorrect selection of rotor bar number can lead to parasitic torques of significant amplitude and, presently, there are no general rules available in the literature which may guide designers towards an optimal design choice for a general number of phases
Optimal Selection of Rotor Bar Number for Minimizing Torque and Current Pulsations due to Rotor Slot Harmonics in Three-Phase Cage Induction Motors
Full text linkThe paper develops a method to choose the number of rotor bars in order to eliminate rotor slot harmonics in stator current spectrum and pulsation torques that are their consequence. Mains-fed, three-phase cage induction motors with the most common number of pole pairs and number of stator slots, that result in integer slot winding, are analyzed. The analysis is based on the recently derived general rule for optimal selection of rotor bars, valid for symmetrical multiphase machine with prime number of phases and integer slot stator winding. As a tool for validation of analytically predicted results, parameterized winding function (PWF) model is used. Electromagnetic torque ripple factor is used as a measure of goodness of the number of rotor bar selection. The practical motivation of the study is an attempt to supersede the many existing rules for rotor bar number selection that, depending on the source, may be different, and provide a unified general approach to the problem. One of the main findings derived in the paper is ascertainment that increasing the number of pole pairs increases the degree of freedom in choosing the proper number of rotor bars. The same applies when the number of motor phases increases
Stator Current Spectral Content of Inverter-Fed Cage Rotor Induction Motor
Full text linkThe paper analyzes the influence of the number of rotor bars on the stator current spectral content in a three-phase cage induction motor fed by a pulse-width modulated (PWM) inverter. It is shown that each of the higher-order time harmonics in the supply voltage produces space harmonics in a rotating magnetic flux density wave, which results in induced rotor slot harmonics (RSHs) in the stator current spectrum. The conditions for the existence of these space harmonics are identical to those applying to a mains-fed motor. In other words, the number of rotor bars of a mains-fed motor yielding an RSH-free stator current spectrum produces the same stator current spectrum even in case the motor is inverter-fed. Additionally, to minimize the adverse effects of RSHs in the stator current spectrum, one must consider not only the number of rotor bars, but also its relationship with the frequency modulation ratio of the PWM inverter. Analytical predictions are presented to illustrate these results supported both by numerical simulations of the induction motor modelled through the winding function theory and experimentally taking the case a two-pole cage induction machine as a case study
Optimal rotor bars number in four pole cage induction motor with 36 stator slots - Part II: Results
No full textThe identification of the optimal number of rotor bars in grid-fed, four-poles, three-phase cage induction motor is addressed in this two-part work for the frequent case of 36 stator slots. The optimization criterion being adopted is the minimization of the electromagnetic torque pulsations in fullload steady-state conditions. In Part I of the work a parameterized winding function (PWF) model was successfully validated through commercially available finite element software. In this Part II of the work, electromagnetic torque is investigated using the validated PWF model as it allows for investigating a wide range of design alternatives in a very short time, taking into account the skewing of rotor bars. Electromagnetic pulsations for different continuously changed number of rotor bars in a rather wide range are numerically evaluated using the torque ripple factor as a performance index. The evaluation is applied to both possible practical cases of unskewed rotor bars and rotor bars that are skewed by one stator slot pitch
Sequence circuit-based modeling of a doubly fed induction wind generator for eccentricity diagnosis by split-phase current signature analysis
No full textA wound-rotor three-phase induction generator with eccentric rotor and parallel connections in both stator and rotor can be represented by five equivalent sequence-circuits of virtually centered-rotor machines, as mathematically proved in previous papers. The five circuits allow the calculation of stator and rotor fault-related currents, in form of 2(p±1)-pole space vectors for 2p-pole machines. The formal calculation of fault-related currents is very useful to define fault indicators and tools for diagnosis, and in case of parallel-connected stators (like in many wind generators) the split-phase current signature analysis becomes viable. In fact, by reversing the equations linking split-phase current amplitudes and eccentricity degrees, original fault indicators are carried out in this paper. The new indicators are validated by simulations of a 1800kW wind induction generator with static, dynamic, and mixed faults in full-load conditions. The simulations are carried out by using a full FEA model of the motor, implemented by time-stepping FEM-circuit co-simulation software (Maxwell-Simplorer). A good match is obtained between theoretical calculations and simulations
Optimal Rotor Bars Number in Four Pole Cage Induction Motor with 36 Stator Slots - Part I: Numerical Modeling
No full textThe identification of the optimal number of rotor bars in grid-fed, four-poles, three-phase cage induction motor is addressed in this two-part work for the frequent case of 36 stator slots. The optimization criterion being adopted is the minimization of the electromagnetic torque pulsations in full-load steady-state conditions. In Part I of the work results from the parameterized winding function (PWF) model are presented and successfully validated against time-stepping finite element simulations. In Part II the electromagnetic torque pulsations obtained from the PWF model are evaluated for different numbers of rotor bars. During this process motor rated power, stator winding design and main machine geometry are maintained invariant. The evaluation is applied to both possible practical cases of unskewed rotor bars and rotor bars that are skewed by one stator slot pitch
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
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