1,195 research outputs found
The Effect of Common-Mode Voltage Elimination on the Iron Loss in Machine Core Laminations of Multilevel Drives
This paper studies the effect of common-mode voltage elimination (CMVE) on the iron loss of electrical machine core laminations under multilevel converter supply. Three identical magnetic ring cores are excited by either a three-level converter or a ïŹve-level voltage source converter to study the behavior of CMVE on a three-phase system. Both multilevel converters are controlled by using a space vector pulse width modulation as it is one of the most often used techniques for CMVE. These experimental results are conïŹrmed numerically with a dynamic iron loss model. In addition, the effect of CMVE, at different switching frequencies, on the core loss of a synchronous machine is numerically studied. The results presented in this paper show that the core loss is considerably increased when the CMVE is implemented. However this iron loss increase in ïŹve-level drive systems is lower compared to the three-level ones. Therefore, it is important that the designers of drive systems take such effects into consideration.Peer reviewe
Symmetries of Electrostatic Interaction between DNA Molecules
We study a model for pair interaction of DNA molecules generated by the
discrete dipole moments of base-pairs and the charges of phosphate groups, and
find noncommutative group of eighth order of symmetries that leave
invariant. We classify the minima using group and employ
numerical methods for finding them. The minima may correspond to several
cholesteric phases, as well as phases formed by cross-like conformations of
molecules at an angle close to , "snowflake phase". The results
depend on the effective charge of the phosphate group which can be modified
by the polycations or the ions of metals. The snowflake phase could exist for
above the threshold . Below there could be several cholesteric
phases. Close to the snowflake phase could change into the cholesteric
one at constant distance between adjacent molecules.Comment: 13 pages, 4 figure
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New mechanism for lightning initiation
This is the final report of a three-year, Laboratory-Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). To distinguish radio-frequency (rf) signals generated by lightning from the electromagnetic pulse produced by a nuclear explosion, it is necessary to understand the fundamental nature of thunderstorm discharges. The recent debate surrounding the origin of transionospheric pulse pairs (TIPPs) detected by the BLACKBEARD experiment aboard the ALEXIS satellite illustrates this point. We have argued that TIPP events could originate from the upward propagating discharges recently identified by optical images taken from the ground, from airplanes, and from the space shuttle. In addition, the Gamma Ray Observatory (GRO) measurements of x-ray bursts originating from thunderstorms are almost certainly associated with these upward propagating discharges. When taken together, these three measurements point directly to the runaway electron mechanism as the source of the upward discharges. The primary goal of this research effort was to identify the specific role played by the runaway-air-breakdown mechanism in the general area of thunderstorm electricity and in so doing develop lightning models that predict the optical, rf, and x-ray emissions that are observable from space
Video and Photometric Observations of a Sprite in Coincidence with a Meteor-triggered Jet Event
Video and photometric observations of a meteor-triggered âjetâ event in association with the occurrence of a sprite were collected during the SPRITES \u2798 campaign. The event raises interest in the question of possible meteoric triggering of upper atmospheric transients as originally suggested by Muller [1995]. The event consisted of three stages: (1) the observation of a moderately bright meteor, (2) the development of a sprite in the immediate vicinity of the meteor as the meteor reached no lower than âŒ70 km altitude, and (3) a slower-forming jet of luminosity that appeared during the late stages of the sprite and propagated back up the ionization trail of the meteor. The event is analyzed in terms of its geometry, its relevance to the meteor, and the implications to existing theories for sprite formation
Conception et rĂ©alisation des capteurs hybrides photovoltaĂŻque-thermiques sous vide ou avec lame dâair confinĂ©e
Cette Ă©tude fait lâanalyse des performances thermiques et Ă©lectriques de deux types de capteurs solaires hybrides photovoltaĂŻque-thermiques Ă air intĂ©grables en toitures des bĂątiments. Pour ces capteurs hybrides, les cellules PV sont isolĂ©es soit avec une lame dâair confinĂ©e ou soit avec un gap vide. La modĂ©lisation des transferts de chaleur dans les systĂšmes est effectuĂ©e en 2D et en rĂ©gime transitoire, suivant lâapproche nodale. Le code numĂ©rique dĂ©veloppĂ© a Ă©tĂ© validĂ© et a permis dâanalyser les comportements thermiques ainsi que les efficacitĂ©s thermique et Ă©lectrique des capteurs. Lâoptimisation des paramĂštres fonctionnels est ensuite effectuĂ©e et prĂ©sentĂ©e.Mots-clĂ©s: Ă©nergie solaire, cellules photovoltaĂŻques, capteurs solaires hybrides (PV/T), transferts thermiques. Conception and realization of hybrid photovoltaic thermal collectors with empty gap or with enclosed air cavityThe present work reports thermal and electrical efficiencies for two solar hybrid photovoltaic-thermal air collectors integrated into the roof of the buildings. In these hybrid collectors, the PV cells are insulated with the enclosed air film or with the empty gap cavity. The unsteady and two-dimensional heat transfer equations are proposed and these equations are discretized using nodal method. The numerical model developed is validated. Then thermal and electrical efficiencies are analyzed for the collectors. The optimization of the characteristics parameters is studied in detail.Keywords: solar energy, photovoltaic cells, hybrid solar collector, heat transfer, nodal method
A twist in chiral interaction between biological helices
Using an exact solution for the pair interaction potential, we show that
long, rigid, chiral molecules with helical surface charge patterns have a
preferential interaxial angle ~((RH)^1/2)/L, where L is the length of the
molecules, R is the closest distance between their axes, and H is the helical
pitch. Estimates based on this formula suggest a solution for the puzzle of
small interaxial angles in a-helix bundles and in cholesteric phases of DNA.Comment: 7 pages, 2 figures, PDF file onl
Medium Modifications of Hadron Properties and Partonic Processes
Chiral symmetry is one of the most fundamental symmetries in QCD. It is
closely connected to hadron properties in the nuclear medium via the reduction
of the quark condensate , manifesting the partial restoration of
chiral symmetry. To better understand this important issue, a number of
Jefferson Lab experiments over the past decade have focused on understanding
properties of mesons and nucleons in the nuclear medium, often benefiting from
the high polarization and luminosity of the CEBAF accelerator. In particular, a
novel, accurate, polarization transfer measurement technique revealed for the
first time a strong indication that the bound proton electromagnetic form
factors in 4He may be modified compared to those in the vacuum. Second, the
photoproduction of vector mesons on various nuclei has been measured via their
decay to e+e- to study possible in-medium effects on the properties of the rho
meson. In this experiment, no significant mass shift and some broadening
consistent with expected collisional broadening for the rho meson has been
observed, providing tight constraints on model calculations. Finally, processes
involving in-medium parton propagation have been studied. The medium
modifications of the quark fragmentation functions have been extracted with
much higher statistical accuracy than previously possible.Comment: to appear in J. Phys.: Conf. Proc. "New Insights into the Structure
of Matter: The First Decade of Science at Jefferson Lab", eds. D.
Higinbotham, W. Melnitchouk, A. Thomas; added reference
Relativistic electron beams above thunderclouds
Non-luminous relativistic electron beams above thunderclouds have been detected by the radio signals of low frequency &sim;40â400 kHz which they radiate. The electron beams occur &sim;2â9 ms after positive cloud-to-ground lightning discharges at heights between &sim;22â72 km above thunderclouds. Intense positive lightning discharges can also cause sprites which occur either above or prior to the electron beam. One electron beam was detected without any luminous sprite which suggests that electron beams may also occur independently of sprites. Numerical simulations show that beams of electrons partially discharge the lightning electric field above thunderclouds and thereby gain a mean energy of &sim;7 MeV to transport a total charge of &sim;â10 mC upwards. The impulsive current &sim;3 &times; 10<sup>&minus;3</sup> Am<sup>â2</sup> associated with relativistic electron beams above thunderclouds is directed downwards and needs to be considered as a novel element of the global atmospheric electric circuit
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