8,721 research outputs found
Control of Five Phase Doubly Fed Induction Machines for Motion and Contactless Power Transmission
In questa tesi si mostra come un motore a induzione di tipo doubly fed (DFIM) possa assolvere contemporaneamente le funzioni di motore e di trasmissione di potenza
In particolare questa peculiarità è particolarmente utile laddove sia necessario alimentare dei dispositivi elettrici a bordo di una macchina automatica a giostra, normalmente alimentati tramite contatti striscianti: da questâultimi infatti spesso derivano innumerevoli problematiche di usura e sicurezza.
In particolare, in questa tesi verrà trattato un azionamento pentafase: tale scelta è stata dettata dalla possibilità di disaccoppiare la trasmissione di potenza elettrica e quella meccanica. Data la particolarità di tale macchina una lunga sezione è dedicata alla sua descrizione.
Si presentano tre diverse metodologie di funzionamento e controllo della macchina doubly fed: in particolare le prime due, che si avvalgono di modulazioni di coppia o di potenza, potrebbero essere implementate anche su un motore trifase qualora risultasse conveniente; al contrario lâultimo controllo proposto, che si fonda sullâutilizzo indipendente delle frequenze di statore, è applicabile solo a un DFIM pentafase.
Dai risultati ottenuti in simulazione, ma comprendenti moltissimi gradi di realismo, appare evidente come questâultimo dia i risultati migliori e sfrutti al meglio la macchina pentafase
A quantum heat engine with coupled superconducting resonators
We propose a quantum heat engine composed of two superconducting transmission
line resonators interacting with each other via an optomechanical-like
coupling. One resonator is periodically excited by a thermal pump. The
incoherently driven resonator induces coherent oscillations in the other one
due to the coupling. A limit cycle, indicating finite power output, emerges in
the thermodynamical phase space. The system implements an all-electrical analog
of a photonic piston. Instead of mechanical motion, the power output is
obtained as a coherent electrical charging in our case. We explore the
differences between the quantum and classical descriptions of our system by
solving the quantum master equation and classical Langevin equations.
Specifically, we calculate the mean number of excitations, second-order
coherence, as well as the entropy, temperature, power and mean energy to reveal
the signatures of quantum behavior in the statistical and thermodynamic
properties of the system. We find evidence of a quantum enhancement in the
power output of the engine at low temperatures.Comment: 15 pages, 14 figures, new references adde
Discreteness-induced resonances and AC voltage amplitudes in long one-dimensional Josephson junction arrays
New resonance steps are found in the experimental current-voltage
characteristics of long, discrete, one-dimensional Josephson junction arrays
with open boundaries and in an external magnetic field. The junctions are
underdamped, connected in parallel, and DC biased. Numerical simulations based
on the discrete sine-Gordon model are carried out, and show that the solutions
on the steps are periodic trains of fluxons, phase-locked by a finite amplitude
radiation. Power spectra of the voltages consist of a small number of harmonic
peaks, which may be exploited for possible oscillator applications. The steps
form a family that can be numbered by the harmonic content of the radiation,
the first member corresponding to the Eck step. Discreteness of the arrays is
shown to be essential for appearance of the higher order steps. We use a
multi-mode extension of the harmonic balance analysis, and estimate the
resonance frequencies, the AC voltage amplitudes, and the theoretical limit on
the output power on the first two steps.Comment: REVTeX, 17 pages, 7 figures, psfig; to appear in J. Applied Physic
Theoretical and experimental study of radiation pressure-induced mechanical oscillations (parametric instability) in optical microcavities
Radiation pressure can couple the mechanical modes of an optical cavity structure to its optical modes, leading to parametric oscillation instability. This regime is characterized by regenerative oscillation of the mechanical cavity eigenmodes. Here, we present the first observation of this effect with a detailed theoretical and experimental analysis of these oscillations in ultra-high-Q microtoroids. Embodied within a microscale, chip-based device, this mechanism can benefit both research into macroscale quantum mechanical phenomena and improve the understanding of the mechanism within the context of laser interferometer gravitational-wave observatory (LIGO). It also suggests that new technologies are possible that will leverage the phenomenon within photonics
Coherent exciton transport in semiconductors
We review the topic of Bose-Einstein condensation of excitons in
semiconductors, focusing on the signatures of the macroscopic order of the
exciton condensate.Comment: Some references were updated with respect to the published version.
appears as Chapter 19 in Novel Superfluids Volume 2, edited by K. H.
Bennemann and J. B. Ketterson, International Series of Monographs on Physics
no. 157, pages 423-474 (Oxford University Press, Oxford, 2014
Undamped nonequilibrium dynamics of a nondegenerate Bose gas in a 3D isotropic trap
We investigate anomalous damping of the monopole mode of a non-degenerate 3D
Bose gas under isotropic harmonic confinement as recently reported by the JILA
TOP trap experiment [D. S. Lob- ser, A. E. S. Barentine, E. A. Cornell, and H.
J. Lewandowski (in preparation)]. Given a realistic confining potential, we
develop a model for studying collective modes that includes the effects of
anharmonic corrections to a harmonic potential. By studying the influence of
these trap anharmonicities throughout a range of temperatures and collisional
regimes, we find that the damping is caused by the joint mechanisms of
dephasing and collisional relaxation. Furthermore, the model is complimented by
Monte Carlo simulations which are in fair agreement with data from the JILA
experiment.Comment: 11 pages, 6 figure
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