1,942 research outputs found
Iron loss in permanent-magnet brushless AC machines under maximum torque per ampere and flux weakening control
The airgap flux density distribution, flux density loci in the stator core, and the associated iron loss in two topologies of brushless AC motor, having a surface-mounted magnet rotor and an interior-mounted magnet rotor, respectively, are investigated when operated under maximum torque per ampere control in the constant torque mode and maximum power control in the flux-weakening mode. It is shown that whilst the interior magnet topology is known to be eminently suitable for flux-weakening operation, due to its high demagnetization withstand capability, its iron loss can be significantly higher than for a surface-mounted magnet machine
Calculation of d- and q-axis inductances of PM brushless ac machines accounting for Skew
A hybrid two-dimensional (2-D) finite-element/analytical technique is described for predicting the d-axis and q-axis inductances of permanent magnet (PM) brushless ac machines, with due account for the influence of skew. Predicted inductances are compared with measured values for two machines having identical stators, which are skewed by one slot-pitch, but which have different rotor topologies, one having surface-mounted magnets and the other having interior magnets
Online optimal flux-weakening control of permanent-magnet brushless AC drives
An enhanced online optimal control strategy, which maximizes the flux-weakening performance of a brushless AC motor, is described, and applied to motors having different rotor topologies: interior (radial or circumferential), inset, and surface-mounted magnet. It enables the maximum inherent power capability of a brushless AC motor to be achieved independent of any variation in its parameters, and facilitates maximum efficiency over the entire speed range. It also results in good transient dynamic performance, since it is coupled with feedforward vector control based on optimal current profiles
First Order Sea Clutter Cross Section for HF Hybrid Sky-Surface Wave Radar
This paper presents a modified method to simulate the first order sea clutter cross section for high frequency (HF) hybrid sky-surface wave radar, based on the existent model applied in the bistatic HF surface wave radar. The modification focuses on the derivation of Bragg scattering frequency and the ionosphere dispersive impact on the clutter resolution cell. Meanwhile, an analytic expression to calculate the dispersive transfer function is derived on condition that the ionosphere is spherical stratified. Simulation results explicate the variance of the cross section after taking account of the influence triggered by the actual clutter resolution cell, and the spectral width of the first order sea clutter is defined so as to compare the difference. Eventually, experiment results are present to verify the rationality and validity of the proposed method
Polarization independent InGaAs/lnP chopped quantum well interferometric space switch at 1.55 mum
A Mach-Zehnder Interferometric (MZI) space switch using a novel CRE grown InGaAs/InP chopped quantum well (CQW) phase section is presented. Each CQW consists of three 3IA. InGaAs strained quantum wells separated by 12A InP barriers. This structure shows a shift of the absorption edge as high as 80nm at lOV reverse bias. The heavy hole and light hole subbands cross at approximately 0.6% tensile strain. Using these chopped quantum wells, we realized MZ/'s with low attenuation and a V2.l product as low as 3.6 V2.cm. Finally, we realized full polarisation independent switching using 0.75% tensile strained CQW's
Perturbative Formulation and Non-adiabatic Corrections in Adiabatic Quantum Computing Schemes
Adiabatic limit is the presumption of the adiabatic geometric quantum
computation and of the adiabatic quantum algorithm. But in reality, the
variation speed of the Hamiltonian is finite. Here we develop a general
formulation of adiabatic quantum computing, which accurately describes the
evolution of the quantum state in a perturbative way, in which the adiabatic
limit is the zeroth-order approximation. As an application of this formulation,
non-adiabatic correction or error is estimated for several physical
implementations of the adiabatic geometric gates. A quantum computing process
consisting of many adiabatic gate operations is considered, for which the total
non-adiabatic error is found to be about the sum of those of all the gates.
This is a useful constraint on the computational power. The formalism is also
briefly applied to the adiabatic quantum algorithm.Comment: 5 pages, revtex. some references adde
Statistical significance of fine structure in the frequency spectrum of Aharonov-Bohm conductance oscillations
We discuss a statistical analysis of Aharonov-Bohm conductance oscillations
measured in a two-dimensional ring, in the presence of Rashba spin-orbit
interaction. Measurements performed at different values of gate voltage are
used to calculate the ensemble-averaged modulus of the Fourier spectrum and, at
each frequency, the standard deviation associated to the average. This allows
us to prove the statistical significance of a splitting that we observe in the
h/e peak of the averaged spectrum. Our work illustrates in detail the role of
sample specific effects on the frequency spectrum of Aharonov-Bohm conductance
oscillations and it demonstrates how fine structures of a different physical
origin can be discriminated from sample specific features.Comment: accepted for publication in PR
Spin-dependent (magneto)transport through a ring due to spin-orbit interaction
Electron transport through a one-dimensional ring connected with two external
leads, in the presence of spin-orbit interaction (SOI) of strength \alpha and a
perpendicular magnetic field is studied. Applying Griffith's boundary
conditions we derive analytic expressions for the reflection and transmission
coefficients of the corresponding one-electron scattering problem. We
generalize earlier conductance results by Nitta et al. [Appl. Phys. Lett. 75,
695 (1999)] and investigate the influence of \alpha, temperature, and a weak
magnetic field on the conductance. Varying \alpha and temperature changes the
position of the minima and maxima of the magnetic-field dependent conductance,
and it may even convert a maximum into a minimum and vice versa.Comment: 19 pages, 9 figure
Disappearance of Ensemble-Averaged Josephson Current in Dirty SNS Junctions of d-wave Superconductors
We discuss the Josephson current in superconductor / dirty normal conductor /
superconductor junctions, where the superconductors have pairing
symmetry. The low-temperature behavior of the Josephson current depends on the
orientation angle between the crystalline axis and the normal of the junction
interface. We show that the ensemble-averaged Josephson current vanishes when
the orientation angle is and the normal conductor is in the diffusive
transport regime. The -wave pairing symmetry is responsible for
this fact.Comment: 8 pages, 5 figure
Optimal control theory for unitary transformations
The dynamics of a quantum system driven by an external field is well
described by a unitary transformation generated by a time dependent
Hamiltonian. The inverse problem of finding the field that generates a specific
unitary transformation is the subject of study. The unitary transformation
which can represent an algorithm in a quantum computation is imposed on a
subset of quantum states embedded in a larger Hilbert space. Optimal control
theory (OCT) is used to solve the inversion problem irrespective of the initial
input state. A unified formalism, based on the Krotov method is developed
leading to a new scheme. The schemes are compared for the inversion of a
two-qubit Fourier transform using as registers the vibrational levels of the
electronic state of Na. Raman-like transitions through the
electronic state induce the transitions. Light fields are found
that are able to implement the Fourier transform within a picosecond time
scale. Such fields can be obtained by pulse-shaping techniques of a femtosecond
pulse. Out of the schemes studied the square modulus scheme converges fastest.
A study of the implementation of the qubit Fourier transform in the Na
molecule was carried out for up to 5 qubits. The classical computation effort
required to obtain the algorithm with a given fidelity is estimated to scale
exponentially with the number of levels. The observed moderate scaling of the
pulse intensity with the number of qubits in the transformation is
rationalized.Comment: 32 pages, 6 figure
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