283 research outputs found

### Improved speed estimation in sensorless PM brushless AC drives

The application of flux-observer-based sensorless control to permanent-magnet brushless AC motor drives is described. Current methods of speed estimation are assessed, both theoretically and experimentally, and an improved method, which combines the best features of methods in which speed is derived from the differential of rotor position and from the ratio of the electromotive force to excitation flux linkage, is proposed. Its performance is verified experimentally

### Sensorless flux-weakening control of permanent-magnet brushless machines using third harmonic back EMF

The sensorless control of brushless machines by detecting the third harmonic back electromotive force is a relatively simple and potentially low-cost technique. However, its application has been reported only for brushless dc motors operating under normal commutation. In this paper, the utility of the method for the sensorless control of both brushless dc and ac motors, including operation in the flux-weakening mode, is demonstrated

### Impurity state in the vortex core of d-wave superconductors: Anderson impurity model versus unitary impurity model

Using an extended Anderson/Kondo impurity model to describe the magnetic
moments around an impurity doped in high-$T_{\text{c}}$ d-wave cuprates and in
the framework of the slave-boson meanfield approach, we study numerically the
impurity state in the vortex core by exact diagonalization of the
well-established Bogoliubov-de Gennes equations. The low-energy impurity state
is found to be good agreement with scanning tunnelingmicroscopy observation.
After pinning a vortex on the impurity site, we compare the unitary impurity
model with the extended Anderson impurity model by examining the effect of the
magnetic field on the impurity state. We find that the impurity resonance in
the unitary impurity model is strongly suppressed by the vortex; while it is
insensitive to the field in the extended Anderson impurity model.Comment: 8 pages, 3 figure

### Analysis of Stator/Rotor Pole Combinations in Variable Flux Reluctance Machines Using Magnetic Gearing Effect

The torque production of variable flux reluctance machines (VFRMs) is explained by the â€śmagnetic gearing effectâ€ť in recent research. Based on this theory, this paper concludes the general principles for feasible stator/rotor pole selection and corresponding winding configuration for VFRMs. The influence of stator/rotor pole combination on torque performance is comprehensively investigated not only in terms of average torque and torque ripple, but also in terms of each single torque component. It is found that the synchronous torque is proportional to the fundamental rotor radial permeance component and has the dominant contribution in average torque for all the VFRMs. The stator slot number and rotor pole number should be close to each other to achieve the highest output torque. Meanwhile, the 6-stator-slot/(6i Â± 2)-rotor-pole (6s/(6i Â± 2)r) and their multiples are large torque ripple origins for VFRMs due to the large reluctance torque ripple. Also, it is proved that a lower stator slot number is preferable choice to obtain higher torque/copper loss ratio, whereas a higher stator slot number is more suitable for large machine scale scenario. Finally, the analyses and conclusions are verified by finite element analysis on the 6-, 12-, 18-, and 24-stator-slot VFRMs and by experimental tests on a 6s/7r and 6s/8r VFRMs

### Thermodynamic properties of the d-density wave order in cuprates

We solve a popular effective Hamiltonian of competing $d$-density wave and
d-wave superconductivity orders self-consistently at the mean-field level for a
wide range of doping and temperature. The theory predicts a temperature
dependence of the $d$-density wave order parameter seemingly inconsistent with
the neutron scattering and $\mu$SR experiments of the cuprates. We further
calculate thermodynamic quantities, such as chemical potential, entropy and
specific heat. Their distinct features can be used to test the existence of the
$d$-density wave order in cuprates.Comment: changed to 4 pages and 4 figures. More reference added. Accepted by
Phys. Rev.

### Localized surface states in HTSC: Alternative mechanism of zero-bias conductance peaks

It is shown that the quasiparticle states localized in the vicinity of
surface imperfections of atomic size can be responsible for the zero-bias
tunneling conductance peaks in high-Tc superconductors. The contribution from
these states can be easily separated from other mechanisms using their
qualitatively different response on an external magnetic field.Comment: REVTeX, 4 pages, 2 figs; to be published in PR

### Power spectrum of many impurities in a d-wave superconductor

Recently the structure of the measured local density of states power spectrum
of a small area of the \BSCCO (BSCCO) surface has been interpreted in terms of
peaks at an "octet" of scattering wave vectors determined assuming weak,
noninterfering scattering centers. Using analytical arguments and numerical
solutions of the Bogoliubov-de Gennes equations, we discuss how the
interference between many impurities in a d-wave superconductor alters this
scenario. We propose that the peaks observed in the power spectrum are not the
features identified in the simpler analyses, but rather "background" structures
which disperse along with the octet vectors. We further consider how our
results constrain the form of the actual disorder potential found in this
material.Comment: 5 pages.2 figure

### Comparative Analysis of Variable Flux Reluctance Machines With Double- and Single-Layer Concentrated Armature Windings

In this paper, the variable flux reluctance machines (VFRMs) with double- and single-layer concentrated armature windings are comparatively analyzed. First, the single-layer winding is found to have an identical winding factor as a double-layer winding, but significantly larger peak value of magneto-motive force, which will result in severe local saturation in cores of VFRMs with single-layer winding. Then, based on the magnetic gearing effect and finite-element analysis, the electromagnetic performances of VFRMs with both winding types are compared. The VFRMs with single-layer winding are proved to be always lower in average torque, higher in torque ripple, larger in iron loss, and lower in efficiency than those with double-layer winding. Nevertheless, better fault-tolerance capability is achieved for a single-layer winding due to its physical separation between phases and larger phase self-inductance. Overall, the double-layer armature winding is the preferable choice for the VFRMs. Finally, a 6-stator-slot/4-rotor-pole VFRM with both double- and single-layer windings is prototyped for verification

### Critical temperature oscillations in magnetically coupled superconducting mesoscopic loops

We study the magnetic interaction between two superconducting concentric
mesoscopic Al loops, close to the superconducting/normal phase transition. The
phase boundary is measured resistively for the two-loop structure as well as
for a reference single loop. In both systems Little-Parks oscillations,
periodic in field are observed in the critical temperature Tc versus applied
magnetic field H. In the Fourier spectrum of the Tc(H) oscillations, a weak
'low frequency' response shows up, which can be attributed to the inner loop
supercurrent magnetic coupling to the flux of the outer loop. The amplitude of
this effect can be tuned by varying the applied transport current.Comment: 9 pages, 7 figures, accepted for publication in Phys. Rev.

### Rotor shaping method for torque ripple mitigation in variable flux reluctance machines

In this paper, four rotor shaping methods, i.e., eccentric circular, inverse cosine, inverse cosine with third harmonic, and multi-step shaping methods, are developed and compared for torque ripple mitigation in variable flux reluctance machines (VFRMs). By using a 6-stator-pole/7-rotor-pole (6/7) VFRM as an example, the design criterions and capabilities of these four methods are illustrated. It is found that all the rotor shaping methods are capable of torque ripple mitigation and applicable to all the VFRMs except those with 6 k /(6 i Â± 2) k ( k , i = 1, 2, 3â€¦) stator/rotor pole combinations. Moreover, the inverse cosine with third harmonic and multi-step shaping methods are found to have the best performance. They are able to reduce the torque ripple by 90% at a cost of only 3% torque density reduction. A 6/7 VFRM with both conventional and shaped rotors is prototyped and tested for verification

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