12,901 research outputs found
A damping method for torsional vibrations in a DFIG wind turbine system based on small-signal analysis
Torsional vibrations present in the drivetrain excited by turbulent winds or grid disturbances (often manifesting as voltage sags) can produce severe stresses on the components of wind turbines. This study presents a damping method for the rotor side converter of a doubly fed induction generator (DFIG)-based wind turbine to improve system damping during small grid disturbances. Within the damping strategy, torsional damper is included into the power loop of the rotor side converter to provide damping for the system. During vibrations, a well-designed low-pass filter (LPF) is employed by the torsional damper to extract the dynamics of the generator speed and to generate a damping signal. To demonstrate the proposed damping approach, the small-signal stability model of a wind turbine connected to an infinite bus system is presented, and the eigenvalue analysis is conducted to verify the damping effect introduced by the torsional damper. In the case study, grid voltage sag at the point of common coupling (PCC) is applied to excite the torsional vibration of the wind turbine shaft. The validity and effectiveness of the proposed damping approach are testified by using the simulation results against theory analysis.</p
Pre--Main-Sequence stellar populations across Shapley Constellation III. I. Photometric Analysis and Identification
We present our investigation of pre--main-sequence (PMS) stellar populations
in the Large Magellanic Cloud (LMC) from imaging with Hubble Space Telescope
WFPC2 camera. Our targets of interest are four star-forming regions located at
the periphery of the super-giant shell LMC 4 (Shapley Constellation III). The
PMS stellar content of the regions is revealed through the differential Hess
diagrams and the observed color-magnitude diagrams (CMDs). Further statistical
analysis of stellar distributions along cross-sections of the faint part of the
CMDs allowed the quantitative assessment of the PMS stars census, and the
isolation of faint PMS stars as the true low-mass stellar members of the
regions. These distributions are found to be well represented by a double
Gaussian function, the first component of which represents the main-sequence
field stars and the second the native PMS stars of each region. Based on this
result, a cluster membership probability was assigned to each PMS star
according to its CMD position. The higher extinction in the region LH 88 did
not allow the unambiguous identification of its native stellar population. The
CMD distributions of the PMS stars with the highest membership probability in
the regions LH 60, LH 63 and LH 72 exhibit an extraordinary similarity among
the regions, suggesting that these stars share common characteristics, as well
as common recent star formation history. Considering that the regions are
located at different areas of the edge of LMC 4, this finding suggests that
star formation along the super-giant shell may have occurred almost
simultaneously.Comment: Accepted for publication in the Astrophysical Journal. 19 pages, 19
figures (three omitted due to size limitations, without affecting the
comprehension of the manuscript
Translational control analysis by translationally active RNA capture/microarray analysis (TrIP–Chip)
We have developed a new approach to systematically study post-transcriptional regulation in a small number of cells. Actively translating mRNAs are associated with polysomes and the newly synthesized peptide chains are closely associated with molecular chaperones such as hsp70s, which assist in the proper folding of nascent polypeptides into higher ordered structures. These chaperones provide an anchor with which to separate actively translating mRNAs associated with polysomes from free mRNAs. Affinity capture beads were developed to capture hsp70 chaperones associated with the polysome complexes. The isolated actively translating mRNAs were used for high-throughput expression profiling analysis. Feasibility was demonstrated using an in vitro translation system with known translationally regulated mRNA transcript thymidylate synthase (TS). We further developed the approach using HCT-116 colon cancer cells with both TS and p53 as positive controls. The steady-state levels of TS and p53 mRNAs were unaltered after 5-fluorouracil treatment as assessed by real-time qRT-PCR analysis. In contrast, the protein expression and polysome-associated mRNA levels of both genes were increased. These differences in translational rate were revealed with our new approach from 500 cells. This technology has the potential to make investigation of translational control feasible with limited quantities of clinical specimens
Probing of local ferroelectricity in BiFeO3 thin films and (BiFeO3)m(SrTiO3)m superlattices
Ferroelectric BiFeO3 thin films and artificial superlattices of
(BiFeO3)m(SrTiO3)m (m~ 1 to 10 unit cells) were fabricated on (001)-oriented
SrTiO3 substrates by pulsed laser ablation. The variation of leakage current
and macroscopic polarization with periodicity was studied. Piezo force
microscopy studies revealed the presence of large ferroelectric domains in the
case of BiFeO3 thin films while a size reduction in ferroelectric domains was
observed in the case of superlattice structures. The results show that the
modification of ferroelectric domains through superlattice, could provide an
additional control on engineering the domain wall mediated functional
properties.Comment: 14 pages, To be published in J. Mag. Mag Mater. proceedings of EMRS
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