Classical Low Frequency Research Articles

The Mechanism of Ultra-Low Frequency Oscillations With the Same Mode Shapes

The ultra-low frequency oscillation (ULFO) problem reappears in China recently. Different from the classical low frequency oscillation (LFO), the generators in ULFO have the same mode shapes, namely oscillating in the same pace. To reveal the reason why ULFO generators do not oscillate inversely as classical LFO does, this paper investigates the mechanism of ULFO based on improved Philips-Heffron model of two-machine system. Through this model, the phase angle relationship between different generators is obtained. Based on the bode diagram, it is found that the phase angle difference of the two machines is around zero in ultra-low frequency band however the generator parameters change. On the contrary, the situation is different in classical low frequency band, which reveals the mechanism that generators in ULFO always oscillate in the same pace. Finally, the analysis above is validated through a 2-area 4-machine system and a 6-machine system established in PSASP software.

Components of arterial systolic pressure and RR-interval oscillation spectra in a case of baroreflex failure, a human open-loop model of vascular control

The baroreflex control of circulation is always operating and modulates blood pressure and heart rate oscillations. Thus, the study of cardiovascular variability in humans is performed in a closed-loop model and the physiology of post-sinoaortic denervation is completely unknown in humans. We dissected for the first time the different components of systolic arterial pressure (SAP) and RR-interval spectra in a patient with 'baroreflex failure' (due to mixed cranial nerve neuroma) who represents a human model to investigate the cardiovascular regulation in an open-loop condition. Interactions among cardiovascular variability signals and respiratory influences were described using the multivariate parametric ARXAR model with the following findings: (1) rhythms unrelated to respiration were detected only at frequencies lower than classical low frequency (LF; Slow-LF, around 0.02 Hz) both in SAP an RR spectra, (2) small high-frequency (HF) modulation is present and related with respiration at rest and in tilt (but for SAP only) and (3) the Slow-LF fluctuations detected both in SAP and RR oscillate independently as the multivariate model shows no relationships between SAP and RR, and these oscillations are not phase related. Thus, we showed that in a patient with impaired baroreflex arc integrity the Slow-LF rhythms for RR have a central origin that dictates fluctuations on RR at the same rhythm but unrelated to the oscillation of SAP (which may be related with both peripheral activity and central rhythms). The synchronization in LF band is a hallmark of integrity of baroreflex arc whose impairment unmasks lower frequency rhythms in SAP and RR whose fluctuations oscillate independently.

Surface analysis on rolling bearings after exposure to defined electric stress

This article gives an overview about classical and frequency converter-induced spurious bearing currents in induction machines and discusses typical damage patterns caused by the current passage. To investigate on the electric damage mechanisms, test bearings are operated in a test rig and exposed to specific (classical low-frequency, and high-frequency) bearing currents. The induced damages to the surfaces are analysed visually and with the help of an atomic force microscope, and compared for the different electric regimes applied. Further, the electrically damaged bearing surfaces are characterized by standard roughness parameters. The surface structure observable on certain test bearings shows good correlation to the structure found with a bearing that had failed in the field under similar electric conditions. One of the investigated electric regimes applying high-frequency currents proved capable of generating fluting patterns - as found in real applications - on the test rig. The experiments also indicate that high-frequency bearing currents, although in total dissipating less energy, are more dangerous to a bearing than continuous current flow. The presented method gives a good starting point for further investigation on electric current damage in bearings, especially regarding high-frequency bearing currents, and on bearing/grease lifetime under specific electric regimes.

Characteristics of the low-temperature-deposited SiO 2-Ga 0.47In 0.53As metal/insulator/semiconductor interface

A critical task in developing a viable Ga 0.47In 0.53As metal/insulator/semiconductor field effect transistor (MISFET) technology for microwave, multigigabit-rate logic and integrated optoelectronic applications is the control of the insulator-semiconductor interface. We have made extensive capacitance-voltage ( C-V) and conductance-voltage measurements as a function of frequency on MIS capacitors fabricated with SiO 2 deposited onto Ga 0.47In 0.53As at low temperature (170°C). The effect of annealing in hydrogen at 300°C was studied. MIS structures with minimum capacitance dispersion, flat-band voltages close to zero, interface state densities of (5–7)x10 10 cm -2 eV -1 at flat bands and an oxide fixed positive charge density of qx4x10 10 cm -2 were achieved. These measurements also showed that classical low frequency effects in C-V data occur at higher frequencies than in the Si-SiO 2 system. The bulk Ga 0.47In 0.53As trapping effects were also more evident. These effects are attributed to the fact that the intrinsic carrier concentration in GaInAs is two orders of magnitude higher than that in silicon. n-channel inversion mode MISFETs fabricated using low-temperature-deposited SiO 2 had field effect mobilities as high as 5200 cm 2 V -1 s -1 and a threshold voltage consistent with the acceptor doping level and the measured interfacial parameters. In early quasi-full-gate MISFETs the drift in the drain current with time, the formation of an anomalous inversion layer with negative gate bias and the hysteresis in the room temperature C-V curves indicated the presence of interfacial trapping. Recent results on self-aligned-gate ion-implanted n-channel inversion mode MISFITs show minimum (less than 2%) drain current drift with time and significant anomalous inversion layer formation.