Der Anlauf von Drehstrom-Asynchronmotoren als Schaltvorgang

Übersicht Die Energieverhältnisse bei Schaltvorgängen sind allgemein zu übersehen, der Verlauf der Einzelgrößen aber ist von der Schaltung abhängig. Für den Anlauf des Drehstrom-Asynchronmotors wird der zeitliche Verlauf der Ströme in Ständer und Läufer, des Luftspaltfeldes, des Drehmomentes und der Drehzahl unter Berücksichtigung sowohl der mechanischen als auch der elektrischen Ausgleichsvorgänge untersucht. Zunächst werden die elektrischen Ausgleichsvorgänge vernachlässigt, dann diese allein bei gleichbleibender Drehzahl behandelt und schließlich der vollständige Anlaufvorgang an einem Beispiel numerisch ausgewertet. Das Auftreten eines zunächst bremsenden Drehmomentes beim Einschalten bei höheren Drehzahlen und das Überschwingen über die synchrone Drehzahl beim Kurzanlauf werden erklärt.Mit 27 Textabbildungen.     

模拟引潮力作用下岩石破坏前兆的实验研究 --加卸载响应比(LURR)理论和能量加速释放(AER)

大地震前加卸载响应比升高和能量加速释放这2种现象可以用来对地震进行中期预报。同时,加卸载响应比升高和能量加速释放有相同的物理机制。实验对于揭示地壳岩石的变形和破坏规律是非常重要的。在三轴应力条件下进行了岩石破坏声发射实验,声发射技术是研究岩石变形破坏微观机理的重要手段。为了模拟日月引潮力对地球的加载和卸载作用,在一常数加载率的轴向压力作用下,叠加上微小的正弦扰动,力求能够模拟地下岩石复杂的受力状态。实验过程中记录到大量的声发射信息,声发射记录包括声发射发生的时间、空间坐标和振幅,它能够反映岩石试件内部每一个损伤(微裂纹)发生的时间、地点和强度。利用声发射记录系统地分析了岩石试件破坏前能量释放及加卸载响应比的演化情况,结果显示,岩石试件宏观破坏前出现了能量加速释放及加卸载响应比剧增这2种前兆现象,从而对地震临界点理论给予了实验支持,同时也为地震预测提供了实验依据。     

Selective lasing of InGaAs quantum-wire array on a V-grooved GaAssubstrate by distributed optical feedback

Distributed-feedback (DFB) lasers were fabricated by using strained InGaAs quantum-wire (QWR) arrays on V-grooved GaAs substrates as an active grating. After characterizing the luminescence from the QWRs and parasitic quantum wells (QWLs), a DFB laser cavity incorporating such a QWR array with its emission wavelength matched to the Bragg wavelength was designed and fabricated. The wavelength selectivity of the DFB cavity was found to strongly support the QWR emission, and DFB lasing from QWR gain up to 145 K has been achieved under pulsed current. The emission from the parasitic QWLs was suppressed by the DFB filtering and the loss induced by coupling to radiation modes. The DFB cavity was shown to be essential for obtaining lasing from QWRs on V-grooved substrates     

Determination of double-K criterion for crack propagation in quasi-brittle fracture, Part II: Analytical evaluating and practical measuring methods for three-point bending notched beams

In this paper, an attempt is made to determine the double-K fracture parameters K _Ic ⁱⁿⁱ and K _Ic ^un using three-point bending notched beams. First, based on the knowledge from extensive investigations which showed that the nonlinearity of P-CMOD curve is mainly associated with crack propagation, a linear asymptotic superposition assumption is proposed. Then, the critical effective crack length a is analytically evaluated by inserting the secant compliance c into the formula of LEFM. Furthermore, an analytical result of a fictitious crack with cohesive force in an infinite strip model was obtained. The double-K fracture parameters K _Ic ⁱⁿⁱ and K _Ic ^un as well the critical crack tip opening displacement CTOD_c were analytically determined. The experimental evidence showed that the double-K fracture parameters K _Ic ⁱⁿⁱ and K _Ic ^un are size-independent and can be considered as the fracture parameters to describe cracking initiation and unstable fracture in concrete structures. The testing method required to determine K _Ic ⁱⁿⁱ and K _Ic ^un is quite simple, without unloading and reloading procedures. So, for performing this test, a closed-loop testing system is not necessary.     

Size effect on the concrete cone pull-out load

In the present paper the failure mechanism and size effect of the concrete cone resistance is reviewed and studied. The influence of material and geometrical parameters on the failure mode and size effect is investigated. In the numerical studies the smeared crack finite element analysis, based on the microplane material model for concrete, was used. Both, experimental and numerical results show that there is a strong size effect on the nominal concrete cone pull-out strength. It is demonstrated that besides the embedment depth the scaling of the head of the stud as well as the scaling of the concrete member influence the nominal strength. This revised version was published online in July 2006 with corrections to the Cover Date.     

Vegetable oils for biofuels versus surfactants: an ecological comparison for energy and greenhouse gases

This paper compares the use of vegetable oils as energy carriers on the one hand and for the production of chemicals on the other hand. The bio-based energy carriers analyzed are rapeseed oil, rapeseed oil methyl ester, and palm oil methyl ester, all of which can be used as substitutes for diesel fuel. The chemicals studied are surfactants. Surfactants can be produced from plant-based feedstocks (oleochemical surfactants) and from petrochemicals. The various options are studied with regard to two ecological indicators, i.e. the consumption of finite energy resources and the global warming potential. First of all, for these two aspects plant-based sources show clear advantages when compared with their fossil counterparts. Secondly, the transesterified types of biofuels are more advantageous than pure vegetable oils. Thirdly, the conservation of finite energy and reduction in emissions of greenhouse gases are higher if vegetable oils are used as a feedstock to produce oleochemical surfactants compared to their use as biofuels. Comprehensive life-cycle analyses must be carried out in order to determine whether these results can also be applied to other ecological indicators. The results presented in this paper support developing strategic goals for bio-based feedstocks, including quantity and cost targets.     

Parturition and hypocalcemia blunts calcium signals in immune cells of dairy cattle

The stress of parturition in the dairy cow is associated with increased susceptibility to infectious disease. During the periparturient period the demands for calcium are increased; these increased demands for calcium can result in subclinical or clinical hypocalcemia. Periparturient cows also experience significant immune suppression. Because intracellular calcium signaling is a key early feature in immune cell activation, we have hypothesized that the increased demand for calcium in periparturient cows may adversely affect intracellular calcium stores of immune cells. This reduction in intracellular calcium stores in immune cells could blunt intracellular calcium release following an activating stimulus, contributing to the immune suppression seen in these animals. To test this hypothesis, peripheral mononuclear cells were obtained from 27 multiparous dairy cows spanning a period of 2 wk before and 2 wk after parturition. Following activation of these cells by anti-CD3 antibodies plus secondary antibodies, intracellular calcium release from intracellular stores was measured. The intracellular calcium released in response to the activation signal declined as calcium demand for lactation became more intense and recovered as plasma calcium normalized. Intracellular calcium stores in peripheral mononuclear cells, estimated by pretreating cells with pervanadate and ionomycin, significantly decreased at parturition and returned to normal levels as the cows’ blood calcium returned to normal levels. Hypocalcemia, which is common in periparturient dairy cows, is associated with decreased intracellular calcium stores in peripheral mononuclear cells. Our data suggest that this is the cause of a blunted intracellular calcium release response to an immune cell activation signal. It is concluded that intracellular Ca stores decrease in peripheral blood mononuclear cells (PBMC) before parturition and development of hypocalcemia. This suggests that systemic calcium stress precedes measurable hypocalcemia, particularly in cows that will develop milk fever. Therefore, PBMC intracellular Ca stores are a more sensitive measure of calcium stresses in transition cow. This decrease in PBMC intracellular Ca stores before parturition and the development of hypocalcemia contributes to periparturient immune suppression.