FAST COMPUTATION OF SYNCHRONOUS MACHINE UNBALANCED TRANSIENTS: APPLICATION TO SHORT CIRCUITS

ABSTRACT

In this paper we propose a model of the synchronous machine for fast digital computation of its transient behaviour. This modelling avoids numerical inversion of the inductance matrix even when some stator phases are open-circuited and allows then a large saving in the computation time. The obtained models are tested in the simulation of some sudden short circuits of an alternator. Simulation and experimental results are compared as well as the computation time using this modelling and the usual one.     

THERMOXIDATIVE AND HYDROLYTIC CHANGES IN OILS USED FOR FRYING OF FROZEN PREFRIED FOODS

Palm olein with 22.9 h of stability at 100 C (by the Rancimat method) and low acid and peroxide values of 0.184% and 1.33 meq/kg, respectively, was used to study the deteriorative effect of frying of frozen prefried potatoes and chicken and compared it with the effect of frying the same fresh foods. Quality characteristics of palm olein were monitored by following the changes in physical and chemical properties; foam height (FH), smoke point (SP), refractive index (RI), viscosity (Vis), acid value (AV), peroxide value (PV), polymer content (%Pol.), oxidized fatty acids (%OFA), thiobarbituric acid (TBA) value and, conjugated acids (absorbance at 232 nm for conjugated dienes and at 268 nm for conjugated trienes) with the progress in frying time of up to 20 h. Results revealed that frying the frozen prefried foods express more deteriorative effect than the fresh foods throughout the frying process.     

Analysis of Field-Assisted Binary Ion Exchange

A differential equation was derived for ionic migration in a system with two mobile ions. Stationary solutions were obtained for the case where the slower ion follows the faster, which reduce to those obtained when diffusion is ignored. When the faster ion is "behind" the slower no stationary solutions are possible, but the diffusion term is not important for this case and appropriate solutions are readily obtained. However, by taking account of diffusion at the boundaries, where it is important, infinite gradients are avoided and concentration equilibrium is permitted at the glass-salt interface. The concentration distributions predicted from this approach are compared with those measured on glass that has been ion exchanged with electric field assistance. Although the agreement is reasonable, there is some discrepancy, due in part to the assumption of concentration-independent species mobility.     

A Mixed-Mode Crack-Growth and Fracture Model for Field-Assisted Ion-Exchanged Glass

It is suggested that shear forces from the compression layer during field-assisted ion exchange of glass cause stable growth of preexisting flaws when the advancing ion-exchanged layer approaches the flaw tip. A quantitative model is presented which explains most of the previously puzzling experimental results, even though it ignores the change in crack direction required if the maximum tensile stress criterion is appropriate for fracture.     

Optimisation of probiotic yoghurt production containing Zedo gum

A Box‐Behnken design was applied to optimise the viability of Lactobacillus acidophilus and Bifidobacterium bifidum in probiotic yoghurt containing a novel exudative Zedo gum. The effect of incubation temperature, probiotic inoculation rate, storage time and Zedo gum concentration on quality indices of the yoghurt were explored. With respect to probiotics viability, probiotic inoculation rate was the most important factor followed by the storage time. Zedo gum did not show any significant effect on probiotics viability. The optimum conditions of probiotic yoghurt production were as follows: probiotic inoculation level, 12.8 g/100 kg of milk; incubation temperature, 41.6 °C; and Zedo gum concentration, 0.13%.     

SPEED AND CURRENT CONTROL OF DC MOTOR IN CONTINUOUS CURRENT OPERATION

ABSTRACT

In this paper we present a non-linear rigurous model of a d.c. motor fed from single phase thyristor bridge. The current and speed control are realized by ; internal current loop with proportional controller and external speed loop with proportional-integral one without zero. This model has been linearised around a steady state operating point to study the behaviour of the system for small perturbations. Analytical formulas are given for the controller parameters for any desired response. A general method of simulation is presented to study the behaviour of the system, taking into account all the discontinuities which may be occur in such a system.     

A SIMULATION STUDY OF AN OPTIMAL NUMERICAL CONTROL OF A THREE-PHASE CYCLOCONVERTER

Abstract

The paper presents a study of the problem of quantization which is linked with the numerical control voltage of a three-phase cycloconverter controlled by a microprocessor. The study makes a compromise between either using a large number of quantizing steps of the desired control voltage, which requires an Increase in microprocessor memory size, or using small number of steps, which leads to a high degree of harmonic presence in load current. In order to carry out such a study, a complete digital simulation of the whole system (source, cycloconverter, load and control devices) is used. Two types of numerical control-voltage are compared subject to the harmonic spectrum of the load current. Calculation is given for the optimum number of steps and the type of the numerical control-voltage     

EQUIVALENT LOAD MODEL OF INDUCTION MACHINES

ABSTRACT

A load model, based on combining two or more induction machines connected to a common bus or substation, either directly or through feeders-, is presented. The model is developed by combining the transient equations in the 0-d-q axes of N-induction machines into that of a single equivalent machine. The parameters of the equivalent machine are then derived from the combined equations. An equivalent mechanical load is also found by combining the loads and the inertias of the various machines. The equivalent is validated by simulating its response to severe voltage and frequency disturbances and comparing the results with the sum of the responses of the individual machines to the same distucbances. For a diversified group of induction machines, this paper illustrates grouping them into two or three equivalent machines would be adequate for modeling the net dynamic response of all the machines     

γ射线通过两类火成岩石材的衰减和散射

由于发射γ射线的同位素在工业、农业和医学上许多新的应用,研究γ射线与物质相互作用变得日趋重要。定量研究了两类火成岩石材料：安山岩和霏细岩对γ射线的屏蔽效果,分别对天然的（整块割制)和（碎粒经）人工压制的石材样品,采用能量在0．36－1．33MeV范围的γ射线,测量了样品的岩粒粗细（石英颗粒直径）（,石材厚度d(mm)和ρ诸因素对石材料吸收γ射线的线性衰减系数μ,质量衰减系数μ/ρ,以及边缘散射系数φs3个主要参数的影响。确定了在石材样品中射线的通量分布和半吸收层厚度X1／2。结果发现：石材的线性衰减系数μ对厚度d和γ射线能量E都有反比关系,而μ与样品密度ρ有正比关系;另一方面,石材样品的边缘散射系数φs却正比于d和E,φs反比于ρ.     

Impact Fracture of Chemically Tempered Glass Helicopter Windshields

Helicopter windshield glass with different degrees of chemical tempering was impact tested by firing spherical projectiles at velocities between 50 and 250 m/s. The residual tempering stresses were measured photoelastically and their values superimposed on the Hertzian impact stresses. Modified forms ol Auerbach's law¹ were utilized to study the experimental results. Lower impact fracture strength and deviations from the modified relations were attributed to changes in surface Flaw distributions.