Some commentary on the choice of rotor bar material for inductionmotors

Almost any metal can be used in the design of the squirrel cage of an induction machine. Obviously lead is not used because of its low melting point and magnesium because of its low flash point. Others are rejected because of the cost, lack of strength, resistance, thermal conductivity, or manufacturability including ease of joining. Basically, the choice fails to two materials, aluminum and copper and their alloys. For lowest cost of manufacture, especially for mass production, aluminum surpasses copper. In order to determine which of the two materials is the best takes the combined skills of an electrical, a mechanical and a metallurgical engineer. This paper discusses some of the considerations in the choice of rotor bar material     

Method of reducing the effect of supply unbalance on inductionmotors

The operation of induction motors under unbalanced supply conditions is well documented. Because of the resulting unbalanced currents, there is an increase in losses and consequent overheating of the windings. IEC Standard 34-1-1996 and NEMA Standard MG-1-1993 stipulate that a motor should be able to withstand continuous operation with 1% negative sequence voltage without derating. Where motors are continuously subjected to unbalance beyond this, the effects become significant and derating the motor becomes essential. Where an unbalance in the supply cannot be easily corrected, the general method adopted is to use a motor of larger rating. Although this solves the problem of the motor getting overheated, the performance parameters of the motor (viz., power factor, efficiency, starting current, etc.) deteriorate. In fact, the negative sequence currents in the supply lines increase, causing additional losses and requiring larger capacity supply cables. Thus, although simpler to implement, use of a larger size motor does not lead to the desirable solution (viz., reduction of unbalance in the motor winding currents). This letter studies the feasibility of modifying the machine in order to minimize the unbalance in the currents of a motor subjected to unbalanced supply voltages     

Performance of hemisphere algorithm for fast form factor calculation

A fast and accurate algorithm is required for carrying out radiative heat transfer simulations designing thermally efficient devices. This paper describes the performance of the hemisphere algorithm, which was originally developed for fast form factor calculation for obtaining photorealistic three‐dimensional computer graphics. We compare the performance of the hemisphere algorithm with that of two conventional algorithms that are widely used for radiative heat transfer simulations. The hemisphere algorithm is found to be significantly faster than the other algorithms, but it has an absolute error of 1.0×10^?5. In addition, the result indicates that the hemisphere algorithm is suitable for simulating the trial and error process in the practical analysis of large‐scale models due to its tolerable visualization of form factor distribution. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20259     

Temperature transients in flowing boreholes

We consider the problem of temperature transients in a borehole which has been drilled into an isothermal homogeneous solid and which is flowing at a constant rate. At a fixed time, the temperature of the fluid entering the borehole at the bottom is raised by a given amount. We derive the theoretical expression for the temperature response of the borehole and give a few numerical data of practical value.     

Calculation of unbalanced magnetic pull in small cage inductionmotors with skewed rotors and dynamic rotor eccentricity

This paper sets out a simple analytical model which is able to account for the damping effects of a cage rotor on the unbalanced magnetic pull (UMP) when the rotor is dynamically eccentric. The algorithm is implemented for a 4-pole cage induction motor with a variety of rotors with different rotor eccentricity and skew. The model is verified by the assessment of the UMP vibration when the different rotors are fitted in the stator. The characteristics are compared and it is found that the predicted and measured UMP characteristics compare well. It is observed that skewing the rotor cage increases the UMP when the motor is loaded     

Placement of vents to reduce shaft flux in two pole inductionmotors with a comment on heating effects

The authors consider the problem of magnetic flux variation in the rotors of vent-cooled, two-pole induction machines. The solution to this problem is important not only to reduce stray losses in such machines but also to identify the location of hot spots to estimate their effects on the design. A second objective behind the research was to reduce the magnitudes and, as a result, the variations in shaft magnetic fluxes. Reducing shaft fluxes will result in reduced shaft failures due to fatigue that is exacerbated by repeated heating and cooling. A procedure is suggested for examining the effects of the placement of the vent holes on flux variations, especially in the shaft. Once the effects of flux variations are known, the heating effect can be deduced     

Effect of adaptive single-pole reclosing on the stochastic behaviorof turbine-generator shaft torsional torques

This paper presents a Monte Carlo based approach to evaluate of the maximum torsional torques induced in turbine-generator shafts during high-speed reclosing of system faults. Two reclosing schemes were considered, namely simultaneous and adaptive reclosing. In the case of a single line-to-ground fault, single-pole reclosing is considered in clearing such a fault. In this context, investigations have been conducted on a large turbine-generator model taking into consideration the uncertainty of several factors associated with the practical operation of a power system. The effect of employing adaptive single-pole reclosing on the expected maximum torsional torques as well as their variances have also been investigated. A risk index which reflects the likelihood that the torque induced in a turbine-generator shaft exceeds its design value is also presented     

Experimental analysis of fast metal hydride reaction bed dynamics

In this paper an experimental investigation of main influencing parameters on the dynamics of a metal hydride reaction bed is presented. The metal hydride used in this work is LmNi_4.91Sn_0.15 and the applied reaction bed is based on a capillary tube bundle heat exchanger with a large heat transfer surface. Based on experimental investigations of the hydrogen distribution in the capillary tube bundle reaction bed a limitation of the reaction bed dynamics due to insufficient hydrogen transport can be excluded. However, it is shown that the desorption dynamics of the reaction bed can be limited by the intrinsic kinetics of the used AB₅ alloy.     

Performance analysis of HT-PEFC stacks

The performance analysis of a five-cell HT-PEFC stack is presented. The stack was operated either with pure hydrogen or synthetic reformate on the anode side and air on the cathode side. The overall electric performance and the heat management were analyzed. The local performance was assessed by current density and temperature distribution measurements. For this purpose, a tailor-made measuring board was integrated into the stack assembly. It is shown how the choice of fuel gas composition, reactant stoichiometry, flow direction and cooling affect the current density and temperature distribution.     

Computer-aided methods of determining thyristor thermal transients

An accurate tracing of the thyristor thermal response for determination of the rating of the thyristor and prediction of its performance is investigated. Several alternatives to thyristors for thermal modeling and analysis by using an electrical circuit analog are offered. These methods are adaptable to numerical solutions and well suited to the use of the digital computer. The thermal analysis of thyristors was performed for the 1000 MVA converter system at the Princeton Plasma Physics Laboratory. Transient thermal impedance curves for individual thyristors in a given cooling arrangement were known from measurements and from manufacturer's data. The analysis pertains to almost any loading case, and the results are obtained in a numerical or a graphic format     

Analysis of effects of three-phase voltage unbalance on inductionmotors with emphasis on the angle of the complex voltage unbalancefactor

Analysis of the steady-state performance of an induction motor connected to unbalanced three-phase voltages is presented. The index of voltage unbalance used in this paper is the complex voltage unbalance factor (CVUF) that consists of the magnitude and the angle. In addition to formulation and discussion of the motor operating characteristics under voltage unbalance, special emphasis is placed on the effect of the angle of the CVUF on the motor. It is found that the angle of the CVUF is also an important factor that merits particular attention in analyzing voltage unbalance issues     

Performance analysis of a solar cooker in Turkey

A box‐type solar cooker is designed and its thermal performance is analysed experimentally. The cooker tracks the sun in two axes, altitude and sun azimuth, by hand control for hourly periods. The experimental results show that the tested cooker may be assumed suitable in some cooking processes for specific country conditions. Copyright © 2002 John Wiley & Sons, Ltd.     

多联机系统中电子膨胀阀作用性能分析

变频多联机系统是以"变频、一拖多或多拖多"为指导思想的节能优化空调系统,它要比普通的定速空调系统节能,并且能够同时满足对不同房间舒适性的要求。在多联机控制系统中通过变频压缩机实现对系统总制冷剂循环量控制的同时,也要实现对进入室内换热器的制冷剂流量的控制,而该控制就在于电子膨胀阀的控制,每个室内换热器对应一个电子膨胀阀,通过控制电子膨胀阀的开度,控制进入每个室内换热器的制冷剂流量。而在多联机系统中,变频压缩机与电子膨胀阀的协调控制也是变频多联机系统的重要研究内容。     

生物质快速热解制生物油的工艺分析

文章利用Aspen Plus软件建立了一个完整的生物质快速热解制生物油的流程模型,并详细描述模型的建立过程,模型包括原料的预处理、快速热解、焦炭和不冷凝气体的燃烧3个部分。通过对日处理2 000 t玉米秸秆的快速热解制生物油工厂各工段进行模拟,结果表明,整个生产过程各种形式的能耗为468.73×109J/h,能量产出为531.6×109J/h,能量产出大于能量消耗;将能量折算成标准煤用量后可知,生产1 kg生物油的能耗相当于0.758 8 kg标准煤,同时产出的能量相当于0.860 6 kg标准煤;焦炭的燃烧量为总量的86%时,可以满足快速热解过程的能量需求。     

Plastic deformation induced by pressure transients in fluid-filled pipes

Severe pressure transients, caused for example by pump breakdown, may induce plastic deformation of a pipe wall. The speed of propagation of waves inducing plastic deformation is of importance in the analysis and design of piping systems. A general expression for the speed of propagation is derived. This also accounts for the type of pipe support. The analysis presented in this paper suggests that there is no propagation at the point of plastic instability.     

Electrical transients of wind turbines in a small power grid

This paper presents field measurements and calculations on the disconnection and braking of wind turbines (WT) with rated power 60 kW and 100 kW in the grid of two Greek Islands. By means of a digital acquisition system with sampling frequency of 130 kHz, voltages and currents of the WT installations have been measured and analyzed just before and during the transient period. A simulation program has been used for this purpose. Results of simulation and measurements show in a wide range a good agreement. The simulation program was further used to investigate the disconnection process of WT under various circumstances. It is shown that during the WT disconnection, a sudden increase in stator currents may appear. So voltage dips may distort the power quality of the grid. Self-excitation of WT, with large induced voltage, occurs when they operate at low-load even with relatively small capacitive compensation. The amount of energy, that the brake of a WT absorbs during the disconnection, has been studied under various disconnection procedures. Suggestions are made for the proper moment of brake application, so that the energy absorbed is minimum     

Temperature transients in spherical medium irradiated by laser pulse

Temperature wave solution predicted by hyperbolic heat conduction model is developed for a hollow sphere exposed to laser pulse heating. Using the obtained analytical solution, the temperature distribution, the propagation and reflection of the temperature wave due to such heat pulse is investigated for different thermal relaxation time and laser pulse duration. The effect of geometry on the temperature profile is also studied and the results of the hyperbolic and Fourier model are compared.