Artifacts in CSI-measurements caused by the drift of the static magnetic field

In chemical shift resolved spectroscopic imaging (CSI) temporal changes in the static magnetic field (drift) can lead to distortions of the phase encoding process. This can result in localization artifacts. The extent of the artifact depends on the size of the drift, the number of acquisitions, as well as on the combination of the size of the field of view and the number of phase encoding gradient steps. Furthermore, it is affected by the succession of the phase encoding gradients. Precautions are described which allow substantial minimization of the artifact.     

磁流体中磁场诱发的气泡偶极子对偏转效应

在以磁流体为介质的气泡上升可视化实验中,首次观察到两两组合而成的气泡对在磁场作用下会发生偏转,最终其中心连线会与磁场方向平行。为解释这一现象,将气泡简化为具有一定等效磁矩的磁偶极子,针对填充磁流体的Hele-Shaw单元中的气泡,求解麦克斯韦方程获得二维等效磁矩计算公式,进而推导出磁场引发气泡之间的相互作用力表达式。相互作用力可分解为径向分量和切向分量,分析后发现切向分量对气泡对运动的影响符合实验中观察到的偏转现象。气泡本身不具有磁性,在磁场作用下也不会被磁化,但是气泡的存在会改变其周围的磁场分布形成"磁穴",从而会诱导出对附近其他气泡的等效磁力。气泡之间的相互作用力根据其相对位置不同而使气泡对之间发生相互吸引或排斥(临界角度为55°或135°),并使气泡对中心连线趋于磁场方向偏转。     

Zero magnetic field type magnetic field sensor

This paper presents a new magnetic field sensor named the zero magnetic field type, on the basis of its principle of operation, which is as follows: when the magnetic field level in the core used as a sensor is shifted by an unknown field to be detected, a controlled field is so superimposed in the opposite direction to the unknown field that the original zero magnetic field state is resotred. Then the unknown field can easily be detected from the controlled field. Consequently, detection performance is expected to be unaffected by geometrical configurations, magnetization properties, and demagnetization effects of the core used as a sensor. Moreover, temperature variations of the core and the existence of resistance and leakage inductance in the windings arranged on the core also do not directly degrade performance. This paper certifies that the proposed sensor certainly has the forementioned properties. In addition, the following are clarified: (1) the upper limit of the detectable range does not exist in principle; (ii) the accuracy is 0.02 percent for the full scale 20 kA/m and realizes the order of 4 × 10^?4 percent/^oC for temperature variations of ?76^o C to 300^oC; (iii) the magnetization property required of the core used as a sensor is that it has as sharply a rectangular B-H loop as possible; (iv) as a result, sensor design is greatly simplified.     

Studies of elastic waves caused by corona discharges in oil

In order to diagnose the deterioration of insulation and evaluate its location in oil-filled pole transformers, measurements of elastic waves brought by corona discharges in oil were performed, based on an acoustic emission (AE) technique. The detected signals in these measurements were analyzed by means of a fast Fourier transform (FFT) and their properties were discussed theoretically. It was found that the AE signals due to the Lamb waves of symmetric mode caused in a thin steel tank could be recognized easily by the FFT analysis of initial stages of the detected AE signals. This suggests that the accuracy on location of the AE sources in the transformer would be improved. Also, it was shown that a low pass filter was effective in improving the location accuracy     

空载线路合闸对发电机励磁回路的影响

通过分析空载线路合闸中发电机励磁电流的暂态特性，解释了励磁电流出现过零趋势的原因，提出计及励磁主回路反向电阻的发电机暂态等值电路。对黑启动模式下发电机投空载长线进行仿真计算，结果表明：空载线路合闸过程中发电机励磁绕组可能会出现过电压，并可能比自励磁现象和线路末端过电压更容易发生；如果励磁回路有允许反向电流通过的能力，则能避免励磁绕组过电压。     

Characteristics of elastic waves caused by corona discharges in anoil-immersed pole transformer

Measurements of elastic waves brought by a pulse of simulated corona discharge in an oil immersed pole transformer and a model tank filled with oil were performed in order to identify the location of insulation deterioration, using an acoustic emission (AE) technique. The detected AE signals were analyzed by means of a fast Fourier transform (FFT), and properties of elastic waves influenced by the existence of insulated windings and magnetic core for power transformation in the transformer were investigated. The result shows that almost all the initial AE signals detected on the-wall of the transformer tank were Lamb waves caused by the oscillation waves passing through the insulating paper and oil with repeating transmission, reflection and refraction. Also, it was possible to recognize whether the measured elastic wave was influenced by windings and their insulating papers or not, by FFT analysis. This suggests that the location accuracy of the corona discharge in the transformers would be improved     

Artefacts in magnetic resonance imaging caused by dental material

A common problem in computer tomography (CT) based imaging of the oral cavity is artefacts caused by dental restorations. The aim of this study was to investigate whether magnetic resonance imaging (MRI) of the oral cavity would be less affected than CT by artefacts caused by typical dental restorative alloys. In order to assess the extent of artefact generation, corresponding MRI scans of the same anatomic region with and without dental metal restorations were matched using a stereotactic frame. MRI imaging of the oral and maxillofacial region could be performed without reduction of the image quality by metallic dental restorations made from titanium, gold or amalgam. Dental restorations made from titanium, gold or amalgam did not reduce the image quality of the MRI sequence used in imaging of the oral and maxillofacial region for dental implant planning. In this respect MRI is superior to CT in implant planning.     

Electric field distortion caused by asymmetric pollution oninsulator surfaces

The pollution on the surface of an insulator is asymmetric, although insulators, in general, are axisymmetric. The distortion of fields on the surface of an asymmetrically polluted axisymmetric insulator is computed in this paper. The surface charge simulation method (SCSM) has been used with cylindrical, conical and toroidal surface elements. The volume resistivity of the insulation also has been taken into consideration, using complex permittivity for the insulating material     

Electromagnetic effects excited by magnetic loops

Contents An analysis is made of the electromagnetic effects in the space around magnetic loops moving in inertial systems. The methods of the relativistic electrodynamics, a four-dimensional potential, and the concepts of a model of a magnetic charge have been used. The obtained results allow to correct certain inaccuracies connected with the application of the magnetic loops theory. Conclusions are made about the applicability and the perspectives concerning the development of the magnetic loops theory.

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Elektromagnetische Effekte die durch magnetische Kreise angeregt werden

Übersicht Es werden elektromagnetische Effekte von sich in Inertialsystemen bewegenden magnetischen Kreisen analysiert. Man wendet Methoden der Relativitätselektrodynamik, das vierdimensionale Potential, die Darstellungsmodelle der magnetischen Ladung und die entsprechenden Tensoren des Elektromagnetischen Feldes an. Die gewonnenen Ergebnisse gestatten es bestimmte Ungenauigkeiten in den existierenden Anwendungen der Theorie der magnetischen Kreise zu korrigieren. Es werden Schlußfolgerungen bezüglich der Praxis und der Perspektiven der Entwicklung und Anwendung der Theorie magnetischer Anregungskreise gemacht.

     

涡流在铁基非晶合金薄带中引起的磁场分布和磁损耗

铁基非晶合金薄带是重要的软磁材料.利用Maxwell方程组求得在弱交变磁场作用下各向同性的铁基非晶合金薄带中的磁场对空间和频率的分布规律,进而推导出平均磁感应强度、平均起始复数磁导率、涡流分布规律和涡流损耗的理论计算公式.     

有限宽金属平板对工频磁场的屏蔽

为了定量评估金属平板对工频磁场的屏蔽效果,在阐述了理想导磁材料和理想导电材料的磁场屏蔽原理基础上,基于无限宽金属平板屏蔽区域磁场解析式,通过计算域映射,推导了有限宽金属平板屏蔽区域磁场解析式.通过仿真分析了对称三相传输线屏蔽系统,讨论了导线组排列位置、屏蔽材料类型对屏蔽效果的影响.结果表明,采用解析式,可对有限宽金属平板的屏蔽效果进行快速计算,其准确性和计算效率优于现有电磁场分析软件所采用的数值求解方法,可对实际屏蔽系统的设计提供参考.     

Computation of the magnetic field generated by an undergroundpipe-type cable

A hybrid computational method for predicting the magnetic field in the vicinity of an underground three-phase pipe-type cable is presented in this paper. This hybrid method combines the numerical techniques of the finite element method and the unimoment method. An iterative solution procedure is developed and employed to treat the nonlinear B-H characteristic of the steel pipe. To validate the computational method, measurements were made and the numerical results are compared with the measured data and with the results obtained previously by using a different numerical method. The result of the comparison is satisfactory     

铁芯式两极纵磁真空灭弧室三维静磁场有限元分析

铁芯式两极纵磁真空来弧室是我国第一个具有自主知识产权具有较高开断能力的真空灭弧室。对其纵向磁场进行数值分析可为进一步提高它的开断能力提供思路。对其三维静磁场的有限元分析结果表明,这种新型真空来弧室可产生很强的纵向磁场,纵向磁场在间隙中及触头表面上分布都较为均匀。结构参数的变化对纵向磁场的影响很大,当触头直径增大时纵向磁场强度增加较大,开距增大时则迅速减小,拐臂宽度增大时其强度呈线性减小。     

五种纵向磁场真空灭弧室触头磁场特性分析比较

用三维有限元法研究了线圈、杯状,两极和四极及双线圈五种纵磁真空灭弧室触头的纵向磁感应强度分布、触头片上涡流分布和纵向磁场滞后时间.研究表明:(1)电流峰值时纵向磁场由强到弱依次排列为:线圈式触头、两极式结构、双线圈式触头、杯状和四极式触头;(2) 电流过零时剩余磁场由弱到强依次为:四极式触头、两极式触头、杯状触头、双线圈式触头和线圈式触头; (3)纵向磁场较强处滞后时间由小到大依次为:两极式触头、四极式触头、线圈式触头、双线圈式触头和杯状纵磁触头.     

Estimation of effective magnetic field in ferromagnetic alloys by magnetoacoustic effect

The magnetic field strength in a nickel rod with an applied uniform external field is studied by transmitting ultrasonic waves. In nondestructive testing (NDT), longitudinal plane waves are transmitted from an ultrasonic transducer to the object, which is characterized by alternating regions of compression and dilation. If the objects are ferromagnetic materials, the compression waves generate the magnetoelastic energy in magnetic substances, which are equivalent to the magnetic anisotropy field. This field passes through magnetic substances with the velocity of ultrasound and is observed as the electromotive force (emf) induced in the sensing coil. As the ultrasonic waves are transmitted continuously to a rod specimen, the ultrasonic stationary wave is formed in it and the high‐frequency magnetic field is generated in nodes of the stationary state by the inverse effect of magnetostriction, then the emf is induced in the nodes. In this paper the strength of the effective magnetic field in the nickel rod is estimated by measuring the electromotive force induced by ultrasonic waves passing through magnetic materials. © 2002 Wiley Periodicals, Inc. Electr Eng Jpn, 140(1): 1–7, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10025     

Electric and magnetic field exposure

The possible health hazards of low-level, power line frequency (50/60 Hz) electric and magnetic fields are considered. The historical background to this concern is briefly discussed. The types of studies being carried out and the results so far are summarized. It is concluded that while the scientific evidence on field effects is inconclusive, inferences of health effects justify further evaluation