高海拔地区玻璃绝缘子的直流污闪特性

为了解决特高压直流输电线路中的外绝缘问题,清华大学深圳研究生院和云南省电力试验研究院合作,通过对两种大吨位玻璃绝缘子在不同盐密、不同灰密、不同染污情况和不同悬挂方式下进行直流污闪特性试验,对其污闪特性作了深入研究。试验表明,玻璃绝缘子和瓷绝缘子直流污闪特性相似;高海拔条件下,由于飘弧现象严重,爬电距离并不能得到有效的利用,故不是影响污闪特性的唯一重要因素;结构高度和绝缘子形状结构在很大程度上可影响飘弧和电弧桥接,因而对污闪特性的影响很大,在一定程度上大于爬电距离对污闪特性的影响;同时分析了非均匀染污情况和不同悬挂方式下的污闪特性及其差异原因。     

基于DSP的交流磁悬浮稳定控制

针对交流谐振磁悬浮难以稳定控制的问题,提出利用DSP控制逆变的输出从而控制悬浮的稳定。理论上通过对交流谐振磁悬浮动态模型的推导,给出控制变量函数。最后通过Matlab仿真和悬浮小球实验,证明了此动态正弦脉宽调制(SPWM)逆交控制能实现稳定交流谐振磁悬浮,从而提高了电路的功率因数。     

Construction and levitation guidance experiment of magnetic levitation system without gap sensors using inductance model of thin steel plates

Recently, the noncontact conveyance methods that are applied magnetic levitation techniques have been studied in many institutes. In this paper, from the view point of practical use, the authors construct a magnetic levitation system without using gap sensors which have been essential for the conventional magnetic levitation of thin steel plates. This system adopts both current sensors and Hall‐effect sensors in place of the gap sensors. In addition, we consider the peculiar inductance characteristics of magnetic levitation for a thin steel plate. Moreover, we conduct the levitation, guidance, conveyance and disturbance experiments.     

Vapor levitation epitaxy reactor hydrodynamics

Vapor Levitation Epitaxy (VLE) is a new epitaxial growth technique for III-V compound semiconductor crystals, characterized by upward stagnation flow of a reactant gas stream against a floating, circular semiconductor substrate wafer. In this work, the hydrodynamics of the process have been mathematically modeled by a complete solution of the governing Navier-Stokes equation to obtain radial and axial velocity profiles, pressure distribution under the wafer, and levitation height as a function of operating parameters. In addition, experimental measurements of levitation height as a function of gas flow rate for different wafers and gases have been performed. By the addition of experimental constants to the analytical solution of levitation height to account for system non-idealities, the functional dependences predicted by theory have been verified. In comparison to the complex flow occurring in other vapor phase epitaxy techniques, the hydrodynamics in the VLE reactor are simple, well behaved, and can be fully described by a few algebraic equations.     

Experimental study of LIM-driven,attraction-type magnetically levitated carrying system

Recently, many kinds of magnetic levitation systems have been proposed and developed for use in factory automation systems. This progress in the development depends on the appearance of new types of permanent magnets and new control theories. On the other hand, some systems have simple structures in which couples roles are given for both levitation and conveyance. As for these systems, however, many trials are needed to establish the system configurations and the control theory. The authors have developed a new magnetically levitated carrying system driven by LIM (linear-induction motor). The system can be completely detached from a guideway both in mechanical and electrical meanings. In this paper, the experimental results, especially the driving characteristics of a vehicle without any contacts to the LIM guideway, are discussed.     

Measurements of thermophysical properties of liquid metals by noncontact techniques

With the advent of containerless processing techniques such as electromagnetic levitation, it is now possible to study the properties of high-temperature liquid metalsin situ by applying sophisticated noncontact diagnostics, such as pyrometry and high-speed videography. Thermophysical properties of interest are, e.g., specific heat, thermal conductivity, and viscosity. Applying containerless processing, it is also possible to undercool the melt because of the lack of container-induced nucleation sites. This gives access to a metastable region of the phase diagram. The knowledge of thermophysical data in this region is very important, because undercooling plays a major role in any solidification process. The degree of undercooling not only determines the growth velocity, but also is crucial in selecting the eventually obtained metastable solid phase. In this paper, some recent developments are surveyed relating to the noncontact measurements of emissivity, specific heat, electrical conductivity, density, surface tension, and viscosity, as well as a discussion of possible experiments in microgravity.Paper presented at the Third Workshop on Subsecond Thermophysics, September 17–18, 1992, Graz, Austria.     

Aspects of high-temperature pyrometry for measurements in ultrahigh vacuum

Containerless materials processing of liquid metals with the use of electromagnetic levitation requires contactless temperature measurement by pyrometry. For high temperatures and under high-vacuum conditions, the vapor pressure of the levitated metal drop increases, leading to evaporation losses of the sample material. This flux condenses on the cold parts of the experimental apparatus including the window in front of the pyrometer. As a result, the intensity of radiation reaching the pyrometer decreases, which is erroneously interpreted as a decrease in temperature. Several methods to protect the pyrometer against contamination have been proposed. In this paper, we report experimental tests of the concepts of shielding windows and mirror optics placed into the optical path between the sample and the pyrometer. Temperature measurements with a periscopic mirror system are also presented.Paper presented at the Second Workshop on Subsecond Thermophysics, September 20–21, 1990, Torino, Italy.     

大间隙混合悬浮系统结构设计与特性分析

研制了一种新型电磁悬浮系统装置,该装置利用电磁铁阵列与永磁铁来提供空间三维方向的悬浮驱动力.与传统的电磁悬浮系统相比,该系统具有悬浮力大、磁场分布规律、运动范围广等特点,可明显提高系统的使用效率.基于有限元数值方法对悬浮电磁力特性以及合成磁场特性进行了详细分析,并进行了相关实验.结果表明,新设计的电磁阵列悬浮系统装置可以在大气隙下使用,具有较好的稳定性和可控性.     

Control of magnetic levitation positioning stage with an eddy current damper

To enhance the system damping,a permanent magnet set which served as an eddy current damper was added to the magnetic levitation positioning stage which consists of a moving table,four Halbach permanent magnetic arrays,four stators and displacement sensors.The dynamics model of this stage was a complex nonlinear,strong coupling system which made the control strategy to be a focus research.The nonlinear controller of the system was proposed based on the theory of differential geometry.Both simulation and experimental results show that either the decoupling control of the movement can be realized in horizontal and vertical directions,and the control performance was improved by the damper,verifying the validity and efficiency of this method.