Model Calculations of the Interaction Between an AC Corotron and a Moving Dielectric

The dependence of the voltage on a moving dielectric exiting an ac corotron on such input variables as corotron frequency, process speed, ac current, dc current, position on the dielectric, and incoming potential are discussed. The voltage excursion occurring as the dielectric passes beneath the ac corotron are shown. The results demonstrate that surprisingly large voltage excursions can occur, with implications for the case where the dielectric is a real photoconductor.     

Corona Current Control by Mesh Grid

Corona charging is widely used for charging dielectric surfaces. It is desirable to attain precise and uniform surface voltage in its application. A corona charging method using a mesh grid is described. It is found that the corona current control characteristics by mesh grid can be expressed by a nondimensional parameter?the corona current transmission rate. This parameter is obtained experimentally. The method of parameter application in dielectric surface charging is expressed by a differential equation. This technique is applied to charging amorphous selenium xerographic plates and to neutralizing the charges on a dielectric surface. It is found that the surface voltage obtained by charging with a mesh grid is far more uniform than that obtained by charging without a mesh grid.     

Conductive fur brush charging of a dielectric surface

A dielectric surface is charged without a corona discharge using a conductive fur brush to which an electrical potential of several hundred volts is applied. The fur brush, which consists of 10-μm-thick conductive fiber, lightly touches the surface of the (Mylar film or photoreceptor) dielectric. It is clear that brush charging is accomplished by direct charge transfer, gas discharge, and triboelectric charging. The direct charge transfer, which is the primary charging process, is analyzed using an ohmic contact model. The charge potential is nearly proportional to that of the brush. When the charging time is longer than 0.3 s, the charged potential on the surface photoreceptor saturates at almost that of the brush, the difference being less than 50 V. This is about the same as in corona charging. Brush charging is, therefore, useful as a low-voltage charging technique     

Effective Ozone Generation With a Wire–Wire-Type Nonthermal Plasma Reactor With a Slit Barrier

The corona discharge and ozone generation characteristics of a wire–wire-type nonthermal plasma reactor (PR), with a slit dielectric barrier, have been experimentally investigated. When a slit mica barrier is installed between corona wires, a significant increase in the generation of ozone could be obtained. From the photographs of the corona discharges, this type of nonthermal PR with a slit barrier (SB) produces two corona discharges, one from the corona wires and another from the slits of the SB. This second corona discharge is caused by the rapid relaxing of the charges, which are migrated from the corona wire and stored on the surface of the barrier, through the slits. As a result, the proposed nonthermal PR has the potential to significantly increase ozone production, and it may be useful as an effective means for removing pollutant gases.      

Corona-Controlled Electrostatic Imaging Processes

Two electrostatic imaging processes, utilizing simultaneous corona charging and image exposure, are described. These processes have been analyzed using a mathematical model based on an equivalent circuit. It can be shown that the photoresponse is controlled by the characteristics of the corona charging device as well as the sensitivity of the photoconductor. However, the ultimate response of the photosensitive element is entirely a function of the corona device. The limiting electrostatic contrast was found to depend primarily upon the corona characteristics and the initial charge on the dielectric surface in the case of dc corona charging. For the ac corona case, the limiting contrast depends primarily upon the initial charge level.     

Electric Charging of Particles Near the Corona Glow Region in Air?CO2 Mixtures

Addition of 3 to 65 percent C02 to air results in the formation of an unusual glow around a positive corona wire and in a corresponding erratic charging of particles traversing this region. The results of the charging experiments for pellets shot in this glow region parallel to the corona wire are presented, as well as a possible explanation of the phenomenon. It is expected that the results will be of particular interest to the electrostatic precipitation and electrostatic painting industries.     

The effect of corona wire heating on the downstream ozoneconcentration profiles in an air-cleaning wire-duct electrostaticprecipitator

The effect of corona wire heating on the downstream ozone concentration profiles in an air-cleaning wire-duct electrostatic precipitator was studied experimentally. The results show that ozone concentration profiles depend significantly on the applied voltage and polarity. Significant ozone concentrations were only observed in a narrow region downstream of the corona wire for positive coronas. For the corona wire surface temperature profile measurements made using a real-time IR thermoimage processing system, it was observed that the heat generated by corona discharge alone only increased the surface temperature of the corona wire by a few degrees centigrade. Ozone generated by corona discharges can be significantly decreased by the external heating of the corona wire     

Phenomenon of skipping sparks in discharges on a dielectric surface

Gliding sparks are associated with channel breakdown in air near a corona precharged dielectric layer backed by ground. Characterized by high current, their bright channels usually end in sudden branching/dimming. Infrequently, there occur cases in which channel brightness is interrupted, with no evident local branching but a new leader continuing on propagating after momentary delay. This skipping-spark phenomenon has been studied in an experimental system intended for taking Kirlian photographs. Operated in a Lichtenberg-figure mode, the method yields evidence in some detail. The authors report on observations of spark-channel touchdown and subsequent rebounding. A model based on nonuniform surface charging is proposed for explanation     

Bi-Ionized Space Charges Generated by Means of Corona

Due to the extensive usage of unipolar corona in electrostatic precipitators, electrostatic painting, mineral ore separation, and other applications, most of the past studies of particle charging and electric fields in space-charge regions have focused on the unipolar corona. As such, while the basic equations for particle charging in bi-ionized fields are known, relatively little experimental work may be found in the literature. A test chamber and a wire probe assembly have been constructed allowing the investigation of electric fields in the bi-ionized space between parallel wire coronas for various gas mixtures, pressures, and spacings. Results are presented for experiments in air, CO2, and CO2-nitrogen mixtures, and they seem to indicate that a substantial amount of recombination of positive and negative ions occurs. The resulting average electric field intensity over a large section of the bi-ionized space between the corona wires was found to be uniform.     

Charging of Polymeric Surfaces by Positive Impulse Corona

This paper focuses on analysis of charging of polymeric surfaces by means of impulse corona discharges in air. Internal (space charge densities and electric fields) as well as external (circuit current) characteristics of corona in a point-plane electrode configuration are investigated by means of computer simulations. Two types of onset positive corona modes, namely positive glow corona and burst pulse corona are identified. The developed and verified computer model is further used to study corona charging of a 2 mm thick polymeric material sample. Both the mechanism of charge deposition and distribution of deposited charges on the surface are dependent on the mode of the corona discharge used. In the case of glow corona, charge generation is limited to the anode region and the generated charges move towards the sample surface under applied electric field. Thereafter the deposited charge cloud expands radially along a portion of the surface with fairly constant concentration. In the case of burst pulse corona, series of positive charge clouds start from the anode and move towards the sample surface in a wave-like manner. Each burst contributes to the deposited charge, which spreads over the surface less extensively than that observed during glow corona charging.     

Particle-Initiated Breakdown Characteristics and Reliability Improvement in SF6 Gas Insulation

Particle-initiated breakdown characteristics in SF6 gas are investigated. Breakdown voltages are greatly influenced by particle diameter especially for AC voltages, and for very fine metallic wire particles are much higher than those for moderately thick ones. The breakdown stress of a gas gap is obtained as a function of particle length and can be estimated fairly well by assuming a corona stabilizing zone. For a spacer, the electric stress which is parallel to its surface dominates surface breakdowns and the relation between the stress and particle length is experimentally determined. In order to increase breakdown voltages, the effects of both the dielectric coating on a high-voltage electrode and spacer shape are examined. Finally, the effectiveness of the dielectric coating on a grounded electrode is demonstrated to prevent particle levitation.     

Optimization of corona electrode position in roll-type electrostatic separators

The position of the corona electrode(s) is known to be a key factor of electrostatic separation efficiency, as it influences both the charging conditions of the granular materials on the surface of the roll electrode connected to the ground, and the magnitude of the electric forces exerted on the particles. Response surface methodology was employed for the design of the experiments performed on a laboratory roll-type corona-electrostatic separators, with samples of chopped electric wire wastes typically processed by such techniques. The results of the electrostatic separation tests are discussed in relation to the data obtained from two other experiments, in which corona current and particle charge measurements were performed for various positions of the standard wire-type electrode. The conclusions of this study enabled the formulation of several recommendations for the improvement of the outcome of industrial separation processes (i.e., maximize the weight percentage as well as the purity of the recovered materials).     

AC Corona Charging of Particles

Measurements of the mobility statistics of dioctyl-phthalate (DOP) particles leaving an ac corona charger with various electrode configurations are reported. Mobilities are determined using a laser-Doppler velocimeter (LDV) to measure particle migration velocities in response to a transverse electric field imposed downstream of the charger. Charging models, which do not include effects of finite ion transit times, nonuniform fields, or differences between the distributions of positive and negative wire coronas, predict some of the mobility statistics, especially below 100 Hz, but cannot explain the high-frequency charging. In the turbulent flow with significant corona wind effects the disparate corona current distributions are shown to have a significant effect on the ac corona charging processes.     

导线全面工频起晕电压测量新方法

电晕放电会带来诸多危害,起始电晕电压的测量对于改进电气设备绝缘结构,提高起晕电压具有重要的意义,为此提出了一种测量导线起始电晕电压的新方法,即在导线附近安装非接触式电压传感器。导线发生电晕放电时电极表面出现的电离层改变了传感器分压比,因此利用分压比导数最大值可确定起始电晕电压。在介绍传感器测量的基本原理并通过工频试验验证传感器的测量精度后根据电晕套理论设计工频起晕试验,借助光子数切线法进行了对比研究。结果表明传感器可以准确测量导线全面起晕电压值,且通过传感器分压比的变化可以描述工频电晕的伏库特性。该方法简单便捷,易于实现,适合进行电晕试验研究。     

Premises for the mathematical modeling of the combinedcorona-electrostatic field of roll-type separators

Corona and induction charging mechanisms are frequently associated in modern roll-type electrostatic separators. Various electrode configurations have been proposed and numerous attempts have been made in order to fully characterize them. This paper approaches this problem from a computational point of view. The boundary-element method is employed for analyzing the electrostatic field distribution generated by a typical arrangement, consisting of a wire-type corona electrode and an ellipse-profile nonionizing electrode. The computed results pointed out the effect of various parameters on the uniformity of the electric field near the ionizing element and at the surface of the grounded rotating roll electrode. These data can be used by the designer in order to improve the electrode configuration, produce a uniform field in the active zone of the separator, and reduce the corona inception voltage. At the same time, they validate two important premises for the mathematical modeling of the combined corona-electrostatic held of a roll-type separator: (1) Peek's law can be used for evaluating the electric field strength at the surface of the wire electrode at corona onset in any of the usual electrode configurations; and (2) the distribution of the electric field in the active zone of the separator is affected only by the geometry of the high-voltage electrode system     

The Use of Graded Nets in Computer Modeling of Corotron Fields

The use of computer modeling with finite-difference techniques to solve for electrostatic fields by successive overrelaxation in regions where no exact analytic solution exists is well documented. However, for irregular Dirchlet boundary curves in two dimensions, which do not contain the nodes of a regular net, various interpolation schemes must be used. It is shown in this paper that these interpolation schemes introduce appreciable error into the solution for a boundary curve with a small radius of curvature, such as is found in the center of a corotron. An alternate technique is described which uses successive graded nets in the immediate region of the boundary. The validity of the method is confirmed in the case of a small wire with an applied voltage concentric with a grounded conducting cylinder, where an analytical solution is then possible. The method is then used for the solution of the Laplacian fields for a corotron having typical dimensions.     

Corona inception in typical electrode configurations forelectrostatic processes applications

The corona inception electric field at the surface of a wire electrode is usually evaluated by an empirical formula established by Peek. That formula was found to be valid in several simple electrode configurations (wire cylinder, wire plate), which are typical to electrostatic precipitators. The aim of this paper is to extend the study to other situations encountered in electrostatic applications. The experimental setup specifically modeled the several electrode arrangements which are commonly used with roll-type electrostatic separators. It consisted of wire-type corona electrodes, connected to a regulated DC high-voltage supply, and a rotating roll electrode, connected to the ground. The effect of associating the corona wire to one or several tubular electrodes of various shapes and sizes was investigated. The experimentally determined corona inception voltage was used as input data of a boundary-element-method program for the electric field computation. The computed values of the electric field were compared with those given by Peek's formula for wire electrodes of the same radius. The derived conclusions can be of help in the custom design of the corona electrode arrangements for various electrostatic applications     

Charging of insulating spheres on the surface of an electrodeaffected by monopolar ions

The charge acquired by an insulating sphere in a uniform monoionized electric field has been accurately evaluated by Pauthenier. In certain electrostatic applications, such as the electroseparation of mixed granular solids, the particles to be charged are on the surface of an electrode. Under these circumstances, Pauthenier's formula is no longer valid, because the field is nonuniform. This paper addresses this problem from both a computational and an experimental point of view. A numerical method of field analysis was employed for the evaluation of the charge acquired by spheres of various dielectric constants, on the surface of a plate electrode. The numerically computed values of the saturation charge in this situation were always greater than those given by Pauthenier's formula. The experiments were carried out on laboratory equipment provided with various types of corona electrodes. An electrometer was used to measure the charge acquired by calibrated spheres of polyamide (3 mm diameter) when subjected to the positive or negative corona discharge generated between these electrodes and a metallic rotating roll electrode (150 mm diameter) connected to the ground. The experimental data were in good agreement with the theoretical predictions; the saturation charge increases linearly with the applied voltage, up to a threshold at which the self discharge of the particle occurs. The efficiency of ionic charging was shown to depend on the type of corona electrode that is employed     

Screen Controlled Corona Device (Scorotron) for Charging in a Xerographic Copier

Special features have been developed to improve scorotron efficiency, manufacturability, reliability, and serviceability for the office copier environment. Unidirectional rotation of the photoreceptor allows the higher efficiency of a combination corotron/ scorotron to be used. A simplified screen configuration, an extruded shield, and high quality molded plastic end blocks simplify manufacture. A wraparound shield, a manual cleaner for corona and screen wires, and a foolproof slide mount, improve reliability and serviceability. Thus a scorotron is able to provide the improved consistency of photoreceptor sensitization necessary for high quality solid area reproduction copies. Charging nonuniformities caused by photoreceptor fatigue, thickness variation, temperature sensitivity, and miscellaneous charge acceptance variables, are significantly reduced.