Charging and discharging of insulating particles on the surface of a grounded electrode

The aim of the present paper is to analyze the corona charging of millimeter-size insulating disks, as well as their discharging when they are no longer exposed to the action of an external electric field. The experiments were carried out on a roll-type electrostatic laboratory separator, equipped with a wire-type corona electrode, simulating the actual charging/discharging conditions in an industrial unit. Disks of various sizes were charged on the surface of the roll electrode, then the high voltage supplied to the corona electrode was turned off and the particles were collected in a Faraday pail, connected to an electrometer. The charge measurements were performed at various time intervals from high-voltage turn-off. In this way, the charge decay could be recorded and the discharge process fully characterized. The measured data show that the discharge process depends on the nature, size, and shape of the particles, as well as on the contact conditions between the particles and the grounded roll electrode. These data could guide the design of the electrostatic separation experiments that precede any new industrial application of this technology.     

Corona discharge from a grounded needle electrode inserted into a charged droplet cloud

The discharge occurring in a space-charge cloud was investigated by using a cloud of charged droplets. To produce a charged cloud with a definite space charge density and to cause a reproducible discharge, droplets ejected from an airless nozzle were induction charged and transported by air flow. The transportation of charged droplets by air flow was found to be quite effective in increasing the charge on the droplets and the charge density. Corona discharge was observed at a grounded needle electrode inserted into a charged droplet cloud with a charge density not exceeding 13 μC/m³. In negatively charged droplets, positive streamer coronas occurred. As the charge density of the cloud increased, the time interval of streamer pulses decreased and the discharge shifted to a glow corona. In positively charged droplets, the height and frequency of negative corona pulses increased with the charge density and the velocity of the charged droplet cloud. © 1998 Scripta Technica. Electr Eng Jpn, 122(4): 1–7, 1998     

Discharge occurring at a grounded electrode located in a charged particle cloud

Electrical discharge phenomena from a grounded electrode located in a charged particle cloud have been investigated. Soil conditioning particles charged by corona charging were blown to form a space charge cloud. A grounded sphere electrode was placed at the center of the charged cloud to produce an electrical discharge. A positive pulse discharge extended from the sphere electrode toward the charged cloud with a strong luminescence. A brushlike streamer with a maximum length of 170 mm appeared at an interval of 30 ms. The maximum pulse height of the discharge was 2.4 A. The distribution of the electric field around the sphere electrode was obtained from field analysis using a concentric spherical model. It was found that the magnitude of the discharge depends on the radius of the grounded electrode and the size and charge density of the clouds. © 2001 Scripta Technica, Electr Eng Jpn, 136(4): 7–14, 2001     

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     

Static electricity hazards in the use of flexible intermediate bulkcontainers (FIBCs)

Flexible intermediate bulk containers (FIBCs), usually made of highly insulating polypropylene fabrics, are used in large numbers worldwide. Concerning electrostatic ignition hazards relating to the insulating fabric, precautions are to be taken in fire and explosion endangered areas. There are different ways to meet this challenge by modifying the FIBCs antistatically. Therefore, tests must be conducted to ensure that the provided measures reliably eliminate electrostatic hazards. Usually, such tests are run in full scale, making them very costly. In addition, the results are difficult to reproduce, as experience shows. A reliable and replicatable method has been found, which is corona charging of the FIBC to simulate the electrostatic charge build up by the bulk. This paper presents and discusses the test method and its results, especially in aspects of essential grounding     

Effect of Moving Speed of Charged Metallic Spherical Electrode on Electrostatic Discharge

In this investigation, the nature of the electrostatic discharge (ESD) that occurs when a charged object moves toward a stationary grounded object is experimentally clarified. The spark lengths, discharge currents, and induced voltages in a magnetic probe were measured when a charged metallic spherical electrode connected to a 422 pF capacitor approached a stationary grounded object, which was the current target, for different moving speeds of the charged metallic spherical electrode in a range of 1 mm/s to 100 mm/s. The charge voltages of the capacitor were +6.5 kV and +10 kV. Based on the results, the average gap length shortened with the speed of the spherical electrode. The average peak values of the discharge current and the induced voltage were likely to increase with the speed of the spherical electrode. The average rise times of the discharge current and the induced voltage were likely to drop with the speed of the spherical electrode. The relation between the spark length and the discharge current due to the ESD can be explained qualitatively by using an equation derived from the spark resistance formula proposed by Rompe and Weizel.     

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     

Studies on electrostatic surface discharges on corona-chargedpolymer surfaces

Electrostatic surface discharge phenomena on corona-charged thin polymer films such as FEP or PTFE Teflon, with surface potentials from 1 to 5 kV in both polarities, were studied. A grounded sphere electrode, 5 mm in diameter, approached the charged surface by a computer-controlled step motor until the surface discharge occurred. Discharge-current waveforms, with a very short risetime (a few nanoseconds), were observed to be related to the length (5-30 cm) or type (single wire or plain mesh) of the earth line. The discharge gap length between the film surface and the grounded electrode was also calculated. The results were strongly dependent on the polarity of the surface charge and were widely divergent, indicating the complicated surface discharge mechanism     

Electrostatic painting of insulating surfaces

The purpose of this work was to develop methods for painting insulating surfaces by the conventional electrostatic painting technique used to coat conducting samples. This technique is not applicable to insulators since the charges carried by the paint accumulate on insulating targets, thereby preventing a proper coating. A detailed investigation of the decay of charges deposited on insulators by a corona discharge simulating the charged paint has confirmed that a grounded counterelectrode (CE) on the back face of the sample speeds up the decay and that the better the contact, the faster the decay. Because a material CE with proper contacts is inconvenient for painting in production configuration, it was suggested that ionized air might behave as a satisfactory CE. Several ionizers have been tested, and good coatings have been obtained, provided that the flows of paint and ionized air are well separated. The possibility of self-creation of ionized air by the voltage of the early paint deposit has also been successfully tested     

Electrostatic Hazards Associated with Powder Handling in Silo Installations

Pneumatic delivery and general movement of powdered products inevitably results in some degree of charge separation between the product and its containing walls. Unlike electrostatic charging associated with high-velocity pumping of insulating liquids, where addition of an antistatic additive is a standard means of quenching the charge, the situation is a little more difficult to control with powder. There is as yet no antistatic additive commercially available for powders, and therefore the standard procedure of grounding all plant hardware does not give sufficient protection against charge accumulation. The powder itself, will still emerge at the end of the delivery sequence in a highly charged state, and it is this which usually results in a potentially hazardous situation. A case study on a powder handling silo installation following a severe explosion during filling operations is described.     

Resolution and signal processing technique of surface charge density measurement with electrostatic probe

When an electrostatic probe is used to measure the surface charge on an insulating plate of constant thickness, the measuring system is regarded a shift-invariant system and the relation between the surface charge density and the probe output can be treated in the spatial frequency domain. The distribution of the surface charge density on an insulating plate just after occurrence of a surface discharge is measured by a Pockels probe, which is regarded as a kind of electrostatic probe without the guard electrode, and restored by Wiener inverse filter. The performance of a Pockels probe and a conventional electrostatic probe are compared quantitatively in terms of the spatial resolution. In the case that the measured object is 3 mm thickness PMMA plate and is charged up to 10 nC/cm/sup 2/ in atmospheric air, it is estimated that the spatial resolution of the Pockels probe with 0.2 mm gap is 1.5 mm and that of the conventional electrostatic probe with the grounded guard electrode with 3 mm gap is 2.2 mm.     

Charge distribution measurement on a truncated cone spacer under DC voltage

A system for measuring charge distribution on an insulating spacer is developed. An electrostatic probe is set close to the spacer, and it moves along its surface maintaining a small gap. The accumulated charge is inversely calculated from the measured data by utilizing the relationship that is obtained through the numerical electric field computation. Using this system, the surface charge distribution on a truncated cone spacer of 80 mm diameter and 15 mm height is measured. The number of measured points is 3402, and the spatial resolution is 3.9 mm. After the application of DC 10 kV for 19 hours, the spacer surface is charged with a spotted pattern. The charge density reaches 60 pC/mm/sup 2/ at its maximum. In addition, the residual charge distribution of partial discharge from metallic particles on the spacer is observed.     

Space charge behavior in an antistatic polymer including polymer solid electrolyte

A polymer dielectric material tends to be easily charged and may cause an electrostatic discharge, due to the serious problems that result when consequent electromagnetic noise damages the electronic circuits in various kinds of apparatus. The antistatic polymers that have been developed to prevent electrostatic discharge contain polymer solid electrolytes so that space charge originally exists inside them. The mechanism of the antistatic discharge effect of the polymer has been investigated by measuring the space charge distribution using the high-resolution pulsed-electroacoustic method. The experimental results suggest that the internal space charge of the polymer accumulates at the surface and forms heterocharge distribution at the interface between the electrodes and the specimen. Thus, internal space charge compensates the applied electric field, resulting in preventing the increase of the surface potential. In addition, internal space charge behavior relates to the dispersion condition of the polymer solid electrolyte that depends on the quantity of the electrolytes and the specimen procedures.     

Surface microstructure of powder layers influenced by the forces ofdeposition and adhesion in electrostatic coating process

In the electrostatic powder coating process, several factors affect the deposition of charged polymer paint particles and the adhesion of the deposited particles on the grounded substrate being coated. In this paper, the roles and relative magnitudes of these forces are discussed. A model on the deposition of a charged particle approaching the surface of the substrate is presented. The electrostatic fields that are considered here to be responsible for particle deposition are the following: (1) corona field between gun tip and grounded plane; (2) image field between a particle and its image charge; (3) field due to the space charge of charged particles; and (4) net repulsive field between a charged particle and the powder layer. Once the particle deposits on the surface and the high voltage is turned off, the particle experiences the following forces: (1) the force due to the image charge of the underlying powder layer; (2) the force due to the image charge of the particle; and (3) the repulsive force between the charged particle and the charged powder layer. The model shows criteria on whether the particles will deposit on the substrate and, if deposited, whether they will remain on the surface of the powder layer once the corona field is turned off. These relative forces influence the microstructure of the powder layer and may affect the ultimate appearance or the texture of paint film after curing     

Steady-state corona charging behavior of a corotron over a movingdielectric substrate

Charging up a dielectric surface through corona discharge from a thin wire has been a common practice in electrophotographic processes. One of the widely used corona charging devices is called a corotron, which consists of a coronating wire enclosed in a rectangular shield with one constituent side being the surface to be charged. Uniform surface charge can be deposited on a dielectric substrate, such as a photoreceptor in the absence of light, by moving the substrate at a constant velocity through a stationary corotron that consistently emits corona current. To design an efficient corotron for charging dielectric substrates, a fundamental understanding of the electrostatic nature of the device is desired. In this paper, the steady-state behavior of corona charging with a corotron over a moving dielectric substrate is analyzed by computationally solving the nonlinearly coupled equation system with Galerkin finite-element method and Newton iterations. The predictions based on a first-principle model are shown to agree well with experimental measurements     

Electrostatic separation of muscovite mica from feldspathic pegmatites

This paper aims at evaluating several electrostatic separation techniques that could be effective for the recovery of mica flakes from pegmatite ores characterized by grain sizes <0.5 mm: 1) triboadhesive separation; 2) dc electric field separation; 3) ac electric field separation; 4) corona field separation using a grounded metallic roll electrode and thermal conditioning; and 5) corona field separation using a grounded metallic roll electrode covered with an insulating layer. The experiments have been carried out on samples containing about 50% mica, 25% feldspar, 15% quartz, 10% iron oxides and hydroxides, as well as iron silicates, with grain sizes ranging between 0.16-0.4 mm. The experimental data confirmed the theoretical predictions: all four separation techniques using high-intensity electric fields improved the grade of muscovite mica concentrate to levels that fulfill the requirements of several industry applications such as welding wire manufacturing. The best results [46.11% SiO/sub 2/, 33.64% Al/sub 2/O/sub 3/, and 11.1% (K/sub 2/O+Na/sub 2/O)] were obtained when performing a three-stage separation using method 5).     

Static electrification of pressboard/oil interface and transientphenomena

The static electrification phenomenon of insulating materials used in power transformers is investigated through two devices. The first is a cell with a rotating disk covered on both sides with a given pressboard and immersed in a metallic tank containing ~41 of oil; and second device enables us to measure the electrostatic charge tendency of oils. The electrostatic charge tendency (ECT) of insulating oils and the leakage current generated by the charge concentration gradient at the oil/pressboard interface are analyzed as function of temperature, water content and aging of oils, the nature of the pressboard, and the rotating speed of the disk. The influence of the surface roughness of the pressboard and antistatic additives on the aging and ECT also are considered. A correlation between ECT and the physico-chemical and electrical characteristics of the oil is established. It is shown that the temperature gradients and the water migration phenomena at the oil/pressboard interface play an important role in the charge separation. Transient currents are strongly affected by the presence of air in the pressboard and the aging of the oil     

Space- and surface-charge analysis of plasma-preprocessed PTFE thinfilms

The space- and surface-charge behavior of corona-charged polytetrafluoroethylene thin films after plasma processing were studied experimentally by space-charge density distribution measurement and thermally stimulated discharge current (TSDC) measurement. The effect of the antistatic process using low-pressure discharge plasma and charge elimination process dipping in tap water was examined. The surface composition of the samples which were plasma- processed was analyzed with X-ray photoelectron spectroscopy (XPS) to study the mechanisms of antistatic process. It was found that charge elimination of plasma-processed samples was enhanced, independently of the kind of processing gas during the plasma processing. Oxygen and nitrogen atoms were observed from the XPS measurement of plasma-processed samples. In particular, a large amount of nitrogen atoms was found on the surface of the samples plasma processed in pure nitrogen gas. The samples plasma processed in pure nitrogen gas showed a large hetero TSDC peak at room temperature before the charge-elimination process. This might be due to nitrogen atoms on the sample surface that were generated during plasma processing     

Particle-triggered pre-breakdown phenomena in atmospheric air gap under AC voltage

Corona discharge mechanism and breakdown voltage characteristics of air gaps in the presence of metallic particles were investigated including the relationship between the corona discharge mechanism and the phase resolved partial discharge patterns. These results are of interest for detection of foreign particles in gas insulated systems (GIS). The results with different shaped particles show that the corona mechanism depends on the particle shape and applied voltage. Moreover the corona discharges on both sides of the particle interfere with each other by inflow of charge carriers into the particle. The breakdown voltage is a minimum when the particle is in the vicinity of the electrode due to active interference of coronas. The phase-charge characteristics as the phase resolved partial discharge patterns vary with the particle shape. This suggests that the corona discharge mechanisms for all types of particles should be considered in the particle-detection method for GIS based on the phase resolved partial discharge patterns.     

Computational estimation of ESD conditions between a charged bodyand a conductor of floating potential

This paper examines the conditions in which a discharge occurs between a charged body and a floating conductor, in air, at atmospheric pressure. The study, which was primarily aimed at simulating a class of hazardous situations quite often met in the operation of electronic circuits, is also of interest to those involved in the research and development of electrostatic technologies for processing of particulate matter: separation of granular mixtures, precipitation of dust, and spraying of powders. The boundary-element method was employed for the analysis of the electric field in the gap between the charged body and the conductive disk at floating potential, located above a grounded plate. The computations were carried out for bodies of various shapes and sizes; some of them addressed the case of a grounded plate covered with an insulating layer of known permittivity. The data on field distribution represented the input data of another program, which estimated the breakdown conditions of that air gap. The study enabled the estimation of the energy involved in this type of electrostatic discharge and the prediction of the related hazards