Experimental Characterization of a Tri-Needle-Type Corona Electrode in View of Improving Charging Uniformity

Previous studies have shown that the distribution of electric charges at the surface of dielectrics exposed to the corona discharge is controlled by several factors, such as the geometry of the electrodes, the level and polarity of the applied voltage, and the environmental conditions. It also depends on the nature of the exposed material and, more specifically, on the chemical and physical composition of its surface. This article aims at evaluating and maximizing the uniformity of the distribution of electric charges on several types of dielectric materials after their exposure to a corona discharge generated by a tri-needle-type electrode system. Two different methods of charging are investigated: “static” for polyvinyl chloride (PVC) plates and “dynamic” for polypropylene (PP) films and particles. In order to estimate the uniformity of the deposited charge on these dielectric materials, two characterization methods were employed: 1) surface potential mappings on PVC plates and PP films, using an electrostatic voltmeter and 2) direct measurement of the charge deposited on PP particles after their passage under the electrode system, using a Faraday cage connected to an electrometer. By modifying the geometrical parameters of the electrode system (height and distance between the needles) and the applied voltage, satisfying nonuniformity rates were recorded: less than 5% for PVC plates and PP films and less than 15% for PP particles. Exposure times longer than 2 s practically affected neither the charge level nor its uniformity.