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Cover Story


Digital operator interface for corona power supply


Corona Discharge Treatment for improving adhesion of various polymer substrates at atmospheric pressure is now standard practice on plastic film extrusion lines, cast/blown/coating, as well as many converting lines.


display panels can show line operators true output. Purely reactive components such as inductors and capacitors do not in essence dissipate power. During one part of an AC cycle, power is stored within the component and during the second part of the cycle the stored power is released. Only resistive components absorb power, pure capacitors and inductors do not absorb power and therefore do not cause any loss in the circuit. All practical inductors and, to a lesser degree, capacitors, have some resistance associated with them. The heat generated by some of these components is caused by this resistive element. However, some of the load power is dissipated in transformer and dielectric losses.


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orona is generated by applying a high frequency, high voltage signal to an electrode, separated from an earthed plane by an air gap, the substrate and a layer of dielectric material. Frequencies in the range of nine to 50kHz, with peak voltages up to 30KV, are generally used in the corona treatment industry. When a voltage is applied to the electrode, virtually no current will pass through the air gap until the electrical breakdown of the air takes place. This occurs when the voltage across the air gap reaches a level of 3,000-5,000 V/mm, the variance being caused by ambient conditions such as humidity. At breakdown potential, free electrons are accelerated towards the positive electrode with such high energy that they are capable of displacing electrons from molecules in the air gap. The consequence is an avalanche effect, with electrons and the corresponding ions being produced, resulting in current flow across the air gap. This breakdown of the air produces large ionisation currents and as the input power to the corona is increased, the potential across the air gap remains essentially constant. As the ionisation current increases, the power in the corona discharge (i.e. the rate at which the charged particles are moving) increases proportionally. For a given power


that produces a relatively high rate of electron bombardment on the surface of the film, a small air gap and its resulting lower breakdown voltage will produce a relatively low energy level. This is the result of a lower breakdown voltage and smaller gap restricting the acceleration of the particles. Increasing the gap increases the breakdown voltage and the energy level of each particle. However, the rate of bombardment of the particles on the film is reduced, therefore increasing the breakdown voltage, and the air gap does not increase the corona power onto the substrate. Many other factors play an important role in a correct and effective corona discharge, such as corona power supply, frequency of operation, air gap settings, dielectric materials and discharge electrode configurations.


CORONA GENERATORS - REACTIVE POWER SUPPLIES


Reactive power systems provide a quantitative measure of the match between the load and the generator. This is a significant step towards more efficient corona treatment and it means that corona generator


The importance of impedance matching cannot be over-emphasised. In this procedure maximum power is delivered to the discharge electrode by tuning the inductance and the capacitance of the generator and electrode station to resonance. Reactive power systems provide a quantitative measure of the match between the generator and the electrode station. The end result of this process is that equivalent treat levels are achieved with significantly less power absorbed.


The CSR plus range of reactive power supplies from Corona Supplies, with integrated PLC and Siemens HMI touch screen, provides significant improvements in feedback to the operator with True Power, reactive power and frequency readouts, machine speed displayed, auto-corona start, on screen real-time visual power trends, datalogging, programmable recipes, fault display, along with cause and solution. Skip-treat function, programmable watt density control, humidity compensation and external alarms are all standard features. A corona treater generator makes repeatability of power used very clear to machine operators, a significant step towards more efficient corona treatment.


Xwww.coronasupplies.co.uk


8


September 2021


www.convertermag.co.uk


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