All corona treating installations require a source of controlled electrical power. Low-voltage 60 hertz electrical power is fed into an electrical device which raises the frequency. This high-frequency electrical power is applied to a step-up transformer that increases the voltage. The high voltage, high-frequency electricity is then discharged from an electrode through the web being treated to the electrically grounded metal roll.
Although the basic principles remain the same, many
improvements have been made over the past 30 years which greatly increase the capability and performance of the power supply. The earliest power supplies were motor generator types that proved unreliable for long-term continuous operation due to mechanical breakdowns. These were replaced by power supplies using a Telsa coil and spark gap to generate the high-frequency, high-voltage, electrical energy. These designs were an improvement over the motor generator systems but still left much to be desired in reliability due to the erosion of the spark gaps.
Next came solid-state power supplies using transistors as the power output device. However, early transistors had a limited output power capacity, and these units required as many as 16 transistors connected in parallel to achieve the required power levels. Although transistors are generally very reliable solid-state devices, the large number of devices required caused a proportional increase in the chance of a random failure.
The natural evolution of power supplies then led to the development of an inverter using silicon controlled rectifiers (SCR’s) as the power output device. SCR-type inverters have been in widespread use for the past 25 years and have proven to be very reliable.
Recent advances in transistor technology have led to a new generation of corona treating power supplies which provide increased power output in smaller sized enclosures. The ability of these new Insulated Gate Bipolar Transistors (IGBT’s) to handle higher current levels yet provide high speed on/off switching through logic control allows more precise control of treating power and frequency.
The advent of logic controllable power devices allowed the use of programmable microprocessor and minicomputer chips that provide reliable treatment levels on a variety of substrates. Improved control circuits for corona treater power supplies are now available which increase the consistency of corona treatment for enhanced adhesion of the extruded material to the substrate and, in post-treating applications, of inks, primers, coating and laminants to the extrusion coated surface. Incorporating microcomputer technology, these treaters with advanced control ensure this advanced control assures corona treating accuracy and consistency by automatically tuning the power supply power and frequency to all variables such as electrode, roll type, web thickness, and air gap (Figure 3).
Part 4 of the Corona Treating Series discusses watt density.