In the paper industry, it is often required on some installations to dry the paper after coating and to control the moisture profile at the end of production line. One technique that is efficient, convenient and safe is short-wave infrared drying.
Depending on the number zones or the power of the different drying zones to be controlled, different thyristor units can be used. However, short-wave infrared heaters are made with tungsten filaments, a material having a high temperature coefficient. When first switching on short-wave infrared heating elements while they are cold, the inrush current can be as much as eight to 10 times the normal current when they are hot. This is a characteristic normally seen when utilizing burst firing (full periods of supply voltage). In this case, high-speed thyristor protection fuses are not used because they would blow at the first half cycle of an on period. Additionally, the thyristor must be sized in order to withstand that first half-cycle of cold inrush current.
Phase-angle firing could be used to avoid inrush current. But this would generate harmonics (electrical noise generated because of altering the sign waves) and power factor deterioration. The use of a fast time based zero-fired controller eliminates the problem of harmonics and poor power factor and still provides good control of a short-wave infrared load. Taking into consideration the high paper speed and the drying rate, the time base (on and off thyristor conduction times) must be as fast as possible to keep the drying rate constant regardless of paper speed. A fast cycle rate or an intelligent half cycle is suitable for this type of application because of its ability to respond quickly.
Two case histories describing typical situations on a paper production line illustrate how communicating SCRs can manage the power control in these types of applications effectively.
Drying and Moisture Profile Control. In the first application, controls were needed for short-wave infrared heaters used to dry the paper after coating (figure 1). A total of six three-phase zones were used, each being controlled with a three-phase thyristor unit.
For the same line, moisture profile control also was needed. As a solution, a total of 20 communicating SCR power controllers, each capable of providing four independent single-phase outputs (for a total of 80 controlled zones) were used. A high speed fuse was not used, and a single cycle firing mode (for a fast time base) was used.
A scanning sensor measures the transversal moisture across the web, feeding the information to a PLC, which controls the 80 digital communicating SCR power controllers.
The single-cycle thyristor firing mode is fully adapted to this short-wave infrared application, allowing it to provide proper drying regardless of paper speed. In addition, utilizing zero-voltage firing means that the power control does not generate destructive harmonics or degraded power factor.
The voltage feedback internal to the thyristor unit provides 1 percent voltage regulation regardless of supply voltage variations (in this case, 500 V, +/-10 percent), giving optimized power control and better moisture control. The maximum RMS voltage applied to short-wave infrared elements can be reduced from 500 V to 470 V using a voltage limit on thyristor unit to promote element life.
The communicating SCRs used provide installation monitoring and a built-in alarm strategy as well as critical operational data such as supply monitoring for over- or under-voltage; short-wave infrared monitoring for indication of lost heaters or overcurrent condition; and thyristor unit monitoring to detect shorted or open SCRs.
Moisture Profiling. In this case, to reduce installation cost and to control each short-wave infrared element, a different control technique was used (figure 2).
Each heating element setpoint, or the power to be dissipated by the element, was sent from the PLC using a communication link to a remote input/output interface. The remote interface used can have as many as 32 outputs, each receiving through communication from the PLC its own setpoint. Several remote interfaces can be used on the same communication link when the number of zones demands it.
Upon receiving a signal from the PLC, the remote interface converts the setpoint into a time-proportioning output designed to control short-wave infrared elements. This output controls a single-phase solid-state contactor. For this application, the controller selected had an intelligent half-cycle feature to ensure the fastest possible cycling time. The thyristor firing controls per half period. This minimizes flickering and provides proper heat control without the drawbacks of phase-angle firing such as harmonics and poor power factor. The power control is able to withstand the cold inrush current because the thyristor utilized has a high surge current rating.
The PLC/control system can switch on the dryer's zones in a programmed sequence for optimum power utilization. High speed thyristor protection fuses are not used. The installation can equipped with alarms such as thyristor short circuit and diagnostic load failure to detect the burnout of one of the short-wave infrared heaters out of six in parallel. PH