Digital Technology Advantages to Power Control in Glassmaking
By Christopher M. McCormick,
Spang Power Electronics
Using advanced Digital Signal Processing technology in high- temperature applications, makers of flat glass and fiberglass can employ multi- zone SCR control for annealing and tempering lehrs, electric forehearths, and float glass (BATH heat). The advantages of precise networked power control include accelerated calibration time, reduced installation and wiring costs, and ability to modify process control settings remotely and in real time.
Advanced process control and fault monitoring: analog vs. digital
Typically, analog designs provide minimal process and fault information. Only critical information is monitored. In an electric forehearth application operating under current control, electrode current is the only parameter being controlled and monitored The process control system will send a signal (typically 4-20 mADC) to set desired zone outputcurrent.
In addition, a transducer (also with 4- 20 mADC output) will be used to monitor output current delivered to the forehearth. Both the output set point control signal and output transducer signal are sent, via wire connections, between the power control panel containing SCR power controllers and the process control system.
Through these individual wires, the operator in the control room sets the desired output and monitors the results. Besides monitoring and controlling specific parameters such as output current, analog equipment uses generic fault indications such as I zone failure’ or ‘power controller failure.’ These single fault indications used to keep equipment and wiring costs to a minimum – have multiple meanings. They could be indicating SCR over temperature, a blown fuse, SCR over current shutdown, load failure, or some other malfunctions.
To further understand the problem, the control room operator must send service or maintenance personnel out to the control panel location to determine exactly what is causing the generic fault indication.
With analog technology, each additional fault requires control relays, wiring and digital inputs on the process controller. If each piece of information is to be displayed in the control room, the cost of showing each fault can be substantial, especially for multi-zone systems.
Digital power controllers have numerous advantages over their analog counterparts. All parameters and faults directly related to each power controller, or zone of control, are monitored, and many are controllable. These parameters include: input and output phase-to-phase voltage, input frequency, input and output per phase current, input kVA and output kW, power factor and kW-hour.
Furthermore, the single-phase and three-phase digital power controllers monitor a variety of faults, including: input high voltage, frequency out of tolerance, over current shutdown, SCR over temperature and phase loss.
Parameters that are digitally controllable include output voltage, current and power. This level of functionality is driven by the nature of the digital design.
In an analog design, each additional parameter to control or monitor current flow meant more circuitry and higher equipment costs. With digital power controllers, that paradigm no longer exists because software in the microprocessor dictates the unit’s functionality.
A better way to achieve local process control
Glass manufacturers are accustomed to using relatively simple methods for achieving local control of SCR power controllers. Typically, on/off pushbuttons, control potentiometers, and analog meters are provided locally and are often located on the enclosure door of the panel contaming the specific SCR power controller. In power control zones for the BATH portion of a flat glass manufacturing line, the local controls for each of the 28-33 zones consist of a power set point potentiometer, power meter (analog or digital), on/off pushbuttons or a selector switch and a ‘power on’ pilot light. These devices allow for individual zone control locally at the power panel during startup, and when there are problems with the central process controller.
As with other aspects of an analog design, functionality is kept to a minimum due to the incremental costs to add features. Local Digital Controllers (LDCs) have forever changed what is expected of local control capabilities. With a mere push of a button, the LDC provides on/off control and set point adjustment, and also displays voltage, current, and power levels
The ideal connectivity package
Analog designs transfer process and control information through control wire (typically ‘twisted-pair’) connections carrying control signals and other relay type contacts representing faults and alarms.
In the BATH application in flat glass manufacturing, each SCR power controller zone accepts a 4-20 mADC control signal for its power set point and sends a 4-20 mADC signal proportional to output power back to the process controller.
In addition, an analog SCR power controller accepts relay contacts for On/off control and provides a relay contact for zone fault. This configuration means that, per zone, there are up to 2 sets of twisted-pair control wires and 3 sets of wires for relay contacts. These wires must be physically ‘run’ from the power panel to the process controller in the control room. Multiply those numbers by the 28-33 zones the application requires, and one can easily see that a lot of wire, conduit, and labor are expended just to achieve a minimal level of control.
In contrast, digital SCR power controllers are capable of continuously monitoring and controlling a multitude of parameters and faults that are unfeasible for analog equipment to detect or regulate
Another benefit of digital power control is that single network connections do not ‘run’ back to the control room. Instead, they are terinitiated at the power panels ‘in network ‘tap’ boxes. A single network cable is then ‘run’ to the control room and to the network interface of the process controller. In BATH applications, a single digital network connection replaces 140-165 analog control wire connections between the process controller and the power panels. The savings in labor and materials are significant.
Single- and three-phase digital power controllers designed to operate on 24 to 600 volt RMS at 50/60 Hz are ideal for glass and fiberglass manufacturing applications. Digital technology allows for independent, remote operation of SCR power controllers and eliminates calibration and hardware considerations that formerly constrained the ability to precisely monitor and control electric current in glassmaking operations.
About the author
Christopher M. McCormick is business manager of power control systems for Spang Power Electronics – Mentor, Ohio