As seen in
Issue 3 - 2000 Spang Power Electronics
Introduces Digital Power Controllers
for Glass Making Applications
By Christopher M. McCormick,
Spang Power Electronics Advanced digital technology for precise networked control in high -temperature glass manufacturing applications is now available from Spang Power Electronics.
The 850 Series Digital SCR Power Controllers offer the latest in Digital Signal Processing (DSP) technology and software. Through networked multi-zone SCR control for annealing and tempering lehrs, electric forehearth and float glass (BATH heat) manufacturers of flat glass and fibreglass can accelerate calibration time, reduce installation and wiring costs, and modify process control settings remotely and in real time.
ADVANCED PROCESS CONTROL AND FAULT MONITORING: ANALOGUE VS. DIGITAL
Typically, analogue designs provide minimal process and fault information. Only the most critical information is monitored. In an electric forehearth application operating under current control, for example, the electrode current is the only parameter being controlled and monitored, The process control system will send a signal (typically 4-20 mADC) to set the desired zone. OUTPUT CURRENT
In addition, a transducer (also with 4-20 mADC output) will be used to monitor the 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 the 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, analogue equipment uses generic fault indications such as "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 defined as SCR over temperature, fuse blown, SCR over current shutdown, load failure or 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 analogue 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.
If money were no object in the world of analogue power control technology, one could monitor all process information, including zone voltage, current, and power, as well as separately annunciate all of the individual zone faults. Because the costs of equipment, installation, wiring and maintenance are significant, only the most critical functions are monitored. THE DIGITAL ADVANTAGE
The 850 Series Digital SCR Power Controllers have numerous advantages over their analogue counterparts. Now, all parameters and faults directly related to each power controller (or each zone of control) are monitored, and many are even 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 controllable by the Digital SCR Power Controller include output voltage, current and power. This level of functionality is driven by the nature of the digital design.
In an analogue design, each additional parameter to control or monitor current flow means more circuitry and higher equipment costs. With digital power controllers, that paradigm no longer exists because the software in the microprocessor dictates the functionality of the unit.
To develop software for the 850 Series, Spang Power Electronics combined its expertise in the application of power control equipment for glass manufacturing with a breadth and depth of knowledge in the general field of power control engineering and equipment design. The result is a product that monitors all the important parameters and faults that affect users' processes. DIGITAL TECHNOLOGY:
A BETTER WAY TO ACHIEVE LOCAL CONTROL OF YOUR PROCESS
Glass manufacturers are accustomed to using relatively simple methods for achieving local control of SCR power controllers. Typically, on/off pushbuttons, control potentiometers, and analogue meters are provided locally and are often located on the enclosure door of the panel containing the specific SCR power controller. In the power control zones for the BATH portion of a flat glass manufacturing line, for example, the local controls for each of the 28- 33 zones consist of a power set point potentiometer, power meter (analogue 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 start-up, and when there are problems with the central process controller.
As with other aspects of an analogue design, functionality is kept to a minimum due to the incremental costs to add features. The 850 Series Local Digital Controller (LDC) has 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.
Additionally, the LDC "flashes" fault and alarm codes for quick diagnosis of problems and has LEDs to indicate the unit's status. When mounted on the enclosure door, the LDC replaces all of the analogue control and metering devices while providing more functionality and many times more operational information than its analogue counterpart.
THE IDEAL CONNECTIVITY PACKAGE
As previously discussed, analogue designs transfer process and control information through control wire (typically "twisted-pair") connections carrying control signals (commonly 4-20 mADC signals) and other relay type contacts representing faults and alarms.
In the BATH application in flat glass manufacturing for example, 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 analogue 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 two sets of twisted-pair control wires and three sets of wires for the 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 labour are expended just to achieve a minimal level of control.
In contrast, the Digital SCR Power Controllers Spang Power Electronics manufactures are capable of continuously monitoring and controlling a multitude of parameters and faults that are unfeasible for analogue equipment to detect or regulate. Using a connectivity package such as DeviceNet or Ethernet (both of which are available for the three-phase Digital SCR Power Controller), digital power controllers transmit key information between each power controller and the central process controller. A single network cable replaces the multiple runs of control wire previously required for each power control zone.
Another benefit of using the connectivity of the 850 Series is that single network connections do not "run" back to the control room. Instead, they are terminated 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, for example, a single digital network connection replaces 140-165 analogue control wire connections between the process controller an the panels. The savings in labour and materials are significant. CONCLUSION
Spang Power Electronics' 850 Series Digital SCR Power Controllers consist of single- and three-phase devices that are designed to operate on 24 to 600 volt RMS at 50/60 Hz. The single- and three- phase controllers are ideal for glass and fibreglass manufacturing applications, and can be used for industrial heating applications in the automotive, chemicals, metals and plastics industries. The use of 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. For more information, contact the author at firstname.lastname@example.org
or visit the company's Web site at www.spangpower.com
ABOUT THE AUTHOR
Christopher M. McCormick is business manager of power control systems for Spang Power Electronics, a designer and manufacturer of SCR power controllers, dry-type transformers, AC & DC power systems, AC & DC drive systems and custom AC & DC motor drives. Spang power control products are manufactured within an ISO 9001 quality system. Spang Power Electronics is headquartered in Mentor, Ohio, USA and is a division of Spang, Inc., which is headquartered in Butler, Pennsylvania, USA, about 60 km from Pittsburgh.