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 spesales@spang.com
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.
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