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When controlling
electrically powered process heating equipment such as ovens,
heaters or washers, the power control scheme employed can make
or break you if you are using analog controls, what you do not
know can hurt you.
Typically,
analog designs provide minimal process and fault information because
only the most critical information is monitored. In most process
heating applications operating under power control, the power
delivered to the load is the only parameter controlled and monitored.
The process control system will send a signal (typically 4 to
20 mA DC) to set the desired zone output power. In addition, a
transducer (also with 4 to 20 mA DC output) will be used to monitor
Output power. Both the output setpoint control signal and output
transducer signal are sent, via wire connections, between the
power control panel containing the SCR controllers and the process
control system. Through these individual wires, the control room
operator sets the desired output and monitors the results.
In addition
to monitoring and controlling specific parameters such as output
power, analog equipment uses generic fault indications such as
zone failure or power controller failure. These single fault indications
have Multiple meanings. They could be defined as SCR overtemperature,
fuse blown, SCR overcurrent 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 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 multizone systems.
If money were
no object in the world of analog 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. But, because the costs of equipment, installation,
wiring and maintenance are significant, many times only the most
critical functions are monitored.
THE
DIGITAL DIFFERENCE
With digital SCR power controllers, all parameters and faults
directly related to each power controller (or each 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/hr
Furthermore,
single-phase and three-phase digital power controllers monitor
a range of faults, including:
- Input high
voltage.
- Frequency
out of tolerance.
- Overcurrent
shutdown.
- SCR overtemperature.
- Phase loss.
Parameters
that are controllable by digital SCR power controllers 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 means more circuitry and higher equipment costs. With digital
power controllers, that paradigm does not exist because the microprocessor's
software dictates the unit's functionality.
ACHIEVING
LOCAL CONTROL
 Traditionally,
manufacturers or processors have used relatively simple methods
to achieve local control of SCR power controllers. On/off pushbuttons,
control potentiometers and analog meters are provided locally
and often are located on the enclosure door of the panel containing
the specific SCR power controller. As with other aspects of an
analog design, monitoring devices - and therefore functionality
- are kept to a minimum due to the incremental costs to add features.
Analog designs
typically transfer process and control information through twisted-pair
control wire connections carrying control signals (commonly 4
to 20 mA DC signals) and other relay-type contacts representing
faults and alarms. In some process heating applications, for example,
each SCR power controller zone accepts a 4 to 20 mA DC control
signal for its power setpoint and sends a 4 to 20 mA DC 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 two sets of twisted-pair
control wires and three sets of wires for the relay contacts.
These wires must be run from the power panel to the process controller
in the control room.
Multiply those numbers by the total number of power control zones
the application requires, and you can see that a lot of wire,
conduit and labor are expended to achieve a minimal control level.
 In
contrast, digital SCR power controllers are capable of continuously
monitoring and controlling a multitude of parameters and faults.
Using a connectivity package such as Device-Net or Ethernet, 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 control zone.
In many applications,
a digital network connection can replace more than 100 analog
control wire connections between the process controller and the
power panels. The savings in labor and materials can be significant.
The use of digital techonology 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.
Christopher
M. McCormick is business manager of power control systems for
Spang Power Electronics, Mentor, OH. For more information about
Spang's power controllers, call (440) 352-8600 or visit www.spangpower.com.
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