Page 363 - Industrial Power Engineering and Applications Handbook

131338 Industrial Power Engineering and Applications HandbGok
Shrouding
Shrouding 7
Control
buses

Front I
Front

Figure 13.6 A non-compartmentalized outdoor-type control panel
(Courtesy: ECS)
auxiliary relays (contactors). The sensing instruments
that are located at the various strategic locations of a
process line or on the machine feeding the line sense the
actual operating conditions continuously and give a signal
through its auxiliary relay in the event of an error beyond
permissible limits.
In a process industry (e.g. chemical, fertilizer, cement, Figure 13.7 Side view of a double front panel with a common
horizontal bus, but separate vertical bus bars (not visible) (Courtesy:
paper, textile, tyre or petrochemicals) such process controls ECS)
are numerous, and to closely monitor and control each
through the conventional method is arduous and may mounting. It is wired internally and eliminates all external
impose many constraints. They also require cumbersome, wiring. There are no moving contacts and hence no
excessive and complex wiring and a very large control phenomenon of operating time, contact arcing, or wear
panel, as shown in Figure 13.6, for a small process line. and tear of contacts.
There will also be limitations in terms of accuracy, A logical controller is a solid-state digitally operated
response time and control besides maintenance problems. control device which can be programmed to follow a set
Thus the method is not suited for critical applications. A of instructions such as logic sequencing, timing, counting
relay may have to activate another before it activates the and arithmetic analysis through digital or analogue input
correction and regulates the machine feeding the process and output signals to respond to the different process
line, hence adding to more delay. A contactor operation, requirements. These may be regulating the speed of the
particularly, introduces an element of time delay (5 to 10 motor to perform inching duties as when a loader crane
ms each relay) in executing correction because of its or an elevator is required to adjust its hoisting height
inherent closing and interrupting times. Also there may precisely at different loading stations or to adjust the
be a number of these operating one after another in tandem movement of a moving platform of a machine tool. Figure
before correcting the process. The time delay may affect 6.49 illustrates a typical process line. It can also perform
the quality of process and may not be warranted. With data handling and diagnostic activities by storing data
advances in solid-state technology control and feedback and messages and also give signals to activate, adjust or
devices can now be in the form of logical controllers, stop a process. A logical controller can thus replace a
which respond instantly and can be programmed in a relay-based circuit more efficiently and also offers many
discrete way, to suit any process needs. A controller is more operational facilities and ease of handling than a
extremely small and can perform the duties of hundreds relay, and all this instantly, without causing a time delay.
of such relays and requires only a small space for It contains three main sections: