Your task: To check the control circuits of items such as valve
actuators, motor drives, SCRs and heaters; manipulate test signals; and
analyze the results. So, you need a run-up box.
Wasn’t in the budget? Make one yourself.
Access to process information and the ability to act on it is becoming
faster and easier year upon year. Control components that make this
possible have digital electronics and sensing devices built in, and
often are called smart or intelligent. These also are marketing words,
so judge each case for yourself. A communications cable leaving the
control room calls at each component, enabling you to watch and
manipulate its parameters. This has bestowed great benefits onto
process diagnostics, preventive maintenance and plant uptime -- and
don’t forget wiring complexity.
Components under discussion include control valves, power-control
devices, temperature sensors, signal converters, transducers and motor
drives. Controllers, indicators and PLCs have long had smart features.
They too are often remote but can still keep in touch with a control
center.
Take an aluminum alloy billet 6" dia. and 20" long. You have to heat one every minute to uniform slush consistency, ready for diecasting. Convection and radiant heating cannot make the
required speed or put the heat uniformly throughout the volume. The
center would still be cold while the outside is melting. Instead, consider
induction heating using a water-cooled coil around the axis of the
billet. This technique can put
heat deep into the billet.
Walk round a few process plants and you will see some 20 years of evolution, says controls guru Arthur Holland. Starting from the tried-and-true using discrete instruments all the way up to SCADA systems with color monitors and operator interfaces, rich in control, protection and data analysis capability, Holland offers a review of control technology.
by ArthurHolland | February 1, 2004 | Comments (0)
Ratio control is used to ensure that two or more process variables such
as material flows are kept at the same ratio even if they are changing
in value. In industrial control, examples of ratio control that come to
mind are burner air/fuel ratio; mixing and blending two liquids; injecting modifiers, pigments, etc., into resins before molding or extrusion; and adjusting heat input in proportion to material flow.
by ArthurHolland | November 1, 2003 | Comments (0)
Now that energy prices are varying as never before, you know that you
have to watch them before they empty your wallet. More than that, you
need a handy way to compare different offers. Whatever you are buying,
your choice of best buy will often be thwarted. Why? Consider these pitfalls.
You can connect two SCRs in just two of the three secondary lines and
use fast-cycle control. You buy only two SCRs and rate them for the
line current and line voltage. This arrangement rules out both
phase-angle control and load balancing because of you cannot use a
neutral conductor.
by ArthurHolland | September 1, 2003 | Comments (0)
The zero sum is mostly true if the SCR-controlled load is switched
in the fast-cycling (burst-firing) mode and all three SCR drive pulses
are in unison.I say mostly because the three currents do not
cease in unison at the end of the control pulse. They cease in turn at
their own next zero crossing -- at slightly different times. These leftover bits of neutral current become more noticeable at short cycle
times, though their heating effect will balance over time due to the
random distribution of which phase is first off.
SCR in the primary means that your SCR is rated for line voltage and
usually a lower current. The firing must be phase-angle with soft
start, for several reasons. First, with fast-cycling, you risk a
current inrush at switch-on and at
every on cycle. This can be up to eight times full load current.
Second, the initial primary current aims to reach V/R -- a very high
value because R is the very low primary winding resistance.
