Process Variables

SETPOINT
The setpoint is a value for a process variable that is desired to be
maintained. For example, if a process temperature needs to kept
within 5 °C of 100 °C, then the setpoint is 100 °C. A temperature
sensor can be used to help maintain the temperature at setpoint.
The sensor is inserted into the process, and a contoller compares the
temperature reading from the sensor to the setpoint. If the temperature
reading is 110 °C, then the controller determines that the process is
above setpoint and signals the fuel valve of the burner to close slightly
until the process cools to 100 °C. Set points can also be maximum or
minimum values. For example, level in tank cannot exceed 20 feet.

MEASURED VARIABLES, PROCESS VARIABLES, AND
MANIPULATED VARIABLES
In the temperature control loop example, the measured variable is
temperature, which must be held close to 100 °C. In this example and
in most instances, the measured variable is also the process variable.
The measured variable is the condition of the process fluid that must
be kept at the designated setpoint.
Sometimes the measured variable is not the same as the process
variable. For example, a manufacturer may measure flow into and out
of a storage tank to determine tank level. In this scenario, flow is the
measured variable, and the process fluid level is the process variable.
The factor that is changed to keep the measured variable at setpoint is
called the manipulated variable. In the example described, the
manipulated variable would also be flow.

ERROR
Error is the difference between the measured variable and the
setpoint and can be either positive or negative. In the temperature
control loop example, the error is the difference between the 110 °C
measured variable and the 100 °C setpoint—that is, the error is +10
°C.
The objective of any control scheme is to minimize or eliminate error.
Therefore, it is imperative that error be well understood. Any error
can be seen as having three major components. These three
components are shown in the figure on the folowing page
Magnitude
The magnitude of the error is simply the deviation between the values
of the setpoint and the process variable. The magnitude of error at any
point in time compared to the previous error provides the basis for
determining the change in error. The change in error is also an
important value.

Duration
Duration refers to the length of time that an error condition has
existed.
Rate Of Change
The rate of change is shown by the slope of the error plot.

OFFSET
Offset is a sustained deviation of the process variable from the
setpoint. In the temperature control loop example, if the control
system held the process fluid at 100.5 °C consistently, even though
the setpoint is 100 °C, then an offset of 0.5 °C exists.
LOAD DISTURBANCE
A load disturbance is an undesired change in one of the factors that
can affect the process variable. In the temperature control loop
example, adding cold process fluid to the vessel would be a load
disturbance because it would lower the temperature of the process
fluid.
CONTROL ALGORITHM
A control algorithm is a mathematical expression of a control
function. Using the temperature control loop example, V in the
equation below is the fuel valve position, and e is the error. The
relationship in a control algorithm can be expressed as:
The fuel valve position (V) is a function (f) of the sign (positive or
negative) of the error (Figure 7.3).
Algorithm Example
Control algorithms can be used to calculate the requirements of much
more complex control loops than the one described here. In more
complex control loops, questions such as “How far should the valve
be opened or closed in response to a given change in setpoint?” and
“How long should the valve be held in the new position after the
process variable moves back toward setpoint?” need to be answered.
MANUAL AND AUTOMATIC CONTROL
Before process automation, people, rather than machines, performed
many of the process control tasks. For example, a human operator
might have watched a level gauge and closed a valve when the level
reached the setpoint. Control operations that involve human
action to make an adjustment are called manual control systems.
Conversely, control operations in which no human intervention is
required, such as an automatic valve actuator that responds to a level
controller, are called automatic control systems.
V = f(± e)

Control algorithms can be used to calculate the requirements of much
more complex control loops than the one described here. In more
complex control loops, questions such as “How far should the valve
be opened or closed in response to a given change in setpoint?” and
“How long should the valve be held in the new position after the
process variable moves back toward setpoint?” need to be answered.

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PROCESS VARIABLE

A process variable is a condition of the process fluid (a liquid or gas)
that can change the manufacturing process in some way. In the example of you sitting by the fire, the process variable was temperature. In the example of the tank in Figure 7.1, the process variable is level. Common process variables include:

• Pressure
• Flow
• Level
• Temperature
• Density
• Ph (acidity or alkalinity)
• Liquid interface (the relative amounts of different liquids that are combined in a vessel)
• Mass
• Conductivity

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The Control Loop

Imagine you are sitting in a cabin in front of a small fire on a cold
winter evening. You feel uncomfortably cold, so you throw another
log on the fire. Thisis an example of a control loop. In the
control loop, a variable (temperature) fell below the setpoint (your
comfort level), and you took action to bring the process back into the
desired condition by adding fuel to the fire. The control loop will
now remain static until the temperature

Control loops in the process control industry work in the same way,
requiring three tasks to occur:

• Measurement
• Comparison
• Adjustment

In Figure 7.1, a level transmitter (LT) measures the level in the tank
and transmits a signal associated with the level reading to a controller
(LIC). The controller compares the reading to a predetermined value,
in this case, the maximum tank level established by the plant
operator, and finds that the values are equal. The controller then
sends a signal to the device that can bring the tank level back to a
lower level—a valve at the bottom of the tank. The valve opens to let
some liquid out of the tank.
Many different instruments and devices may or may not be used in
control loops (e.g., transmitters, sensors, controllers, valves, pumps),
but the three tasks of measurement, comparison, and adjustment are
always present.

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The Importance of Process Control

Refining, combining, handling, and otherwise manipulating fluids to profitably produce end products can be a
precise, demanding, and potentially hazardous process. Small changes in a process can have a large impact
on the end result. Variations in proportions, temperature, flow, turbulence, and many other factors must be
carefully and consistently controlled to produce the desired end product with a minimum of raw materials and
energy. Process control technology is the tool that enables manufacturers to keep their operations running
within specified limits and to set more precise limits to maximize profitability, ensure quality and safety.

Process as used in the terms process control and process industry,
refers to the methods of changing or refining raw materials to create
end products. The raw materials, which either pass through or remain
in a liquid, gaseous, or slurry (a mix of solids and liquids) state
during the process, are transferred, measured, mixed, heated or
cooled, filtered, stored, or handled in some other way to produce the
end product.
Process industries include the chemical industry, the oil and gas
industry, the food and beverage industry, the pharmaceutical industry,
the water treatment industry, and the power industry.

Reducing variability can also save money by reducing the need for Activities
product padding to meet required product specifications. Padding
refers to the process of making a product of higher-quality than it
needs to be to meet specifications. When there is variability in the end
product (i.e., when process control is poor), manufacturers are forced
to pad the product to ensure that specifications are met, which adds
to the cost. With accurate, dependable process control, the setpoint
(desired or optimal point) can be moved closer to the actual product
specification and thus save the manufacturer money.

Manufacturers control the production process for
three reasons:

1. Reduce variability
2. Increase efficiency
3. Ensure safety

• Reduce Variability

Process control can reduce variability in the end product, which
ensures a consistently high-quality product. Manufacturers can also
save money by reducing variability. For example, in a gasoline
blending process, as many as 12 or more different components
may be blended to make a specific grade of gasoline. If the refinery
does not have precise control over the flow of the separate
components, the gasoline may get too much of the high-octane
components. As a result, customers would receive a higher grade
and more expensive gasoline than they paid for, and the refinery
would lose money. The opposite situation would be customers
receiving a lower grade at a higher price.

• Increase Efficiency

Some processes need to be maintained at a specific point to maximize
efficiency. For example, a control point might be the temperature at
which a chemical reaction takes place. Accurate control of temperature
ensures process efficiency. Manufacturers save money by minimizing
the resources required to produce the end product.

• Ensure Safety

A run-away process, such as an out-of-control nuclear or chemical
reaction, may result if manufacturers do not maintain precise control
of all of the processg variables. The consequences of a run-away
process can be catastrophic.
Precise process control may also be required to ensure safety. For
example, maintaining proper boiler pressure by controlling the inflow
of air used in combustion and the outflow of exhaust gases is crucial
in preventing boiler implosions that can clearly threaten the safety of
workers.

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Introduction To Process Control

Control in process industries refers to the regulation of all aspects of the process. Precise control of level,
temperature, pressure and flow is important in many process applications.

Process control refers to the methods that are used to control process
variables when manufacturing a product. For example, factors such
as the proportion of one ingredient to another, the temperature of the
materials, how well the ingredients are mixed, and the pressure under
which the materials are held can significantly impact the quality of
an end product.

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VFD- VFD OPERATION

Understanding the basic principles behind VFD operation requires understanding the three basic sections of the VFD: the rectifier, dc bus, and inverter.
The voltage on an alternating current (ac) power supply rises and falls in the pattern of a sine wave
(see Figure 1). When the voltage is positive, current
flows in one direction; when the voltage is negative,
the current flows in the opposite direction. This type
of power system enables large amounts of energy to
be efficiently transmitted over great distances             .
The rectifier in a VFD is used to convert incoming
ac power into direct current (dc) power. One rectifier
 will allow power to pass through only when the
voltage is positive. A second rectifier will allow
power to pass through only when the voltage is negative.
 Two rectifiers are required for each phase of
power. Since most large power supplies are three
phase, there will be a minimum of 6 rectifiers used
(see Figure 2). Appropriately, the term “6 pulse” is
used to describe a drive with 6 rectifiers. A VFD
may have multiple rectifier sections, with 6 rectifiers per section, enabling a VFD to be “12 pulse,”
“18 pulse,” or “24 pulse.” The benefit of “multipulse” VFDs will be described later in the harmonics section.
Rectifiers may utilize diodes, silicon controlled rectifiers (SCR), or transistors to rectify power. Diodes
are the simplest device and allow power to flow any
time voltage is of the proper polarity. Silicon controlled rectifiers include a gate circuit that enables a
microprocessor to control when the power may
begin to flow, making this type of rectifier useful for
solid-state starters as well. Transistors include a gate
circuit that enables a microprocessor to open or
close at any time, making the transistor the most
useful device of the three. A VFD using transistors
in the rectifier section is said to have an “active
front end.”
After the power flows through the rectifiers it is
stored on a dc bus. The dc bus contains capacitors
to accept power from the rectifier, store it, and later
deliver that power through the inverter section. The
dc bus may also contain inductors, dc links, chokes,
or similar items that add inductance, thereby
smoothing the incoming power supply to the dc bus.
The final section of the VFD is referred to as an
“inverter.” The inverter contains transistors that
deliver power to the motor. The “Insulated Gate
Bipolar Transistor” (IGBT) is a common choice in
modern VFDs. The IGBT can switch on and off several thousand times per second and precisely control
the power delivered to the motor. The IGBT uses a
method named “pulse width modulation” (PWM)
to simulate a current sine wave at the desired frequency to the motor.
Motor speed (rpm) is dependent upon frequency.
Varying the frequency output of the VFD controls
motor speed:
Speed (rpm) = frequency (hertz) x 120 / no. of poles

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Variable Frequency Drives (VFD)

A variable-frequency drive (VFD) (also termed adjustable-frequency drive, variable-speed drive, AC drive, micro drive or inverter drive) is a type of adjustable-speed drive used in electro-mechanical drive systems to control AC motor speed and torque by varying motor input frequency and voltage.

VFDs are used in applications ranging from small appliances to the largest of mine mill drives and compressors. However, about a third of the world's electrical energy is consumed by electric motors in fixed-speed centrifugal pump, fan and compressor applications and VFDs' global market penetration for all applications is still relatively small. This highlights especially significant energy efficiency improvement opportunities for retrofitted and new VFD installations.

Over the last four decades, power electronics technology has reduced VFD cost and size and improved performance through advances in semiconductor switching devices, drive topologies, simulation and control techniques, and control hardware and software.

VFDs are available in a number of different low and medium voltage AC-AC and DC-AC topologies

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