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1.1 For the purpose of AUTOMATION specialized hardened computers, referred
to as programmable logic controllers (PLCs), are frequently used to synchronize the
flow of inputs from (physical) sensors and events with the flow of outputs to
actuators and events. This leads to precisely controlled actions that permit a tight
control of almost any industrial process. Human-machine interfaces (HMI) or
computer human interfaces (CHI), formerly known as man-machine interface, are
usually employed to communicate with PLCs and other computers, such as entering
and monitoring temperatures or pressures for further automated control or
emergency response. Service personnel who monitor and control these interfaces
are often referred to as stationary engineers.
1.2 Automation has had a notable impact in a wide range of highly visible
industries beyond manufacturing. Once-ubiquitous telephone operators have been
replaced largely by automated telephone switchboards and answering machines.
Medical processes such as primary screening in electrocardiography or radiography
and laboratory analysis of human genes, sera, cells, and tissues are carried out at
much greater speed and accuracy by automated systems. Automated teller machines
have reduced the need for bank visits to obtain cash and carry out transactions. In
general, automation has been responsible for the shift in the world economy from
agrarian to industrial in the 19th century and from industrial to services in the 20th
century.
1.3 The widespread impact of industrial automation raises social issues, among
them its impact on employment. Historical concerns about the effects of automation
date back to the beginning of the industrial revolution, when a social movement of
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1.6 Another major shift in automation is the increased emphasis on flexibility and
convertibility in the manufacturing process. Manufacturers are increasingly
demanding the ability to easily switch from manufacturing Product A to
manufacturing Product B without having to completely rebuild the production lines.
Flexibility and distributed processes have led to the introduction of Automated
Guided Vehicles with Natural Features Navigation.
1.7 The widespread impact of industrial automation raises social issues, among
them its impact on employment. Historical concerns about the effects of automation
date back to the beginning of the industrial revolution, when a social movement of
English textile machine operators in the early 1800s known as the Luddites
protested against Jacquard's automated weaving looms often by destroying such
textile machines that they felt threatened their jobs. One author made the
following case. When automation was first introduced, it caused widespread fear. It
was thought that the displacement of human operators by computerized systems
would lead to severe unemployment.
1.8 At first glance, automation might appear to devalue labor through its
replacement with less-expensive machines; however, the overall effect of this on
the workforce as a whole remains unclear. Today automation of the workforce is
quite advanced, and continues to advance increasingly more rapidly throughout the
world and is encroaching on ever more skilled jobs, yet during the same period the
general well-being and quality of life of most people in the world (where political
factors have not muddied the picture) have improved dramatically. What role
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automation has played in these changes has not been well studied. Currently, for
manufacturing companies, the purpose of automation has shifted from increasing
productivity and reducing costs, to broader issues, such as increasing quality and
flexibility in the manufacturing process. Different types of automation tools exist
Block Diagram Of Industrial Automation
5
Field Equipmentsand Machineries
ProgrammableLogic Controller
AC OR DCDrives
Auxiliaries Sensors
SCADA Systemwith HMI
Screens
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a) A Human-Machine Interface or HMI is the apparatus which presents process
data to a human operator, and through this, the human operator, monitors and
controls the process.
b) A supervisory (computer) system, gathering (acquiring) data on the process
and sending commands (control) to the process.
c) Remote Terminal Units (RTUs) connecting to sensors in the process,
converting sensor signals to digital data and sending digital data to the supervisory
system.
d) Programmable Logic Controller (PLCs) used as field devices because they are
more economical, versatile, flexible, and configurable than special-purpose RTUs.
e) Communication infrastructure connecting the supervisory system to the
Remote Terminal Units
There is, in several industries, considerable confusion over the differences between
SCADA systems and Distributed control systems (DCS). Generally speaking, a
SCADA system usually refers to a system that coordinates, but does not control
processes in real time. The discussion on real-time control is muddied somewhat by
newer telecommunications technology, enabling reliable, low latency, high speed
communications over wide areas. Most differences between SCADA and
Distributed control system DCS are culturally determined and can usually be
ignored. As communication infrastructures with higher capacity become available,
the difference between SCADA and DCS will fade.
The term SCADA usually refers to centralized systems which monitor and control
entire sites, or complexes of systems spread out over large areas (anything between
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an industrial plant and a country). Most control actions are performed automatically
by remote terminal units ("RTUs") or by programmable logic controllers ("PLCs").
Host control functions are usually restricted to basic overriding orsupervisory level
intervention. For example, a PLC may control the flow of cooling water through
part of an industrial process, but the SCADA system may allow operators to change
the set points for the flow, and enable alarm conditions, such as loss of flow and
high temperature, to be displayed and recorded. The feedback control loop passes
through the RTU or PLC, while the SCADA system monitors the overall
performance of the loop.
Data acquisition begins at the RTU or PLC level and includes meter readings and
equipment status reports that are communicated to SCADA as required. Data is
then compiled and formatted in such a way that a control room operator using the
HMI can make supervisory decisions to adjust or override normal RTU (PLC)
controls. Data may also be fed to a Historian, often built on a commodity Database
Management System, to allow trending and other analytical auditing.
2.1.1 Human Machine Interface:
A Human-Machine Interface or HMI is the apparatus which presents process data to
a human operator, and through which the human operator controls the process.
An HMI is usually linked to the SCADA system's databases and software programs,
to provide trending, diagnostic data, and management information such as
scheduled maintenance procedures, logistic information, detailed schematics for a
particular sensor or machine, and expert-system troubleshooting guides.
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The HMI system usually presents the information to the operating personnel
graphically, in the form of a mimic diagram. This means that the operator can see a
schematic representation of the plant being controlled. For example, a picture of a
pump connected to a pipe can show the operator that the pump is running and how
much fluid it is pumping through the pipe at the moment. The operator can then
switch the pump off. The HMI software will show the flow rate of the fluid in the
pipe decrease in real time. Mimic diagrams may consist of line graphics and
schematic symbols to represent process elements, or may consist of digital
photographs of the process equipment overlain with animated symbols.
The HMI package for the SCADA system typically includes a drawing program
that the operators or system maintenance personnel use to change the way these
points are represented in the interface. These representations can be as simple as an
on-screen traffic light, which represents the state of an actual traffic light in the
field, or as complex as a multi-projector display representing the position of all of
the elevators in a skyscraper or all of the trains on a railway.
An important part of most SCADA implementations are alarms. An alarm is a
digital status point that has either the value NORMAL or ALARM. Alarms can be
created in such a way that when their requirements are met, they are activated. An
example of an alarm is the "fuel tank empty" light in a car. The SCADA operator's
attention is drawn to the part of the system requiring attention by the alarm. Emails
and text messages are often sent along with an alarm activation alerting managers
along with the SCADA operator.
2.1.2 Hardware solutions:
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SCADA solutions often have Distributed Control System (DCS) components. Use
of "smart" RTUs or PLCs, which are capable of autonomously executing simple
logic processes without involving the master computer, is increasing. A functional
block programming language, IEC 61131-3 (Ladder Logic), is frequently used to
create programs which run on these RTUs and PLCs. Unlike a procedural language
such as the C programming language or FORTRAN, IEC 61131-3 has minimal
training requirements by virtue of resembling historic physical control arrays. This
allows SCADA system engineers to perform both the design and implementation of
a program to be executed on an RTU or PLC. Since about 1998, virtually all major
PLC manufacturers have offered integrated HMI/SCADA systems, many of them
using open and non-proprietary communications protocols. Numerous specialized
third-party HMI/SCADA packages, offering built-in compatibility with most major
PLCs, have also entered the market, allowing mechanical engineers, electrical
engineers and technicians to configure HMIs themselves, without the need for a
custom-made program written by a software developer.
2.2 Around the world, SCADA systems control:
Electric power generation, transmission and distribution: Electric utilities
use SCADA systems to detect current flow and line voltage, to monitor the
operation of circuit breakers, and to take sections of the power grid online or
offline.
Water and sewage: State and municipal water utilities use SCADA to monitor
and regulate water flow, reservoir levels, pipe pressure and other factors.
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Where are you lacking accurate, real-time data about key processes that affect
your operations?
Real-Time Monitoring and Control Increases Efficiency and Maximizes
Profitability
2.4 A SCADA system performs four functions:
1. Data acquisition
2. Networked data communication
3. Data presentation
4. Control
2.4.1 Data Acquisition:
First, the systems you need to monitor are much more complex than just one
machine with one output. So a real-life SCADA system needs to monitor hundreds
or thousands of sensors. Some sensors measure inputs into the system (for example,
water flowing into a reservoir), and some sensors measure outputs (like valve
pressure as water is released from the reservoir).
Some of those sensors measure simple events that can be detected by a
straightforward on/off switch, called a discrete input (or digital input). For example,
in our simple model of the widget fabricator, the switch that turns on the light
would be a discrete input. In real life, discrete inputs are used to measure simple
states, like whether equipment is on or off, or tripwire alarms, like a power failure
at a critical facility.
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closed proprietary protocols, but today the trend is to open, standard protocols and
protocol mediation.
Sensors and control relays are very simple electric devices that cant generate or
interpret protocol communication on their own. Therefore the remote telemetry unit
(RTU) is needed to provide an interface between the sensors and the SCADA
network. The RTU encodes sensor inputs into protocol format and forwards them to
the SCADA master; in turn, the RTU receives control commands in protocol format
from the master and transmits electrical signals to the appropriate control relays.
2.4.3 Data Presentation:
The only display element in our model SCADA system is the light that comes on
when the switch is activated. This obviously wont do on a large scale you cant
track a light board of a thousand separate lights, and you dont want to pay someone
simply to watch a light board, either.
A real SCADA system reports to human operators over a specialized computer thatis variously called a master station, an HMI (Human-Machine Interface) or an HCI
(Human-Computer Interface).
The SCADA master station has several different functions. The master
continuously monitors all sensors and alerts the operator when there is an alarm
that is, when a control factor is operating outside what is defined as its normal
operation. The master presents a comprehensive view of the entire managed system,
and presents more detail in response to user requests. The master also performs data
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processing on information gathered from sensors it maintains report logs and
summarizes historical trends.
An advanced SCADA master can add a great deal of intelligence and automation to
your systems management, making your job much easier.
2.4.4 Control:
Unfortunately, our miniature SCADA system monitoring the widget fabricator
doesnt include any control elements. So lets add one. Lets say the human
operator also has a button on his control panel. When he presses the button, it
activates a switch on the widget fabricator that brings more widget parts into the
fabricator.
Now lets add the full computerized control of a SCADA master unit that controls
the entire factory. You now have a control system that responds to inputs elsewhere
in the system. If the machines that make widget parts break down, you can slow
down or stop the widget fabricator. If the part fabricators are running efficiently,you can speed up the widget fabricator.
If you have a sufficiently sophisticated master unit, these controls can run
completely automatically, without the need for human intervention. Of course, you
can still manually override the automatic controls from the master station.
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3.1 Basic PLC Operation: PLCs consist of input modules or points, a Central
Processing Unit (CPU), and output modules or points. An input accepts a variety of
digital or analog signals from various field devices (sensors) and converts them into
a logic signal that can be used by the CPU. The CPU makes decisions and executes
control instructions based on program instructions in memory. Output modules
convert control instructions from the CPU into a digital or analog signal that can be
used to control various field devices (actuators). A programming device is used to
input the desired instructions. These instructions determine what the PLC will do
for a specific input. An operator interface device allows process information to be
displayed and new control parameters to be entered.
Pushbuttons (sensors), in this simple example, connected to PLC inputs, can be
used to start and stop a motor connected to a PLC through a motor starter (actuator).
Prior to PLCs, many of these control tasks were solved with contactor or relay
controls. This is often referred to as hardwired control. Circuit diagrams had to be
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InputModule
OperatorInterface
ProgrammingDevice
OutputModule
CPUCentral
processing unit
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designed, electrical components specified and installed, and wiring lists created.
Electricians would then wire the components necessary to perform a specific task.
If an error was made, the wires had to be reconnected correctly. A change in
function or system expansion required extensive component changes and rewiring.
3.2 Advantages of PLCs:
The same, as well as more complex tasks can be done with a PLC. Wiring between
devices and relay contacts is done in the PLC program. Hard-wiring, though still
required to connect field devices, is less intensive. Modifying the application and
correcting errors are easier to handle. It is easier to create and change a program in
a PLC than it is to wire and re-wire a circuit.
Following are just a few of the advantages of PLCs:
Smaller physical size than hard-wire solutions.
Easier and faster to make changes.
PLCs have integrated diagnostics and override functions.
Diagnostics are centrally available.
Applications can be immediately documented.
3.3 Logic 0, Logic 1:
Programmable controllers can only understand a signal that is On or Off (present or
not present). The binary system is a system in which there are only two numbers, 1
and 0. Binary 1 indicates that a signal is present, or the switch is On. Binary 0
indicates that the signal is not present, or the switch is Off.
The language of PLCs consists of a commonly used set of terms; many of which are
unique to PLCs.
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3.4 In order to understand the ideas and concepts of PLCs, an understanding of
these terms is necessary.
3.4.1 Sensor: A sensor is a device that converts a physical condition into an
electrical signal for use by the PLC. Sensors are connected to the input of a PLC. A
pushbutton is one example of a sensor that is connected to the PLC input. An
electrical signal is sent from the pushbutton to the PLC indicating the condition
(open/ closed) of the pushbutton contacts.
3.4.2 Actuators: Actuators convert an electrical signal from the PLC into aphysical condition. Actuators are connected to the PLC output. A motor starter is
one example of an actuator that is connected to the PLC output. Depending on the
output PLC signal the motor starter will either start or stop the motor.
3.4.3 Discrete Input: A discrete input also referred to as a digital input, is an
input that is either in an ON or OFF condition. Pushbuttons, toggle switches, limit
switches, proximity switches, and contact closures are examples of discrete sensors
which are connected to the PLCs discrete or digital inputs. In the ON condition a
discrete input may be referred to as a logic 1 or a logic high. In the OFF condition a
discrete input may be referred to as a logic 0 or a logic low.
A Normally Open (NO) pushbutton is used in the following example. One side of
the pushbutton is connected to the first PLC input. The other side of the pushbutton
is connected to an internal 24 VDC power supply. Many PLCs require a separate
power supply to power the inputs. In the open state, no voltage is present at the PLC
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input. This is the OFF condition. When the pushbutton is depressed, 24 VDC is
applied to the PLC input.
3.4.4 Analog Inputs: An analog input is a continuous, variable signal. Typical
analog inputs may vary from 0 to 20 milliamps, 4 to 20 milliamps, or 0 to 10 volts.
In the following example, a level transmitter monitors the level of liquid in a tank.
Depending on the level transmitter, the signal to the PLC can either increase or
decrease as the level increases or decreases.
3.4.5 Discrete Outputs: A discrete output is an output that is either in an ON orOFF condition. Solenoids, contactor coils, and lamps are examples of actuator
devices connected to discrete outputs. Discrete outputs may also be referred to as
digital outputs. In the following example, a lamp can be turned on or off by the PLC
output it is connected to.
3.4.6 Analog Outputs: An analog output is a continuous, variable signal. The
output may be as simple as a 0-10 VDC level that drives an analog meter. Examples
of analog meter outputs are speed, weight, and temperature. The output signal may
also be used on more complex applications such as a current-to-pneumatic
transducer that controls an air-operated flow-control valve.
3.4.7 CPU: The central processor unit (CPU) is a microprocessor system that
contains the system memory and is the PLC decision making unit. The CPU
monitors the inputs and makes decisions based on instructions held in the program
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memory. The CPU performs relay, counting, timing, data comparison, and
sequential operations.
3.5 Programming:
A program consists of one or more instructions that accomplish a task.
Programming a PLC is simply constructing a set of instructions. There are several
ways to look at a program such as ladder logic, statement lists, or function block
diagrams.
3.5.1 Ladder Logic: Ladder logic (LAD) is one programming language usedwith PLCs. Ladder logic uses components that resemble elements used in a line
diagram format to describe hard-wired control. The left vertical line of a ladder
logic diagram represents the power or energized conductor. The output element or
instruction represents the neutral or return path of the circuit. The right vertical line,
which represents the return path on a hard-wired control line diagram, is omitted.
Ladder logic diagrams are read from left-to-right, top-to-bottom. Rungs are
sometimes referred to as networks. A network may have several control elements,
but only one output coil.
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CHAPTER - 4
DRIVES
4.1 AC DRIVESAC MOTORS BASICS
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In an induction motors, when the 3-phase stator windings, are fed by 3 phase AC
supply then, a magnetic flux of constant magnitude, but rotating at synchronous
speed, is set up. The flux passes through the air gap; sweeps past the rotor surface
and so cuts the rotor conductors, which as yet, are stationary. Due to the relative
speed between the rotating flux and the stationary conductors, an E.M.F. is induced
in the letter according to Faradays law of ElectroMagnetic induction. The
frequency of the induced E.M.F. is the same as the supply frequency. Its magnitude
is proportional to the relative velocity between the flux and the conductors and
Flemings Right Hand Rule gives its directions. The Synchronous Speed (Ns) of an
induction motor is given by,
Ns = (120*f) / P
Where,
F= frequency
P= nos of Pole.
In an induction motor, the motors run at a speed, which is always less than the
speed of the stator field. The difference in speeds depends upon the load on the
motor. The difference between the synchronous speed Ns & the actual speed N of
the rotor is known as Slip.
Therefore, Slip (S) = (Ns - N) / Ns
Where, N is the rotor speed.
Therefore, Actual speed of shaft (N) = Ns * (1- S).
The torque equation of an AC motor is given as:
Torque (T) = Ia *
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Where, Ia = stator current. = Air gap flux.
4.2 VOLTAGE/FREQUENCY CONCEPT:
The V/F concept is mainly used in AC drives. Therefore AC drives are also known
as V/F DRIVES.
In drives it is necessary for a motor to deliver rated torque at set speed. In order to
change the speed of AC motor stator frequency is to be changed. Since torque
delivered by motor is proportional to the product of the stator current and flux, it is
essential that motor flux be to be kept constant. This means at any speed, motor can
deliver torque (maximum up to rated torque) demanded by load and is roughly
proportional to the product of stator current and motor flux. So we have,
Torque = Ia *
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Where, Ia = Armature current which varies with load
= Motor flux which remains constant
VOLTAGE / FREQUENCY CURVE:
The EMF generated is proportional to the rate at which conductors cut the flux. Sowe have,
EMF = Rate of change of flux = V / F
i.e. V = d / dt
d = V * dt
= V * T
i e. = V / FTherefore, in order to maintain constant flux in motor, the ratio of voltage to frequency is
always maintained constant so that motor can deliver rated torque through out the speed
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reduces the pulsation or the ripples contents in the rectified output and gives reasonably
constant dc output. Then true inverter function occurs i.e. Variable Voltage Variable
Frequency control. The main role is performed by the switching element which is
invariably a semiconductor device.i.e.BJTs, IGBTs.
TESTING PROCEDURE
During testing of the AC Drive following tests are carried out:
1. Visual checks
2. Electrical checks
Visual checks
Carry out visual inspection as per Inspection Report for AC drives.
Output and Input supply terminals of panels should be distinctly identified and
output terminals of inverter are connected to the motor.
Check correctness and firmness of wires, cables and earth of the panel.
Electrical checks:
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Give the power supply to the panel according to the scheme.
Check logic circuit as per scheme.
Check phase sequence of auxiliary supplies.
Verify that the direction of airflow of panel +fan is upward.
Put the inverter ON.
Set the parameters
Check flash ID.
Set control circuit's terminals according the scheme.
Connect test motor at parameter outgoing terminals of panel and check RUN,
SPEED RAISE, SPEED LOWER, STOP commands in all possible selections
according to the scheme.
Check Forward and Reverse RUN commands
Check the operation by varying the reference (4-20 mA or 0-10V) in Remote mode.
Check correctness and firmness of wires, cables and earth of the panel.
4.4 DC MOTOR
DC MOTOR BASICS
An electrical motor is a machine, which converts electrical energy into mechanical
energy. The basic principle is that when a current carrying conductor is placed in a
magnetic field it experiences a mechanical force whose direction is given by
Flemings left hand rule. There is no basic difference between the construction of adc generator and dc motor; the same machine can be used as a generator or a motor.
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In case of a dc motor the field electromagnet and armature conductors are supplied
with the current from mains supply and mechanical force is obtained by rotation of
armature. In case of dc motor, the e.m.f (E) is less than the applied voltage (V) and
the direction of the current (Ia) is the reverse of that when the machine is used as a
generator.
E = V IaRa OR V = E + IaRa
As the e.m.f. generated in the armature of a motor is in opposition to the
applied voltage, it is also referred as Back emf.
4.5 WHY WE USE A DC DRIVE?
Basically, DC drive is used due to following things: -
DC drive has precise control on speed & torque.
DC drive is a soft starter means it has ramp input. It is useful in order to minimize
the maintenance of the DC motor.
DC drive has good efficiency, which is an around 80 % to 95 % giving good result
during running condition of DC motor.
DC drive gives good speed regulation means it can sense load variation (from no-
load to full-load) in proper manner & maintain the same speed.
DC drive has speed controlling range from 0% to 100%, so it can control speed
from 0 rpm to rated rpm of the motor.
DC drive has 0.01% accuracy which means motor can run at 0.01% of its rated rpm
speed.
DC drive gives various types of protection over the motor control like Feedback
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loss, Integrated Overload, Phase sequence failure, Under Voltage, Over Voltage,
Over Current, Over Speed, over temperature etc.
4.6 CLASSIFICATION OF DC DRIVES:
There are two types of converter used in DC Drives. These are following: -
DC Thyristor converter drives
DC Transistor converter drives.
4.6.1 DC Thyristor Converter Drives:
These drives are available in rating from a few hundred watts up to several
megawatts and have a great variety of applications in industries. But these drives
have certain advantages & disadvantages:
Advantages:
1. These are simple and highly efficient than their transistor equivalents.
2. Thyristors are available with very high current and voltage ratings.
Disadvantages:
1. Because of delay in thyristor operation (3.3ms), the current control loop
bandwidth of the thyristor converter is limited to approximately 25Hz, which is too
low for many servo drive applications.
2. Thyristor phase control rectifiers have poor input power factor, particularly at
low output voltages.
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3. Electronic short circuit protection is not possible with thyristorised converters.
Fuses normally accomplish protection.
4.6.2 DC Transistor Converter Drives:
These drives are usually of low power rating and are typically used in rather
specialist applications. The main advantage of DC transistor drives is that, they can
be battery supplied or mains supplied.
Advantages:
1. Due to ability of transistor to interrupt current, it operate from battery or DCsupply.
2. Transistor phase control rectifiers have high input power factor, particularly at
low output voltages. Electronic short circuit protection is possible with
transistorized converters.
3. Fuses normally accomplish protection.
Disadvantage:
1. These are more complex and less efficient than their thyristor equivalents.
2. Transistors are not available with very high current and voltage ratings.
4.7 SPEED CONTROL OF DC MOTOR USING DC DRIVES:
The speed control of DC motor is given by
N = (Va IaRa) /
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From the above equation we can say that, the speed of separately excited DC motor
can be varied in two ways:
1 .Field current is kept constant while the armature voltage is varied from zero to
rated value.
2. Armature voltage is kept constant at the rated value and field current is varied
from maximum to minimum.
These two speed control result in speed-torque characteristics, which are different
from each other. Armature voltage control gives constant torque and variable
power characteristics while variable field flux gives constant power and variable
torque characteristics.
4.7.1 Armature Voltage Control:
This method is used for controlling speed up to base speed of the motor. Base
speed is the speed at which the motor delivers the rated power and torque at rated
armature and field current. Since the field flux is kept constant, the torque is
entirely dependent on the value of armature current. Once the value of starting
torque i.e. starting current is determined, the armature voltage can be varied
smoothly up-to base speed, keeping the armature current within the fixed limit. As
the motor speeds up, Eb increases and the current tends to lower but since the
voltage is also increasing, the current level can be maintained. As the current and
the flux are kept constant the motor has a constant torque characteristics and power
of the machine rises.
By varying the armature voltage below the nominal rated voltage, motor can be
made to operate at various speeds in a wider range delivering full torque and reduce
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4.8 DRIVES ADVANTAGES AND DISADVANTAGES:
34
Advantages Disadvantages
1.Potentially lower installed cost above
50 HP.
Brush Maintenance.
2. Good energy efficiency &
regeneration of power can possible by 4-
Quadrant method.
Higher Repair Costs.
3. Speed control of DC Drive is better
than AC Drive.
Limited Dynamic Response due to line
commutation restrictions, coupled with
higher mass moments of inertia imposed
by the wound field armature.
4. DC motor tuning is good in DC Drive
means current auto tuning is done in
proper manner & also all gains are set by
auto tuning.
Limited range to 5,000-hp, due to
commutation restrictions.
5. Few distance limitations. Potential for rapid acceleration to
destructive velocities upon loss of the
stationary field.
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CHAPTER 5
SENSORS AND AUXILLIARIES
Technical Education Program, designed to prepare our distributors to sell Energy &
Automation products more effectively. This course covers Sensors and related
products. SENSORS Welcome to another course in the STEP 2000 series, upon
completion of Sensors you should be able to describe advantages, disadvantages,
and applications of limit switches, photoelectric sensors, inductive sensors,
capacitive sensors, and ultrasonic sensors. Describe design and operating principles
of mechanical limit switches.
Identify components of International and North American mechanical limit
switches describe design and operating principles of inductive, capacitive,
ultrasonic, and photoelectric sensors and describe differences and similarities.
Apply correction factors where appropriate to proximity sensors Identify the
various scan techniques of photoelectric sensors Identify ten categories of
inductive sensors and sensors in each category. Describe the effects of dielectric
constant on capacitive proximity sensors.
Identify environmental influences on ultrasonic sensors. Identify types of ultrasonicsensors that require manual adjustment, can be used with SONPROG, and require
the use of a signal evaluator. Describe the difference between light operate and dark
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operate modes of a photoelectric sensor. Describe the use of fiber optics and laser
technology used in Siemens photoelectric sensors.
Select the type of sensor best suited for a particular application based on material,
sensing distance, and sensor load requirements. This knowledge will help you better
understand customer applications. In addition, you will be better able to describe
products to customers and determine important differences between products. You
should complete Basics of Electricity and Basics of Control Components before
attempting Sensors. An understanding of many of the concepts covered in Basics
of Electricity and Basics of Control Components is required forSensors.
5.1 Types of switch
5.1.1 Limit Switch
High Current Capability
Low Cost
Familiar "Low- Tech" Sensing
Requires Physical Contact with Target
Very Slow Response
Contact Bounce
Interlocking
Basic End-of- Travel Sensing
5.1.2 Photoelectric
Senses all Kinds of Materials
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Long Life
Longest Sensing Range
Very Fast Response Time
Lens Subject to Contamination
Sensing Range Affected by Color and Reflectivity of Target
Packaging
Material Handling
Parts Detection
5.1.3 InductiveResistant to Harsh Environments
Very Predictable
Long Life.
Easy to Install.
Distance Limitations.
Industrial and Machines.
Machine Tool.
Senses Metal- Only Targets.
5.1.4 Capacitive
Detects Through Some Containers.
Can Detect Non-Metallic Targets.
Very Sensitive to Extreme Environmental Changes.
Level Sensing.
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5.1.5 Ultrasonic
Senses all Materials
Resolution
Repeatability
Sensitive to Temperature Changes
Anti-Collision
Doors
Web Brake
Level Control
5.2 Contact Arrangement: Contacts are available in several configurations.
They may be normally open (NO), normally closed (NC), or a combination of
normally open and normally closed contacts.
Circuit symbols are used to indicate an open or closed path of current flow.
Contacts are shown as normally open (NO) or normally closed (NC). The standard
method of showing a contact is by indicating the circuit condition it produces whenthe contact actuating device is in the DE energized or nonoperatic state. For the
purpose of explanation in this text a contact or device shown in a state opposite of
its normal state will be highlighted. Highlighted symbols used to indicate the
opposite state of a contact or devices are not legitimate symbols.
They are used here for illustrative purposes only. Mechanical limit switches, which
will be covered in the next section, use a different set of symbols. Highlighted
symbols are used for illustrative purposes only.
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5.3 Limit Switches: A typical limit switch consists of a switch body and an
operating head. The switch body includes electrical contacts to energize and DE
energizes a circuit. The operating head incorporates some type of lever arm or
plunger, referred to as an actuator. The standard limit switch is a mechanical device
that uses physical contact to detect the presence of an object (target). When the
target comes in contact with the actuator, the actuator is rotated from its normal
position to the operating position. This mechanical operation activates contacts
within the switch body.
Principle of Operation A number of terms must be understood to understand howa mechanical limit switch operates. The free position is the position of the actuator
when no external force is applied. Pretravel is the distance or angle traveled in
moving the actuator from the free position to the operating position. The operating
position is where contacts in the limit switch change from their normal state (NO or
NC) to their operated state. Over travel is the distance the actuator can travel safely
beyond the operating point. Differential travel is the distance traveled between the
operating position and the release position. The release position is where the
contacts change from their operated state to their normal state. Release travel is the
distance traveled from the release position to the free position.
Snap-Action Contacts There are two types of contacts, snap-action and slow-
break. Snap-action contacts open or close by a snap action regardless of the actuator
speed. When force is applied to the actuator in the direction of travel, pressure
builds up in the snap spring. When the actuator reaches the operating position of
travel, a set of moveable contacts accelerates from its normal position towards a set
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of fixed contacts. As force is removed from the actuator it returns to its free
position. When the actuator reaches the release position the spring mechanism
accelerates the moveable contact back to its original state.
Since the opening or closing of the contacts is not dependent on the speed of the
actuator, snap-action contacts are particularly suited for low actuator speed
applications. Snap action contacts are the most commonly used type of contact.
Slow-Break Contacts Switches with slow-break contacts have moveable contacts
that are located in a slide and move directly with the actuator. This ensures the
moveable contacts are forced directly by the actuator. Slow-break contacts can
either be break-before-make or make-before-break. In slow-break switches with
break-before-make contacts, the normally closed contact opens before the normally
open contact closes. This allows the interruption of one function before
continuation of another function in a control sequence. In slow-break switches with
make-before-break contacts, the normally open contact closes before the normally
closed contact opens. This allows the initiation of one function before the
interruption of another function.
NO NC NO NC
Free Position Open Closed Open Closed Transition Open Open Closed Closed
Operated State Closed Open Closed Open
Break-Before-Contact State Make Make-Before-Break
Contact Arrangements There are two basic contact configurations used in limit
switches: single-pole, double-throw (SPDT) and double-pole, double-throw
(DPDT). This terminology may be confusing if compared to similar terminology for
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other switch or relay contacts, so it is best just to remember the following points.
The single-pole, double-throw contact arrangement consists of one normally open
(NO) and one normally closed (NC) contact. The double-pole, double-throw
(DPDT) contact arrangement consists of two normally open (NO) and two normally
closed (NC) contacts. There are some differences in the symbology used in the
North American and International style limit switches. Make Break
5.4 Actuators:Several types of actuators are available for limit switches, some
of which are shown below. There are also variations of actuator types. Actuators
shown here are to provide you with a basic knowledge of various types available.The type of actuator selected depends on the application.
Flexible Loop Flexible loop and spring rod actuators can be actuated from all
Spring Roddirections, making them suitable for applications in which the direction
of approach is constantly changing.
Plungers Plunger type actuators are a good choice where short, controlled machine
movements are present or where space or mounting does not permit a lever type
actuator. The plunger can be activated in the direction of plunger stroke, or at a
right angle to its axis.
Mounting Considerations When using plain and side plunger actuators the cam
should be operated in line with the push rod axis. Consideration should be given so
as not to exceed the over travel specifications. In addition, the limit switch should
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not be used as a mechanical stop for the cam. When using roller top plunger the
same considerations should be given as with lever arm actuators.
CONCLUSION
5.1 Automation plays an increasingly important role in the global economy and in
daily experience. Engineers strive to combine automated devices with mathematical
and organizational tools to create complex systems for a rapidly expanding range of
applications and human activities.
5.2Automation provides 100% accuracy all time. So the failures and mismatch in
production completely eliminates. It makes the systems efficiency higher than
manual as well as it controls wastages. So the overall savings increases. It provides
safety to human being. By that industry can achieves the safety majors and ISO and
OHSAS reputation. It makes the operation faster than manual which causes higher
production and proper utilization of utilities. It increases the production by which
the cost of each product decreases and industry profit increases. It provides smooth
control on system response. It provides repeatability, so that the same kinds of
products are easier to manufacture at different stages without wasting time. It
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provides quality control, so that the products become reliable which improves
industrial reputation in market. It provides integration with business systems. It can
reduce labor costs, so the final profit increases.
5.3 Industrial automation is very compulsory need of industries in todays scenario
to meet market competition.
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