project report on industrial automation

7/27/2019 Project Report on Industrial Automation

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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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http://en.wikipedia.org/wiki/Data_acquisitionhttp://en.wikipedia.org/wiki/Data_acquisition


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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.

project report on industrial automation

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