CHAPTER – 1
INTRODUCTION
Industrial automation or numerical control is the use of control systems such as computers to control industrial machinery and processes, reducing the need for human intervention. In the scope of industrialization, automation is a step beyond mechanization. Whereas mechanization provided human operators with machinery to assist them with the physical requirements of work, automation greatly reduces the need for human sensory and mental requirements as well. Processes and systems can also be automated. 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. Many roles for humans in industrial processes presently lie beyond the scope of automation. Human-level pattern recognition, language recognition, and language production ability are well beyond the capabilities of modern mechanical and computer systems. Tasks requiring subjective assessment or synthesis of complex sensory data, such as scents and sounds, as well as high-level tasks such as strategic planning, currently require human expertise. In many cases, the use of humans is more cost-effective than mechanical approaches even where automation of industrial tasks is possible.
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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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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. 1.4 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. The old focus on using automation simply to increase productivity and reduce costs was seen to be short-sighted, because it is also necessary to provide a skilled workforce who can make repairs and manage the machinery. Moreover, the initial costs of automation were high and often could not be recovered by the time entirely new manufacturing processes replaced the old. (Japan's "robot junkyards" were once world famous in the manufacturing industry.) 1.5 Automation is now often applied primarily to increase quality in the manufacturing process, where automation can increase quality substantially. For example, automobile and truck pistons used to be installed into engines manually. This is rapidly being transitioned to automated machine installation, because the error rate for manual installment was around 1-1.5%, but has been reduced to 0.00001% with automation. Hazardous operations such as oil refining, the manufacturing of industrial chemicals, and all forms of metal working, were always early contenders for automation.
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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
SCADA System with HMI Screens
Programmable Logic Controller
Field Equipments and Machineries
AC OR DC Drives
Sensors
Auxiliaries
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MAIN BODY OF AUTOMATION •
SCADA - Supervisory Control and Data Acquisition
•
PLC - Programmable Logic Controller
•
DRIVES - Variable Speed Drives
•
SENSORS – Transducers, Feedback equipment.
•
AUXILIARIES – Converters, Power Supplies, Different Communication
mediums etc.
CHAPTER – 2
SCADA
SCADA stands for “Supervisory Control And Data Acquisition.” It generally refers to an industrial control system: a computer system monitoring and controlling a process. The process can be industrial, infrastructure or facility based as described below. Industrial processes include those of manufacturing, production, power generation, fabrication, and refining, and may run in continuous, batch, repetitive, or discrete modes. Infrastructure processes may be public or private, and include water treatment and distribution, wastewater collection and treatment, oil and gas pipelines, electrical power transmission and distribution, and large communication systems. Facility processes occur both in public facilities and private ones, including buildings, airports, ships, and space stations. They monitor and control HVAC, access, and energy consumption.
2.1 A SCADA System usually consists of the following subsystems: 6
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 or supervisory 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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•
Buildings, facilities and environments: Facility managers use SCADA to
control HVAC, refrigeration units, lighting and entry systems. •
Manufacturing: SCADA systems manage parts inventories for just-in-time
manufacturing, regulate industrial automation and robots, and monitor process and quality control. •
Mass transit: Transit authorities use SCADA to regulate electricity to subways,
trams and trolley buses; to automate traffic signals for rail systems; to track and locate trains and buses; and to control railroad crossing gates. •
Traffic signals: SCADA regulates traffic lights, controls traffic flow and detects
out-of-order signals.
2.3 Importance of SCADA: Maybe you work in one of the fields I listed; maybe you don’t. But think about your operations and all the parameters that affect your bottom-line results: •
Does your equipment need an uninterrupted power supply and/or a controlled
temperature and humidity environment? •
Do you need to know — in real time — the status of many different components
and devices in a large complex system? •
Do you need to measure how changing inputs affect the output of your
operations? •
What equipment do you need to control, in real time, from a distance? 11
•
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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Some sensors measure more complex situations where exact measurement is important. These are analog sensors, which can detect continuous changes in a voltage or current input. Analog sensors are used to track fluid levels in tanks, voltage levels in batteries, temperature and other factors that can be measured in a continuous range of input. For most analog factors, there is a normal range defined by a bottom and top level. For example, you may want the temperature in a server room to stay between 60 and 85 degrees Fahrenheit. If the temperature goes above or below this range, it will trigger a threshold alarm. In more advanced systems, there are four threshold alarms for analog sensors, defining Major Under, Minor Under, Minor Over and Major Over alarms.
2.4.2 Data Communication: In our simple model of the widget fabricator, the “network” is just the wire leading from the switch to the panel light. In real life, you want to be able to monitor multiple systems from a central location, so you need a communications network to transport all the data collected from your sensors. Early SCADA networks communicated over radio, modem or dedicated serial lines. Today the trend is to put SCADA data on Ethernet and IP over SONET. For security reasons, SCADA data should be kept on closed LAN/WANs without exposing sensitive data to the open Internet. Real SCADA systems don’t communicate with just simple electrical signals, either. SCADA data is encoded in protocol format. Older SCADA systems depended on
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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 can’t 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 won’t do on a large scale — you can’t track a light board of a thousand separate lights, and you don’t want to pay someone simply to watch a light board, either. A real SCADA system reports to human operators over a specialized computer that is 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 doesn’t include any control elements. So let’s add one. Let’s 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 let’s 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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CHAPTER –3 PROGRAMMABLE LOGIC CONTROLLERS
Programmable Logic Controllers (PLCs), also referred to as programmable controllers, are in the computer family. They are used in commercial and industrial applications. A PLC monitors inputs, makes decisions based on its program, and controls outputs to automate a process or machine. This course is meant to supply you with basic information on the functions and configurations of PLCs.
Drive Motors
Photo Sensor s
Pumps
Other equipments
PLC
Start Push Button s
Lights
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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.
Input Module
CPU Central processing unit
Programming Device
Output Module
Operator Interface
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 17
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. 18
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 a physical 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 19
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 or OFF 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 used with 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 DRIVES AC 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 Faraday’s law of Electro–Magnetic 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 Fleming’s 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’= no’s 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 * Φ 23
Where,
‘Ia’ = stator current.
‘Φ’= 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 * Φ
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Where, Ia = Armature current which varies with load Φ = Motor flux which remains constant
VOLTAGE / FREQUENCY CURVE:
The EMF generated is proportional to the rate at which conductors cut the flux. So we have, EMF = Rate of change of fluxΦ = V / F i.e.
V = dΦ / dt dΦ = V * dt Φ=V*T
i e.
Φ=V/F
Therefore, 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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range.
4.3 BASIC THEORY OF INVERTER: Inverter is what industry sees as an ultimate and supreme controller of AC drives system. With micro controller based logical controls, the inverter is very fast, efficient safe and easily operable device. The range of application of inverter is enhancing day by day, so it is imperative to study inverter. 4.3.1 DEFINITION: The DC to AC power converter is known as INVERTER. In other words, an inverter is a circuit, which converts a fixed dc power in to an ac power at desired output voltage and frequency. 4.3.2 INVERTER SCHEMATIC DIAGRAM:
Since the inverter is dc to ac converter it has to have a constant dc source. Normally the power available in industry is ac, so to derive dc power, we need to rectify available ac power. The rectifier produces pulsating dc output voltage from ac power. The filter
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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 Fleming’s left hand rule. There is no basic difference between the construction of a dc 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 noload 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 29
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 DC supply. 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) /φ 31
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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power output. It is not possible to operate the motor at higher than the base speed by increasing the armature voltage above nominal rated voltage. This method of speed control is used in cranes, rolling mills etc. Thus up to base speed the motor can be controlled easily by controlling the armature voltage, called as ‘constant torque application’. 4.7.2 Field current control: Up to the base speed, the motor is controlled by armature voltage control. Now if the speed required is more than the base speed and the armature voltage is not being increased beyond the rated voltage, the choice is to decrease the field flux. To achieve this, the field current is to be decreased. This is called ‘constant power application’ since power remains constant. This is also termed as field weaking of the system.
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4.8 DRIVES ADVANTAGES AND DISADVANTAGES: Advantages
Disadvantages
1.Potentially lower installed cost above
Brush Maintenance.
50 HP. 2. Good
Higher Repair Costs.
energy
efficiency
&
regeneration of power can possible by 4Quadrant method. 3. Speed control of DC Drive is better
Limited Dynamic Response due to line
than AC Drive.
commutation restrictions, coupled with higher mass moments of inertia imposed by the wound field armature.
4. DC motor tuning is good in DC Drive
Limited range to 5,000-hp, due to
means current auto tuning is done in
commutation restrictions.
proper manner & also all gains are set by auto tuning. 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 ultrasonic sensors 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 for Sensors.
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 Inductive •Resistant 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 when the 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 how a 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 slowbreak. 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 39
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 Rod directions, 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. 5.2 Automation provides 100% accuracy all time. So the failures and mismatch in production completely eliminates. It makes the system’s 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 42
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 today’s scenario
to meet market competition.
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