The following line diagram illustrate how a normally open and a normally closed pushbutton might be connected to control a three -phase AC motor as shown above figure.
In this example , a motor starter coil (M) is wired in series with a normally open, momentary start
pushbutton, a normally closed , momentary stop pushbutton , and normally closed overload relay (OL) contacts.
Momentarily pressing the start push button complete the path for current flow and energies the motor starter (M). This close the associated M and Ma (auxiliary contact located in the motor starter ) contacts. When the start button is released, current
Continues to flow through the stop button and the Ma contacts, and the M coil remains energized. The motor will run until the normally closed stop button is pressed, unless the overload relay (OL) contacts open . when the stop button is pressed, the path for current is interrupt , opening the associated M and Ma contacts and the motor stop
.
30SCADA
Introduction
SCADA is not a specific technology, but a type of application
“SCADA” stands for Supervisory Control And Data Acquisition –any application can gets data about a system in order to control the system is a SCADA application’’ it is a purely software package that is positioned on top of hardware to which it is interfaced , in general via
Programmable logic controller (PLCs), or other commercial hardware modules.
In other words
SCADA: the term refers to a large scale, distributed measurement (and control) system. SCADA are used to monitor or to control chemical, physical or transport process.
SCADA it is refers to an industrial control system a computer system monitoring and controlling process. The process can be industrial, infrastructure or facility describe below
Industrial process: it is includes those of manufacturing, production, power generation,
fabrication, and refining and process may be in continuous, batch, repetitive or discrete or modules.
Infrastructure process: it may be public or private and water treatment and distribution,
wastewater collection, and treatment, oil and gas pipelines , electric power transmission and distribution, and large communication systems.
Facility Process: it is occur both in public facilities and private ones, including building,
airports, ship and space station. The monitor and control HVAC, access and energy consumption
A SCADA system usually consists of following subsystem
A Human – Machine interface (HMI) is the apparatus which present process data tohuman operator, and through this, the human operator monitor and control the process.
A supervisory (computer) system, gathering (acquiring) data on the process and sending commands.
Remote terminal unit (RTU) connection to sensors in the process and converting sensors signals to digital data and sending digital data to the supervisory system.
Programmable Logic Controller (PLC) used as field devices because they are more economical, versatile, flexible configurable than special purpose -RTUs.
Communication infrastructure connection the supervisory system to the Remote Terminal Unit. 31Systems Concepts
The terms SCADA usually refer to a central system that monitors and controls a complete site. The bulk of the site control is actually performed automatically by a Remote Terminal Unit (RTU) or by a programmable Logic controller (PLC). Host control functions are almost always restricted to basic site over- ride or supervisory level capability.
EX: 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 condition, 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
TAGS (POINTS)
It is logical name of a variable or object that be used in our animation of a plant. Tags are two types
System defined Tags User defined tags
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System defined tags
It is represent the system value in the SCADA it is represented by dollar sing $ User defined tags
This tag are defined by the user or programmer Except (A,S) are not defined.
Trends:
Two types
Real times trends
Historical Times trends
Classification of tags
Direct tags Indirect tags
Direct tags
These tags are directly used by the programmer to represent the value of plant or variable
Indirect tags
These tags are used by the another tags to show or represent the value of plant or variable Direct tags
Memory tags : these tags having no connection with process the plant.
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ALARM
An important part of most SCADA implementation is alarm handling. The system monitors whether certain alarm conditions are satisfied, to determine when an alarm event has occurred. Once alarm event has been detected, one or more action are taken (such as the activation or remote SCADA operators are informed) in many case, a SCADA operate may have to knowledge the alarm event, these may be deactivate some alarm indicators whereas other indicators remain active until the alarm conditions are cleared. Alarm conditions can be explicit - for example, an alarm point is a digital status point that has either the value NORMAL or ALARM that is calculated by a formula based on the values in other analogue and digital points- or implicit: the SCADA system might automatically monitor whether the value in an analogue point lies outside high and low
limit values associated with that point. Examples of alarm indicators include a siren, a pop- up box on a screen, or a colored or flashing area on a screen (that might act in a similar way to the "fuel tank empty" light in a car); in each case, the role of the alarm indicator is to draw the operator's attention to the part of the system 'in alarm' so that appropriate action can be taken. In designing SCADA systems, care is needed in coping with a cascade of alarm events occurring in a short time, otherwise the under lying cause (which might not be the earliest event detected) may get lost in the noise. Unfortunately, when used as a noun, the word 'alarm' is used rather loosely in the industry; thus, depending on context it might mean an alarm point, an alarm indicator, or an alarm event.