Electrical Eqipments


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Electrical Eqipments

  1. 1. Thursday, November 12, 2009 VACUUM CIRCUIT BREAKER VACUUM CIRCUIT BREAKERS Circuit breakers are devices which can automatically break a circuit under fault conditions and it can make the circuit manually after the fault. Circuit breakers are classified into different types. One of the main circuit breaker among them is Vacuum circuit breaker. Here vacuum is used to extinct the arc. Arc extinction is the main process of a circuit breaker Before that, it is needy to explain how an arc is produced. Circuit breaker has two contacts- Fixed and movable, otherwise called as electrodes. Normally the contacts remain closed but if any fault occurs these contacts open automatically and of course can be opened manually. We know that circuit breakers work under the instructions from relay. When the trip coils from relay energize, circuit breaker identifies fault occurred and hence the contacts are pulled apart. A high intensity current flows through the contacts of the circuit breaker before they are opened by the system. At the same instant contacts begin to separate and causes heavy heat and temperature in between the contacts. This is termed as an arc. This heat is highly dangerous and can cause heavy damage to the whole system. So it has to be extinguished in no time. Vacuum is having superior qualities over other arc quenching medium. It has high insulating strength. When the contacts are opened in vacuum, the arc is produced in between them. It got extinguishes quickly since the metallic vapours and ions produced during the arc rapidly condense on the surface of the circuit breaker contacts, resulting in quick recovery of dielectric strength. The contacts of Vacuum circuit breaker and arc shied is mounted inside a vacuum chamber. The chamber is connected to the control mechanism by means of stainless steel bellows. A glass vessel is used as the outer insulating body. This is all about its construction. Vacuum circuit breaker is really the most reliable one with low cost, no fire hazards, no gas generation, low arc energy, little maintenance and low inertia. Sunday, November 8, 2009
  2. 2. What is an Actuator? Actuator is a typical mechanical device used to move or control a mechanism or a system. This device gains energy for its working from air, electricity or liquid and then converts it into some kind of motion. In engineering field actuators have many more applications in different sectors. In this field actuators are used as mechanisms to provide motion and are again used to stop the motion of that particular device. So it is clear that actuator just actuates the motion. May be start or stop, depends on the machine. In electronics engineering, actuators are subdivisions of transducers. They are used to transform input signal into motion. Normally input signal would be electrical. E.g. electrical motors, pistons, relays, pneumatic actuators, piezo electric actuators etc. Actuators can be sometimes used as hardware components. Different types of actuators are available like plasma actuators, pneumatic actuators, electric actuators, linear actuators. It’s a type of tool used to put something into automatic action. Actually actuators are used with number of sources. Depending on the type of actuator we are using, different tools will be assisting us to put the device to motion. Mostly they are used in office work area since most of them are used to move valve door in systems. Sometimes they are used to maneuver certain mechanical devices on work. Depending on the shape and style actuators are divided into different classes. Linear actuators, valve actuators, hydraulic actuators are the best known and used among them. Each of them has assigned functions e.g. hydraulic actuators are used to allow more pressure, size and movement with the object. All of these different types of actuators are best when knowing what type of motion control you need. They also provide speed, Belt drive, acceleration and accuracy with linear motion No matter what type of actuator is needed, there is a tool that makes it easier to maneuver a certain object or space in your work area. DIGITAL SIGNAL PROCESSING (DSP) What is Digital Signal Processing? It’s a major branch of science that deals with the representation of signals as symbols or sequences and after that these signals are getting processed. There are two subfields of processing available- Analog signal processing and Digital signal processing. This digital signal processing also includes subfields like audio and speech processing, sonar and radar processing, sensor array processing, spectral estimation, statistical signal processing, digital image processing, signal processing for communication systems, biomedical signal processing, seismic signal processing etc. So from the above
  3. 3. description it’s clear that DSP is a vast branch in engineering field that requires genuine attention and development. Generally these processing measures the continuous analog signals and is then converted into digital form by means of an Analog to Digital converter (A-D converter). For certain, the required output would be in Analog form, so the signal is then converted to Analog using Digital to Analog converter (D-A converter). We know that this process is very complex when compared with analog processing and hence error detection and correction methods are used to clarify the process. The main notable advantage of DSP is that it is less affected to noise and hence non vulnerable. DSP algorithms are carried out in special processors like Digital Signal Processors which is inbuilt in the systems. Since the process is very lengthy and time consuming, we usually undergo the process in this type of processors. DSP’s include microprocessors, digital signal controllers, field programmable gate arrays etc. DSP’s can be implemented using specialized microprocessors like DSP56000, TMS320 and the SHARC. Usually the digital signals are denoted in time domain, frequency domain, spatial domain etc. With the available characteristics, it is decided that on which domain the signal can be denoted. Sampling is another important point to be specified. Sampling can be done in signal processing during two stages discretization and quantization. In the dicretization stage the space of signals are divided into equivalence classes and quantization is carried out by replacing the signals with representative signals. DSP has applications in the fields of audio signal processing, audio compression and in the fields as mentioned earlier. What are Lightning Arresters? LIGHTNING ARRESTER Lightning arresters or surge diverters are used to protect the earth screen and ground wires in an electrical system against direct lightning strokes. They conduct the high voltage surges to the ground without getting affected to the system. The lightning arrestor provides a cone of protection which has a ground radius approximately equal to its height above the ground. The surge diverters consist of a spark gap in series with a nonlinear resistor. The function of non linear resistor is very important. As the gap sparks over, due to the over voltage, the arc would get short circuited and may cause power flow current in the arrester. Since the characteristic of the resistor is to offer high resistance to high voltage, it prevents the effect of a short circuit. After the surge is over, the resistor offers high resistance to make the gap non-conducting. One end of the arrestor would be connected to the terminal of the equipment to be protected and other end would be grounded. The length of the spark gap is set in such a
  4. 4. way that normal line voltage is enough to cause an arc across the gap but high voltage would break down the air insulation and form an arc. Different types of surge diverters are available depending on the purpose. Some of them are Rod gap arrester, Horn gap arresters, Multigap arresters, Expulsion type lightning arrester and valve gap arrester. Each of them has variety of range of operating conditions. Wednesday, October 28, 2009 What are Acoustic Couplers? ACOUSTIC COUPLER The term acoustic means sound. This is a device used in telecommunication field for coupling electrical signals by acoustical means. Its main working is related to our telephone. Acoustic coupler is a terminal and interface device to link data terminals and radio sets with telephone. Acoustic couplers are used to transmit data over telephone lines using modems. This is a device onto which telephone handset is placed to connect computer to the network, a modem will be also there. Modems that do not use acoustic coupler can some times be called as Direct Connected Modems The link between the terminals and telephones can be achieved by means of acoustic signals rather than through electrical connections. In 1956 acoustic couplers came into use in our telephone industry till then this industry was a closed system wholly controlled and owned by Bell systems. At first acoustic couplers were sensitive to external disturbances and noises. Direct electrical connections to telephone networks were made legal in early times. But rapidly preference got secured to the modem connecting method and hence acoustic couplers are used widespread. At times acoustic couplers are no longer used since now a days, telephones can be directly connected to modem instead of connecting to modular telephone connectors. Better connections can be established by avoiding unwanted breakage in signals. Still acoustic coupler modems can be useful in some situations like, in a hotel room where telephone cable is anchored in the walls. Wednesday, October 28, 2009 VARISTORS What is a VARISTOR? The name varistor came from variable resistor. Varistor is a pure electronic component used to protect circuits from transient over voltages making the circuit triggered. The
  5. 5. function of a varistor is to provide increased current when voltage becomes excessively high. Varistor is also known as Voltage Dependent Resistor (VDR) since it depends on voltage. The most common type we are dealing with now days is Metal Oxide Varistor (MOV). It is just like a Unijunction Transistor, since it allows current flow only in one direction. It contains mass of ceramic grains enclosed between two metal plates. The boundary between and its neighbour forms a diode junction. So when a small voltage is applied across the electrodes, only a small current passes due to the reverse break down at the junctions. Similarly when a large voltage is applied, due to thermionic emission large current passes through the junction. This behavior results in nonlinear current voltage characteristic. Varistor can absorb surge in some ways but to some to extent. A typical surge protector power strip can be made using MOV’s. A varistor provides no equipment protection from inrush surge current. A varistor doesn’t control such events. Varistors have many applications on different electronic circuits. It can replace ordinary resistor in almost all functions, basically it can vary its resistance. So using varistors we can control voltage and current to the circuit. Thus we can protect the circuit from internal damage and severe short circuits. SCADA SCADA SCADA is the acronym of Supervisory Control And Data Acquisition. It is used in the fields of data acquisition and monitoring in industries. SCADA is a pure software package and not having any hardware components. It is the monitoring section of a process and the decision making section. It controls all the process taken while doing a particular task. Normally we use couplet of software, e.g. SCADA- PLC couplet. Here SCADA acts as master and PLC acts as slave. SCADA gives instructions and PLC obeys as per the order. In short control is safe in SCADA hands. It’s more convenient if we use the couplets even if SCADA alone can do all the operations alone. SCADA operations are categorized under three below explaining sections. Such as- industrial, infrastructure and facility based processes. Industrial process includes manufacturing, production, power generation etc. Infrastructure based process means water treatment, waste water collection, refining etc. Facility process includes building, airport and all public sector processes. SCADA mainly consists of five subsystems. Each is assigned to perform a particular task. The five subsystems are: + HMI or Human Machine Interface- This is an apparatus used to supply data to the human operator and through this he could able to monitor and control
  6. 6. + A supervisory computer system- This is used to acquire data and to send commands to the process. + RTU or Remote Terminal Unit- These are connected to sensors to do a process, Converts sensor signals to digital data and sends this data to the supervisory system. + PLC or Programmable Logic Controller- used as field devices in a process, since they are economical, flexible, versatile and much configurable than general purpose RTU’s. + Communication- Many modes of communication are available. Communication connects supervisory systems to RTU’s. Three generation SCADA software is available. They are First generation or Monolithic, Second generation or distributed, Third generation or Integrated. Now third generation is in use. SCADA has applications on energy management system, multi tasking, automation, industrial control systems, data exchange, pipeline transport, graphical design etc. SCADA made almost all sections under its control tactically. Mainly we use this software in automation purposes. This software is freely available and another advantage is no programming is needed. So even a person who is not good in programming can handle SCADA well and efficiently.