the open circuit voltages and short circuit currents are the general terms which we here usually.
here is an explanation on them
V AND I
here is an explanation on them
V AND I
Sunday 30 September 2012
WIND POWER
have you ever worried about wind power ???
Haven't you have a question on wind power as "How much amount of power does a wind turbine produce?"
If you have that query here is an excellent answer for it.
click on the below link to download
Wind Power
Haven't you have a question on wind power as "How much amount of power does a wind turbine produce?"
If you have that query here is an excellent answer for it.
click on the below link to download
Wind Power
Thursday 27 September 2012
Why Armature is placed on Stator for Synchronous machines
Opeation principle of synchronous machine is quite similar to dc machine. However difference exists in the synchronous machine as there is no need to rectify the time varying emf generated in the armature winding by commutator. Hence in place of commutator slip rings are provided in synchronous machine.
For alternators we see in big power plants armature will be on stator and field winding will be on rotor. Some of the questions to be answered is what happen when the field is placed on stator and armature is on rotor like dc machine? What are the merits and demerits in placing armature on stator?
Why Rotating Magnetic field:
Like dc machine armature of the machine can be placed on the rotor and still we can operate the synchronous machine. Main problem arises for large rating machines when the armature is placed on the rotor shaft large currents are to be collected by the brushes of the machine at higher voltages, which results in sparking and wear and tear of the brushes. More than the above reasons the cost of the brushes employed will be huge as it has to able to carry large currents. Therefore for large rating machines (500kVA to 750MVA) armature is placed on the stator which requires no brushes to collect the large currents. For lower rating machines (up to 250kVA) armature of the machine can be placed on the rotor as the current to be collected will be small.
Salient Pole vs Non Salient Pole Synchronous Generator Difference
Salient Pole Synchronous Alternator:
- Salient pole Generators will have large diameter and short axial length
- Pole shoes cover 2/3 of the pitch
- Salient Poles are laminated in order to reduce eddy currents
- They are used in hydraulic turbines or diesel engines
- Salient pole generators will have typical speed about 100 to 375 rpm.
- As the speed of the water turbine is slow hence more number of poles are required to attain the frequency. Therefore Salient pole machines will have typically number of poles will be between 4 to 60.
- Cheaper compared to cylindrical rotor machines for speeds below 1000rpm.
- Causes excessive windage losses
- Flux distribution is not uniform due to the presence of salient poles, hence emf waveform generated is not good compared to cylindrical machine
- Salient Pole Synchronous Generators are employed in Hydro-Power plants.
- Non-Salient pole generators will have smaller diameter and longer axial length
- They are used for High speed operation (typically speed will be 1500 and 3000 rpm)
- Better in dynamic balancing because of absence of salient poles.
- Less windage loss
- Robust construction and noiseless operation
- Nearly sinusoidal flux distribution around the periphery, therefore gives a better emf waveform than salient pole machine
- No need to provide damper windings (except in special case to assist the synchronising) because the field poles themselves acts as efficient dampers.
- Non-Salient pole generators are used in Thermal,Gas and in Nuclear Power plants.
Distributed Control System (DCS) in Power Plant
Usage of Single Computer for control of all the process variables of plant would result in the loss of control action in the plant if failure occurs to the master single computer. Also, time required for accessing large data, processing the received data and controlling of the entire plant by single computer will takes more time and slows down the process system
Distributed Control System:
In distributed control system, different process systems in the plant will have their control system loop. Thus in distributed control system number of individual control loops will be present. Several Microprocessors were employed at the different locations of the power plant and dedicated to provide services for individual process system of the plant and monitor and control each process of the plant (This is unlike SCADA master-slave architecture, in DCS all microprocessors will be masters). Every processor at different locations of the plant acquire the data from its process system, process the received data and provides control functions and also stores the required data. These microprocessors are connected to each other and also connected to the station master computer through which operator can see the information and take control actions if necessary. Control system in power plant will have distribution in both topologically and functionally. Functional distribution such as operation of boiler side values, fans, turbine values, boiler oil firing, boiler coal firing for carrying out such different functions Microprocessors are distributed.
Topological distibutuion is achieved by employing closed loops for operation of different equipment such as boiler steam A equipment, boiler steam B equipment, boiler oil firing AB/CD/EF elevations will have dedicated control loop
This type of distribution results in increase in speed and reliability of the control system.
Why Efficiency of Thermal Power Plant is low (30-35)%
Thermal efficiency of Steam Power Plant:
In steam power plant overall efficiency will be in the order of about 30%. In case of modern thermal power plats where super critical parameters (pressure and temperature) are employed and by employing number heat savig devices the power plant efficiency will be raised to around 40%. Quality of the coal also plays significant role in improving the efficiency of the Thermal Plant.
Some the losses occurring i the steam power plants are:
Loss at Boiler - 15%
Loss at the Turbo-Generator
Heat Rejection to the condenser - 55%
Alternator loss - 1%
From above mentioned figures we understand that more than 50% of total heat combustion is lost as heat rejected to the condenser. This loss cannot be avoided as heat energy cannot be converted into mechanical energy without (some heat rejected) drop in temperature (From the second law of thermodynamics-A fraction of heat can only be converted to mechanical work and remaining heat must be rejected). Steam in the condenser is at lowest temperature.
Thermal efficiency of the power plant mainly depends on three factors.
a) Pressure of the steam entering the turbine
b) Temperature of the steam entering the turbine
c) Pressure in the condenser
Thermal efficiency of the plant can be improved by increasing the temperature and pressure of the steam. For this reason high temperature and pressure is used and also thermal efficiency is improved by decreasing the pressure in the condenser
Also efficiency can be improved by reheating the steam between the turbine stages
Thermal Power Plant Advantages and Disadvantages
Thermal power plat has the following merits and demerits:
Advantages:
- Fuel used is cheaper
- Smaller space is required compared to hydro power plant
- Economical in initial cost compared to hydro plants and running costs are less compared to gas plants or diesel plants
- Thermal plants can be placed near load centers unlike hydro and nuclear plants. Hence transmission of power losses can be minimized
- Thermal plants are able to respond to the load demand more effectively and supports the performance of the electrical grid
- Steam plants can withstand for overload for certain extent
Disadvantages:
- Higher maintenance and operational costs
- Pollution of the atmosphere
- Huge requirement of water
- Handling of coal and disposal of ash is quite difficult and requires large area
- Gestation period (period for commissioning of plant) takes long time
- Efficiency of thermal plant is quite less (30-35%)
- Operational cost of thermal plant is more costlier compared to hydro and nuclear plant
Thermal Power Plant Interview Questions-Answers
What are the major electric systems in Thermal Power Plant?
Answer: Major electrical equipment in thermal power plant are
- Turbine Generator
- Exciter System
- Generator Protection System
- Generator Transformer
- HT/LT switch gear
- Electrical Switch-yard
What are the different Generator Protections employed in Thermal Power Plants?
Answer: Faults in the windings, Over load protection, Over heating of windings or bearings, Over speed protection, Loss of Excitation protection, Motoring operation protection, Inadvertent energisation, single phase or unbalanced current protection, out of step operation protection, sub-synchronous oscillations protection and earth fault protection
How Generator Transformer is cooled in Thermal Power Plant?
Answer: MVA power rating of the Generator Transformer will be equal to the alternator. Hence lot of heat will be generated while generator transformer is under operation. Oil Forced Air Forced (OFAF) type of cooling is employed for generator transformer
What are different types of circuit breakers employed in thermal power plant?
Answer: For low voltage operation 415/220V vacuum circuit breakers or air break circuit breakers are employed. For voltage ratings about 6.6kV and beyond SF6 circuit breakers are employed.
What type of cooling is provided for Generator in power plant?
Answer: Hydrogen gas cooling is employed for large size generators because of better heat carrying ability of the hydrogen. Hydrogen cooling is provided for rotors and core of the generator. Water cooling is provided for the stator of the alternator.
Emergency Auxiliary Equipment in Thermal Power Plant
Power required for avoiding the damage to the equipment in case of power failure is called survival power. Consider a total grid collapse, which results in tripping both offsite power (electrical power coming from the grid to the thermal power plant) and onsite power (power generated by the thermal power plant). During this emergency condition some electrical power is required to operate certain pumps and other auxiliaries to keep the major equipment in the thermal power plant in safe condition
Some of the auxiliaries which require power during emergencies in thermal power plant are
- Turbine emergency lube oil pump: This is battery driven centrifugal oil pump designed to operate at 40% of required oil to turbine and ensures the turbine coast down smoothly
- Lubricating oil pumps
- Compressors for Air blast circuit breakers
- Emergency lighting
- Control room controls
- Communication and telemetry equipment in the power plant
These emergency auxiliaries operate when power black out occurs and protect the major equipment of the plant. This load will be supplied by battery banks in thermal power plant. This is the major reason why battery banks are provided in thermal power plants. This load will be around 0.25% to 0.3% of the total unit capacity.
Difference between Thermal and Nuclear Power Plants
Thermal Power Plant:
- In Thermal Power Plant Reactor follows Turbine. When load demand changes governor system of turbine operates accordingly depends on load demand by opening or closing steam value. Hence more or less inputs are taken to boiler according to the load demand.
- Thermal Power Plants basically are peak load plants. However due to lack of power generation sources, thermal plants are forced to operate as base load plants.
Nuclear Power Plant:
- In Nuclear Plants after reactor shut down still decay heat is produced due to the fission of the daughter nuclei and gamma radiation. Hence in nuclear power plant decay heat is to be removed continuously even the reactor is shutdown. Reactor cooling system must be continuously operated.
- In Nuclear Plants safety is prime criteria than production of electricity
- Nuclear Power Plants are base load plants
- In Nuclear Plants Turbine follows reactor. No variation of the turbine speed according to load demand. This is designed in order to avoid tripping of reactor.
Tuesday 25 September 2012
Tuesday 11 September 2012
THE HINDU-YOUNG WORLD
Young scientists’ day out
http://www.thehindu.com/todays-paper/tp-features/tp-youngworld/article3884005.ece
Young World Quiz
http://www.thehindu.com/todays-paper/tp-features/tp-youngworld/article3884052.ece
Save ourselves
Monday 10 September 2012
Root Locus
An applet for Root locus
check it http://users.ece.gatech.edu/bonnie/book/OnlineDemos/InteractiveRootLocus/applet.html
check it http://users.ece.gatech.edu/bonnie/book/OnlineDemos/InteractiveRootLocus/applet.html
Wednesday 5 September 2012
Tuesday 4 September 2012
Monday 3 September 2012
Meaning Of GOOGLE
Have you ever wondered what is the meaning of Google? Where did the name google come from? What does google stand for? Google is the name of internet’s most popular search engine and the word Google is derived from GOOGOL.
Why is Google Called Google?
Googol is the mathematical term for a 1 followed by 100 zeros. The term was coined by Milton Sirotta, nephew of American mathematician Edward Kasner, and was popularized in the book, “Mathematics and the Imagination” by Kasner and James Newman. Google’s play on the term reflects the company’s mission to organize the immense amount of information available on the web.”
Goooooooooo,oooooooooo,oooooooooo,oooooooooo,oooooooooo,
oooooooooo,oooooooooo,oooooooooo,oooooooooo,ooooooooooGle
oooooooooo,oooooooooo,oooooooooo,oooooooooo,ooooooooooGle
is indeed derived from Googol
10000000000,0000000000,0000000000,0000000000,0000000000,
0000000000,0000000000,0000000000,0000000000,0000000000.
Yes, that is 100 zeros!
0000000000,0000000000,0000000000,0000000000,0000000000.
Yes, that is 100 zeros!
EXAMINATION TIPS
- Get a normal night’s sleep before the examination date
- Reach exam centre early and relax or discuss with close friends
- Read the question paper and the instructions thoroughly. Mark the questions which you find you can answer.
- Ask the invigilator for clarifications, if any.
- Make a rough time scheme and decide how much time you should allot to each question.
- Leave 15 minutes at the end to review the answers and make corrections, as needed.
- Before you start on any particular question, make sure you understand it/interpret it correctly.
- Answer the easiest and shortest question first.
- Answers should be brief and conforming to what is asked in the question.
- Clearly understand the difference between the usages Define, Distinguish, Compare, List, Enumerate, Explain, Write short notes on, Differentiate, Sketch and explain etc.
- If you get stuck up , proceed to the next question and come back at the end if there is time.
- Leave a margin of 3 cm at the left side of the answer sheet, if there is no printed margin.
- Mark the question numbers clearly in the left margin.
- ORGANISE YOUR ANSWER SHEET: write legibly; use reasonably large size letters; don’t cram; space the lines reasonably apart; use a lined sheet below the answer sheet to ensure steady writing; use pen or dot pen with reasonably thick writing point; use blue black ink; do not use washable ink; use pencil and instruments for drawing figures, tables etc.
- Indicate your numerical answers clearly and with proper units
- Eg. Maximum stress = 23000 kN/mm2 …. Ans
- Re-read your paper and make corrections, if needed, before you hand over the paper.
- Never do any post mortem on examination till the last exam is over. Go home and prepare for the next examination.
Sunday 2 September 2012
Saturday 1 September 2012
Universal Current Sensor
INTRODUCTION to the Universal Current Sensor
The measurement of electric current strength is not always easy, especially when the measured signal requires further electronic conditioning. Simply connecting an ammeter to an electrical circuit and reading out the value is no longer enough. The current signal must be fed into a computer in which sensors convert current into a proportional voltage with minimal influence on the measured circuit. The basic sensor requirements are galvanic isolation and a high bandwidth, usually from DC up to at least 100 kHz. Conventional current measurement systems therefore tend to be physically large and technically complex.
MAGNETORESISIVE SENSORS
MAGNETORESISTIVE EFFECT
The anisotropic magnetoresistive effect is known to be present in a whole family of ferromagnetic alloys. Most of these alloys are composed of iron, nickel, and chromium, and may be primary or ternary. They have in common a more or less strong anisotropy in their magnetic properties. Whenever these materials are exposed to a magnetic field during crystal formation, a preferred orientation in magnetization will result. The same happens when the materials are forced into shape that is a mechanical anisotropy is imposed.
It is found that changing the orientation of the magnetic moment in the wire caused a current passing through it to change correspondingly. The orientation could be changed by apply in an external magnetic field, and generally an increase in current was observed. This phenomenon is called anisotropic magnetoresistive effect.
The ferromagnetic materials can be deposited as thin films and structured into small strips that are typically 40mm thick,10mm wide, and 100mm long. In most general case, the electrical resistance of AMR material depends on the angle between the direction of the magnetization, and the direction of the current going through it. When the current and magnetic moment are parallel, the resistance of the strip is greatest; when they are at a 90 degree angle to each bother, it is smallest.
Magnetoresistive field sensors are usually configures as a half or full bridge. The barber poles are positioned such that in the presence of magnetic field the value of first resistor increases and that of second decreases.
Ultrasonic Motors
All of us know that motor is a machine which produces or imparts motion, or in detail it is an arrangement of coils and magnets that converts electric energy into mechanical energy and ultrasonic motors are the next generation motors.
In 1980,the world's first ultrasonic motor was invented which utilizes the piezoelectric effect in the ultrasonic frequency range to provide its motive force resulting in a motor with unusually good low speed, high torque and power to weight characteristics.Electromagnetism has always been the driving force behind electric motor technology. But these motors suffer from many drawbacks. The field of ultrasonic seems to be changing that driving force.
DRAWBACKS OF ELECTROMAGNETIC MOTORS
Electromagnetic motors rely on the attraction and repulsion of magnetic fields for their operation. Without good noise suppression circuitry, their noisy electrical operation will affect the electronic components inside it. Surges and spikes from these motors can cause disruption or even damage in nonmotor related items such as CRTs and various types of receiving and transmitting equipments. Also , electromagnetic motors are notorious for consuming high amount of power and creating high ambient motor temperatures. Both are undesirable from the efficiency point of view. Excessive heat energy is wasted as losses. Even the efficiently rated electromagnetic motor has high input to output energy loss ratios.
Replacing these by ultrasonic motors would virtually eliminate these undesirable effects. The electromagnetic motors produce strong magnetic fields which cause interference. Ultrasonic motors use piezoelectric effect and hence no magnetic interference.
PRINCIPLE OF OPERATION
PIEZOELECTRIC EFFECT
Many polymers, ceramics and molecules are permanently polarized; that is some parts of the molecules are positively charged, while other parts are negatively charged. When an electric field is applied to these materials, these polarized molecules will align themselves with the electric field, resulting in induced dipoles within the molecular or crystal structure of the material. Further more a permanently polarized material such as Quartz(SiO2) or Barium Titanate(BaTiO3) will produce an electric field when the material changes dimensions as a result of an imposed mechanical force. These materials are piezoelectric and this phenomenon is known as Piezoelectric effect. Conversely, an applied electric field can cause a piezoelectric material to change dimensions. This is known as Electrostriction or Reverse piezoelectric effect. Current ultrasonic motor design works from this principle, only in reverse.
When a voltage having a resonance frequency of more than 20KHz is applied to the piezoelectric element of an elastic body (a stator),the piezoelectric element expands and contracts. If voltage is applied, the material curls. The direction of the curl depends on the polarity of the applied voltage and the amount of curl is determined by how many volts are applied.
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