Electrical QAS

7/28/2019 Electrical QAS

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Question: What is electric traction?

A: Electric traction means using the electric power for traction system (i.e. for railways, trams,

trolleys etc.). Electric traction means use of the electricity for all the above machines. Now a day,

magnetic traction is also used for bullet trains. And basically dc motors are used for electric traction

systems.

Question: Why link is provided in neutral of an ac circuit and fuse in phase of ac circuit?

A: Link is provided at a Neutral common point in the circuit from which various connections are taken

for the individual control circuit and so it is given in a link form to withstand high Amps. But in the

case of Fuse in the Phase of AC circuit it is designed such that the fuse rating is calculated for the

particular circuit (i.e. load) only. So if any malfunction happens the fuse connected in the particular

control circuit alone will blow off.

Question: What is the difference between electronic regulator and ordinary electrical rheostat

regulator for fans?

A:The difference between the electronic and ordinary electrical regulator is that in electronic

regulator power losses are less because as we decrease the speed the electronic regulator gives

the power needed for that particular speed but in case of ordinary rheostat type regulator, the power

wastage is same for every speed and no power is saved. In electronic regulator, triac is employed

for speed control by varying the firing angle speed and it is controlled but in rheostatic control

resistance is decreased by steps to achieve speed control.

Question:: what is MARX CIRCUIT?

A: It is used with generators for charging a number of capacitor in parallel and discharging them in

series. It is used when voltage required for testing is higher than the available.

Question: What is vacuum circuit breaker? Define with cause and where we use this Device?

A: A breaker is normally used to break a circuit. While breaking the circuit, the contact terminals will

be separated. At the time of separation an air gap is formed in between the terminals. Due to

existing current flow the air in the gap is ionized and results in the arc. Various mediums are used to

quench this arc in respective CB's. But in VCB the medium is vacuum gas. Since the air in the CB is

having vacuum pressure the arc formation is interrupted. VCB's can be used up to 11kv.

AC/DC Circuit

Question: How can you start-up the 40w tube lite with 230v AC/DC without using any

choke/Coil?

A: Its possible by means of Electronic chokes; otherwise it's not possible to ionize the particles intube light with normal voltage.

Question:: How tube light circuit is connected and how it works?


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A: A choke is connected in one end of the tube light and a starter is in series with the circuit. When

supply is provided, the starter will interrupt the supply cycle of AC. Due to the sudden change of

supply the chock will generate around 1000volts. This volt will capable of to break the electrons

inside the tube to make electron flow. Once the current passes through the tube the starter circuit

will be out of part. Now there is no change of supply causes choke voltage normalized and act as

minimize the current.

Question: Why Human body feel Electric shock ? and in an Electric train during running, We

didnt feel any Shock ? why?

A: Unfortunately our body is a pretty good conductor of electricity, The golden rule is Current takes

the lowest resistant path if you have insulation to our feet as the circuit is not complete (wearing

rubber footwear which doing some repairs is advisable as our footwear is a high resistance path not

much current flows through our body).The electric train is well insulated from its electrical system.

Question: Why, when birds sit on transmission lines or current wires doesn't get shock?

A: Its true that if birds touch the single one line (phase or neutral) they don't get electrical shock... ifbirds touch 2 lines than the circuit is closed and they get electrical shock. so if a human touch single

one line (phase) then he doesn't get shock if he is in the air (not touching - standing on the ground if

he is standing on the ground then touching the line (phase) he will get a shock because the ground

on what we standing is like line (ground bed - like neutral) and in the most of electric lines the

neutral is grounded.so that means that human who touch the line closes the circuit between phase

and neutral.

Question: What happen if we give 220 volts dc supply to a bulb or tube light?

A: Bulbs [devices] for AC are designed to operate such that it offers high impedance to AC supply.

Normally they have low resistance. When DC supply is applied, due to low resistance, the current

through lamp would be so high that it may damage the bulb element.

Question: What will happen when power factor is leading in distribution of power?

A: If there is high power factor, i.e. if the power factor is close to one:

1. Losses in form of heat will be reduced,2. Cable becomes less bulky and easy to carry, and very cheap to afford, &3. It also reduces over heating of transformers.

Power System

Question: What is "pu" in electrical engineering?

A: PU stands for per unit and this will be used in single line diagram of power distribution and it is

like a huge electrical circuit with no of components (generators, transformers, loads) with different

ratings (in MVA and KV). To bring all the ratings into common platform we use pu concept in which,

in general largest MVA and KV ratings of the component is considered as base values, then all other

component ratings will get back into this basis. Those values are called as pu values. (p.u=actual

value/base value).


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Question: What is ACSR cable and where we use it?

A: ACSR means Aluminium conductor steel reinforced; this conductor is used in transmission &

distribution.

Power Plant Engineering

Question: What operation carried out in Thermal power stations?

A: The water is obtained in the boiler and the coal is burnt so that steam is obtained this steam is

allowed to hit the turbine; the turbine which is coupled with the generator generates the electricity.

Communication

Question: What is encoder, how it function?

A: An encoder is a device used to change a signal (such as a bit stream) or data into a code. The

code may serve any of a number of purposes such as compressing information for transmission or

storage, encrypting or adding redundancies to the input code, or translating from one code toanother. This is usually done by means of a programmed algorithm, especially if any part is digital,

while most analog encoding is done with analog circuitry.

Power Electronics

Question: What are the advantages of speed control using thyristor?

A: Advantages:

1. Fast Switching Characteristics than Mosfet, BJT, IGBT2. Low cost3. Higher Accuracy.

Question: whats the one main difference between UPS & inverter ? And electrical

engineering & electronics engineering?

A: uninterrupted power supply is mainly use for short time. Means according to ups VA it gives

backup. Ups is also two types: on line and offline. Online ups having high volt and amp for long time

backup with high dc voltage. But ups start with 12v dc with 7 amp. but inverter is start with 12v,24,dc

to 36v dc and 120amp to 180amp battery with long time backup.

Electronics Engineering falls roughly into the overall category of electrical engineering.

Electrical engineering deals with ALL aspects of electrical things, from power generation &

transmission, to superconductors. In a nutshell, electrical engineers work on power distribution,

electrical controls in factories, motors & their control, transformers, and things like the electrical

requirements for a new mall (just to mention a few of the many areas of EE).

Electronics Engineers deal more with things at the component level, primarily Integrated circuits,

signal processing, gadgets like your GPS, and iPOD.


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On board a cruise ship for example, an Electrical engineer would see over the motor generator sets,

the power distribution, the computer controls at the 'control' level (steering & propulsion), while an

Electronics Engineer would deal with this like troubleshooting an single circuit board and the

communications equipment.

Machine

Question: What is the principle of motor?

A: Whenever a current carrying conductor is placed in an magnetic field it produce turning or twisting

movement is called as torque.

Fleming's left-hand rule (for motors), and Fleming's right-hand rule (for generators) are a pair of

visual mnemonics.

Question: What is meant by armature reaction?

A: The effect of armature flux to main flux is called armature reaction. The armature flux may support

main flux or opposes main flux.

Question: Which motor has high Starting Torque and Staring current DC motor, Induction

motor or Synchronous motor?

A: DC Series motor has high starting torque. We cannot start the Induction motor and Synchronous

motors on load, but cannot start the DC series motor without load.

Question: What is 2 phase motor?

A: A two phase motor is a motor with the the starting winding and the running winding have a phase

split. e.g;ac servo motor. Where the auxiliary winding and the control winding have a phase split of

90 degree.

Question: Advantages of vvvf drives over non vvvf drives for EOT cranes?

A:

1. smooth start and stop.2. No jerking of load.3. Exact positioning4. Better protection for motor.5. high/low speed selection.6. Reliability of break shoe.7. Programmable break control.8. Easy circuitry9. Reduction in controls10.Increases motor life

Question: What is the significance of vector grouping in Power Transformers?


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A: Every power transformer has a vector group listed by its manufacturer. Fundamentally it tells you

the information about how the windings are connected (delta or wye) and the phase difference

between the current and voltage. EG. DYN11 means Delta primary, Wye Secondary and the current

is at 11 o clock referred to the voltage.

Question: Which type of A.C motor is used in the fan (ceiling fan, exhaust fan, pedestal fan,bracket fan etc.) which are find in the houses?

A: Its Single Phase induction motor which mostly squirrel cage rotor and are capacitor start capacitor

run.

Question: Give two basic speed control scheme of DC shunt motor?

A:

1. By using flux control method: In this method a rheostat is connected across the field winding tocontrol the field current. So by changing the current the flux produced by the field winding can be

changed, and since speed is inversely proportional to flux, speed can be controlled2. Armature control method: In this method a rheostat is connected across armature winding by

varying the resistance the value of resistive drop(IaRa)can be varied, and since speed is directly

proportional to Eb-IaRa the speed can be controlled.

Question: What is meant by armature reaction?

A: The effect of armature flux to main flux is called armature reaction. The armature flux may support

main flux or opposes main flux.

Question: What is the difference between synchronous generator & asynchronous

generator?

A: In simple, synchronous generator supply's both active and reactive power but asynchronousgenerator (induction generator) supply's only active power and absorb reactive power for

magnetizing. These types of generators are used in windmills.

Question: What is the Polarization index value? (pi value)and simple definition of polarization

index ?

A: Its ratio between insulation resistance (IR) i.e meggar value for 10min to insulation resistance for

1 min. It ranges from 5-7 for new motors & normally for motor to be in good condition it should be

Greater than 2.5.

Question: Why sync. generators are used for the production of electricity?

A: Synchronous machines have capability to work on different power factor (or say Different

imaginary power varying the field emf. Hence syn. generators are used for the production of

electricity.

Problems on Machine

Parallel operation of transformer (single-phase)


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Ex.1: Two 1-phase transformers with equal turns have impedance of (0.5 + j3) ohm and (0.6 +

j10) ohm with respect to the secondary. If they operate in parallel, determine how they will

share a total load of 100 kW at p.f 0.8 lagging?

Solution:

ZA = 0.5 + j3 = 3.04


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field winding are stationary and armature rotates. Initial voltage is built up by residual magnetism. It

gives the starting torque to the generator.

Q: Why use the VCB at High Transmission System? Why can't use ACB?

A: Actually the thing is vacuum has high arc quenching property compare to air because in VCB, the

die-electric strengths equal to 8 times of air. Thats why always vacuum used as in HT breaker and

air used as in LT.

Q: What is the difference between surge arrestor and lightning arrestor?

A: LA is installed outside and the effect of lightning is grounded, whereas surge arrestor installed

inside panels comprising of resistors which consumes the energy and nullify the effect of surge.

Q: What happens if I connect a capacitor to a generator load?

A: Connecting a capacitor across a generator always improves power factor, but it will help depends

up on the engine capacity of the alternator, otherwise the alternator will be over loaded due to the

extra watts consumed due to the improvement on pf. Secondly, don't connect a capacitor across an

alternator while it is picking up or without any other load.

Q: Why the capacitors work on ac only?

A: Generally capacitor gives infinite resistance to dc components (i.e., block the dc components). It

allows the ac components to pass through.

Q: Explain the working principal of the circuit breaker?

A: Circuit Breaker is one which makes or breaks the circuit. It has two contacts namely fixed contact

& moving contact. Under normal condition the moving contact comes in contact with fixed contact

thereby forming the closed contact for the flow of current. During abnormal & faulty conditions (when

current exceeds the rated value) an arc is produced between the fixed & moving contacts & thereby

it forms the open circuit. Arc is extinguished by the Arc Quenching media like air, oil, vacuum etc.

Q:How many types of cooling system in transformers?

A:1. ONAN (oil natural, air natural)

2. ONAF (oil natural, air forced)

3. OFAF (oil forced, air forced)

4. ODWF (oil direct, water forced)

5. OFAN (oil forced, air natural)

Q: What is the function of anti-pumping in circuit breaker?

A:when breaker is close at one time by close push button, the anti-pumping contactor prevent re

close the breaker by close push button after if it already close.


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Q: what is stepper motor? What are its uses?

A: Stepper motor is the electrical machine which act upon input pulse applied to it. It is one type of

synchronous motor which runs in steps in either direction instead of running in complete cycle.so, in

automation parts it is used.

Q: How to calculate capacitor bank value to maintain unity power factor with some suitable

example?

A:KVAR= KW(TAN(COS(-1)#e)- TAN(COS(-1)#d) )

#e= EXISTING P.F.

#d= DESIRED P.F.

Q: Tell me in detail about c.t. and p.t.?(Company: reliance)

A: The term C.T means current transformer, and the term P.T means potential transformer. In circuit

where measurements of high voltage and high current is involved they are used there. Particularlywhen a measuring device like voltmeter or ammeter is not able to measure such high value of

quantity because of large value of torque due to such high value it can damage the measuring

device.so, CT and PT are introduced in the circuits. They work on the same principle of transformer,

which is based on linkage of electromagnetic flux produced by primary with secondary. They work

on the ratio to they are designed .E.g. if CT is of ratio 5000\5A and it has to measure secondary

current of 8000A.then ANS=8000*5\5000=8Aand this result will be given to ammeter. And after

measuring 8A we can calculate the primary current. Same is the operation of PT but measuring

voltage.

Q: There are a Transformer and an induction machine. Those two have the same supply. For

which device the load current will be maximum? And why?

A: The motor has max load current compare to that of transformer because the motor consumes

real power. And the transformer is only producing the working flux and its not consuming. Hence theload current in the transformer is because of core loss so it is minimum.

Q: what is power factor? whether it should be high or low? why?

A: Power factor should be high in order to get smooth operation of the system. Low power factor

means losses will be more. it is the ratio of true power to apparent power. It has to be ideally 1. If it is

too low then cable over heating & equipment overloading will occur. if it is greater than 1 then load

will act as capacitor and starts feeding the source and will cause tripping. (If pf is poor ex: 0.17 to

meet actual power load has to draw more current (V constant), result in more losses if pf is good ex:

0.95 to meet actual power load has to draw less current(V constant),result in less losses).

Q: What is the difference between Isolator and Circuit Breaker?

A: Isolator is an off load device which is used for isolating the downstream circuits from upstream

circuits for the reason of any maintenance on downstream circuits. It is manually operated and does

not contain any solenoid unlike circuit breaker. It should not be operated while it is having load. First


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the load on it must be made zero and then it can be safely operated. its specification only rated

current is given. But circuit breaker is unload automatic device used for breaking the circuit in case

of abnormal conditions like short circuit, overload etc., it is having three specification 1 is rated

current and 2 is short circuit breaking capacity and 3 is instantaneous tripping current.

Q: what is boucholz relay and the significance of it in to the transformer?

A: Boucholz relay is a device which is used for the protection of transformer from its internal faults; it

is a gas based relay. whenever any internal fault occurs in a transformer, the boucholz relay at once

gives a horn for some time, if the transformer is isolated from the circuit then it stop its sound itself

otherwise it trips the circuit by its own tripping mechanism.

Q:What is SF6 Circuit Breaker?

A:SF6 is Sulpher hexa Flouride gas.. if this gas is used as arc quenching medium in a Circuitbreaker

means SF6 CB.

Q:what is ferrantic effect?

A:Output voltage is greater than the input voltage or receiving end voltage is greater than the

sending end voltage.

Q: what is meant by insulation voltage in cables? explain it?

A:It is the property of a cable by virtue of it can withstand the applied voltage without rupturing it is

known as insulation level of the cable.

Q: Why we do 2 types of earthing on transformer (ie:)body earthing & neutral earthing , what

is function. i am going to install a 5oo kva transformer & 380 kva DG set what should the

earthing value?

A:The two types of earthing are Familiar as Equipment earthing and system earthing. In Equipment

earthing: body ( non conducting part)of the equipment shouldd be earthed to safegaurd the human

beings.system Earthing : In this neutral of the supply source ( Transformer or Generator) should be

grounded. With this,in case of unbalanced loading neutral will not be shifted.so that unbalanced

voltages will not arise. We can protect the equipment also. With size of the equipment( transformer

or alternator)and selection of relying system earthing will be further classified into directly

earthed,Impedance earthing, resistive (NGRs) earthing.

Q:What is the difference between MCB & MCCB, Where it can be used?

A:MCB is miniature circuit breaker which is thermal operated and use for short circuit protection insmall current rating circuit. MCCB moulded case circuit breaker and is thermal operated for over

load current and magnetic operation for instant trip in short circuit condition.under voltage and under

frequency may be inbuilt. Normally it is used where normal current is more than 100A.

Q:Where should the lighting arrestor be placed in distribution lines?


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A:Near distribution transformers and out going feeders of 11kv and incomming feeder of 33kv and

near power transformers in sub-stations.

Q:Define IDMT relay?

A:It is an inverse definite minimum time relay.In IDMT relay its operating is inversely proportional

and also a characteristic of minimum time after which this relay operates.It is inverse in the sense

,the tripping time will decrease as the magnitude of fault current increase.

Q:What are the transformer losses?

A:TRANSFORMER LOSSES - Transformer losses have two sources-copper loss and magnetic loss.

Copper losses are caused by the resistance of the wire (I2R). Magnetic losses are caused by eddy

currents and hysteresis in the core. Copper loss is a constant after the coil has been wound and

therefore a measurable loss. Hysteresis loss is constant for a particular voltage and current. Eddy-

current loss, however, is different for each frequency passed through the transformer.

Query:What is meant by regenerative braking?

Resolution:When the supply is cutt off for a running motor, it still continue running due to inertia. In

order to stop it quickly we place a load(resitor) across the armature winding and the motor should

have maintained continuous field supply. so that back e.m.f voltage is made to apply across the

resistor and due to load the motor stops quickly.This type of breaking is called as "Regenerative

Breaking".

Query:Why is the starting current high in a DC motor?

Resolution:In DC motors, Voltage equation is V=Eb-IaRa (V = Terminal voltage,Eb = Back emf in

Motor,Ia = Armature current,Ra = Aramture resistance).At starting, Eb is zero. Therefore, V=IaRa, Ia

= V/Ra ,where Ra is very less like 0.01ohm.i.e, Ia will become enormously increased.

Query:What are the advantages of star-delta starter with induction motor?

Resolution:(1). The main advantage of using the star delta starter is reduction of current during the

starting of the motor.Starting current is reduced to 3-4 times Of current of Direct online starting.(2).

Hence the starting current is reduced , the voltage drops during the starting of motor in systems are

reduced.

Query:Why Delta Star Transformers are used for Lighting Loads?

Resolution:For lighting loads, neutral conductor is must and hence the secondary must be star

winding. and this lighting load is always unbalanced in all three phases. To minimize the currentunbalance in the primary we use delta winding in the primary. So delta / star transformer is used for

lighting loads.

Query:Why in a three pin plug the earth pin is thicker and longer than the other pins?

Resolution:It depends upon R=rho l/a where area(a) is inversely proportional to resistance (R), so if

(a) increases, R decreases & if R is less the leakage current will take low resistance path so the


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earth pin should be thicker. It is longer because the The First to make the connection and Last to

disconnnect should be earth Pin. This assures Safety for the person who uses the electrical

instrument.

Query:Why series motor cannot be started on no-load?

Resolution:Series motor cannot be started without load because of high starting torque. Series

motor are used in Trains, Crane etc.

Query:Why ELCB can't work if N input of ELCB do not connect to ground?

Resolution:ELCB is used to detect earth leakage fault. Once the phase and neutral are connected

in an ELCB, the current will flow through phase and that much current will have to return neutral so

resultant current is zero. Once there is a ground fault in the load side, current from phase will directly

pass through earth and it will not return through neutral through ELCB. That means once side

current is going and not returning and hence because of this difference in current ELCB wil trip and it

will safe guard the other circuits from faulty loads. If the neutral is not grounded, fault current will

definitely high and that full fault current will come back through ELCB, and there will be no differencein current.

Q:Why an ac solenoid valve attract the plunger even though we interchanges the terminal?

Will the poles changes?

A:Yes because the poles changes for every half-cycle of ac voltage so the polarity of AC voltage is

continuously changing for every half cycle. so, interchanging of terminals in ac system does not

show any difference. That's why the ac solenoid attract the plunger even though it's terminals are

interchanged

Q:What is derating?, why it is necessary, it is same for all means for drives, motors,and

cables.

A:The current currying of cables will change depending upon the site temperature (location of site),

type of run (it will run through duct, trench, buried etc.), number of tray, depth of trench, distance

between cables. Considering this condition actual current currying capacity of cable reduce than

current currying capacity (which given to cable Catalogue) this is called derating.

Q:What is derating?, why it is necessary, it is same for all means for drives, motors,and

cables.

A:The current currying of cables will change depending upon the site temperature (location of site),

type of run (it will run through duct, trench, buried etc.), number of tray, depth of trench, distance

between cables. Considering this condition actual current currying capacity of cable reduce thancurrent currying capacity (which given to cable Catalogue) this is called derating.

Q:Why temperature rise is conducted in bus bars and isolators?

A:Bus bars and isolators are rated for continuous power flow, that means they carry heavy currents

which rises their temperature. so it is necessary to test this devices for temperature rise.


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Q:When voltage increases then current also increases then what is the need of over voltage

relay and over current relay? Can we measure over voltage and over current by measuring

current only?

A:No.We can't sense the over voltage by just measuring the current only because the current

increases not only for over voltages but also for under voltage(As most of the loads are non-linear in

nature).So,the over voltage protection & over current protection are completely different. Overvoltage relay meant for sensing over voltages & protect the system from insulation break down and

firing. Over current relay meant for sensing any internal short circuit, over load condition ,earth fault

thereby reducing the system failure & risk of fire.So, for a better protection of the system.It should

have both over voltage & over current relay.

Q:How do you select a cable size (Cu & Al) for a particular load?

A:At first calculate the electrical current of the load, after that derate the electrical current

considering derating factor(depending on site condition and laying of cable) after choose the cable

size from cable catalog considering derating electrical current.After that measure the length of cable

required from supply point of load to load poin. Calculate the voltage drop which will max 3%

(resistance and reactance of cable found from cable catalog of selecting cable) if voltage drop>3%then choose next higher size of cable.

Q:What are HRC fuses and where it is used?

A:HRC stand for "high rupturing capacity" fuse and it is used in distribution system for electrical

transformers.

Q:Which power plant has high load factor?

A:All base load power plants have a high load factor. If we use high efficiency power plants to supply

the base load,we can reduce the cost of generation.Hydel power plants have a higher efficiency than

thermal & nuclear power plants.

Q:Mention the methods for starting an induction motor?

A:The different methods of starting an induction motor

DOL:direct online starter

Star delta starter

Auto transformer starter

Resistance starter

Series reactor starter

Q:What is the difference between earth resistance and earth electrode resistance?

A:Only one of the terminals is evident in the earth resistance. In order to find the second terminal we

should recourse to its definition: Earth Resistance is the resistance existing between the electricallyaccessible part of a buried electrode and another point of the earth, which is far away.

The resistance of the electrode has the following components:

(A) the resistance of the metal and that of the connection to it.

(B) the contact resistance of the surrounding earth to the electrode.

Q:What is use of lockout relay in ht voltage?

A:A lock-out relay is generally placed in line before or after the e-stop switch so the power can be


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shut off at one central location. This relay is powered by the same electrical source as the control

power and is operated by a key lock switch. The relay itself may have up to 24 contact points within

the unit itself. This allows the control power for multiple machines to be locked out by the turn of a

single key switch.

Q:What is the power factor of an alternator at no load?A:At no load Synchronous Impedance of the alternator is responsible for creating angle difference.

So it should be zero lagging like inductor.

Query:How to determine capacitor tolerance codes?

Resolution:In electronic circuits, the capacitor tolerance can be determined by a code that appears

on the casing. The code is a letter that often follows a three-digit number (such as 130Z).The first

two are the 1st and 2nd significant digits and the third is a multiplier code. Most of the time the last

digit tells you how many zeros to write after the first two digits and these are read as Pico-Farads.

Query:Why most of analog o/p devices having o/p range 4 to 20 mA and not 0 to 20 mA?

Resolution:4-20 mA is a standard range used to indicate measured values for any process. Thereason that 4ma is chosen instead of 0 mA is for fail safe operation .For example- a pressure

instrument gives output 4mA to indicate 0 psi, up to 20 mA to indicate 100 psi, or full scale. Due to

any problem in instrument (i.e) broken wire, its output reduces to 0 mA. So if range is 0-20 mA then

we can differentiate whether it is due to broken wire or due to 0 psi.

Query:Two bulbs of 100w and 40w respectively connected in series across a 230v supply

which bulb will glow bright and why?

Resolution:Since two bulbs are in series they will get equal amount of electrical current but as the

supply voltage is constant across the bulb(P=V^2/R).So the resistance of 40W bulb is greater and

voltage across 40W is more (V=IR) so 40W bulb will glow brighter.

Query:What is meant by knee point voltage?

Resolution:Knee point voltage is calculated for electrical Current transformers and is very important

factor to choose a CT. It is the voltage at which a CT gets saturated.(CT-current transformer).

Query:What is reverse power relay?

Resolution:Reverse Power flow relay are used in generating stations's protection. A generating

stations is supposed to fed power to the grid and in case generating units are off,there is no

generation in the plant then plant may take power from grid. To stop the flow of power from grid to

generator we use reverse power relay.

Query:What will happen if DC supply is given on the primary of a transformer?

Resolution:Mainly transformer has high inductance and low resistance.In case of DC supply there

is no inductance ,only resistance will act in the electrical circuit. So high electrical current will flow

through primary side of the transformer.So for this reason coil and insulation will burn out.


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What is transposition of a transmission line?

The balancing effect on account of irregular spacing of conductors can be avoided by changing the

position of the conductors at regular distances. this is called transposition of conductors. In case of

3- phase transmission lines, the inductance and capacitance of each phase will be different if the

three conductors are spaced differently (each phase to the other two phases, and each phase toground). the apparent resistance of the conductors is also affected on account of transfer of power

between the phases which occur due to mutual coupling. so all the three parameters of the

transmission lines are affected by irregular spacing of the conductors.

Alternator/Generator vector diagram

Synchronous motor vector diagram

Transformer Diagram
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Word Bank

Recapitulate - To repeat in concise form.

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==

Interview

Economizer

Solar cell working principle and raw materials

Globalization

Global warming and its cause

Name of the liquid mixture in lead-acid battery

Application of capacitors

BCD Code (valid/invalid)

Flip-flop conditions

Transistor (, )

Conditions for parallel operation of two alternators

Conditions for parallel operation of two transformers

============================================================================

==

Power plant

Unit generated per day = Max. demand (KW) Load factor 24 hours

Units generated / annum = Max. demand Load factor Hours in a year

Demand factor = (Max. demand) / (connected load)
http://lh5.ggpht.com/-yRsFgr-7CfA/UCalys3gRwI/AAAAAAAAHAY/kV8u3c50ShE/s1600-h/clip_image0033.gif


16/46

Average demand = (units generated per annum) / (hours in a year)

Load factor = average demand / Max. demand

Plant capacity factor = average demand / Plant capacity = (Units generated per annum) / (Plant

capacity Hours in a year)

Reserve capacity of plant = Plant capacity Max. demand

(Straight line method) Annual depreciation charges = (P-S) / n, P = initial cost of equipments, n =

useful life, S = Salvage or scrap value

(Sinking fund method) Annual deposit, q = (P-S)[r / {(1+r)n - 1}], r = compound interest

Sinking fund at the end of mth years = q {(1+r)m -1}/r

Demand charge = tariff on maximum demand maximum demand

Energy charge = tariff on energy consumption annual consumption

Annual bill = Demand charge + Energy charge

Flat rate of energy consumption = Flat rate / unit = (annual bill) / (annual consumption)

1 calorie = 4.1858 Joules

Overall efficiency = Thermal efficiency Electrical efficiency

overall = (Electrical output in heat units) / (Heat of combustion)

Thermal efficiency = boiler turbine

For hydroelectric plant, Electrical energy available = W H overall, W = mg

For hydroelectric plant, the volume of water which can be utilized per annum =

Catchment area Annual rainfall Yield factor

For hydroelectric plant, work done / sec. = W H = gross plant capacity

Firm capacity = Plant efficiency gross plant capacity

For hydroelectric plant, output per annum = Firm capacity Hours in a year

For hydroelectric plant, overall efficiency, overall= penstock turbine generator

Diversity factor = (Sum of individual max. demand) / (Max. demand on power plant)

Daily average load = (No. of units generated in a day) / (24 hours)

1 BTU(British Thermal Unit) = 1053 Jouls


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1 KWh = 3418 BTU

Efficiency of steam power station is about 29%

From the time the power station is installed, its equipment steadily deteriorates due to wear and

tear so that there is a gradual reduction in the value of the plant. This reduction in the value of plant

every year is known as depreciation. Due to depreciation, the plant has to be replaced by the new

one after its useful life. Thus while determining the cost of production of electrical energy, annual

depreciation charge must be included.

AC

Active power = VA cos

Reactive power = VA sin

Q = (S2 P2), Q = reactive power, S = apparent power, P = active power

Power factor = (active power) / (apparent power)

For fixed P and VL, P.F 1 / IL, so, IL P.F ]

Alternators

Balanced earth-fault protection is generally provided for small size alternators.

Overheating caused by overloads, short-circuit, failure of cooling systems.

The most dangerous fault in an alternator is the stator winding fault.

Faults of alternator 1. Failure of prime mover 2. Failure of field 3. Over current 4. Over speed 5.Over voltage 6. Unbalanced loading 7. Stator winding faults

Protection of alternator 1. Differential protection 2. Balanced earth fault protection 3. Stator inter-turn protection

Inverted running of alternator => When input to the prime-mover fails, the prime-mover stops the

conversion of mechanical energy into electrical energy. If the alternator works in parallel with other

alternators, then it will start working as a synchronous motor running at synchronous speed. Thismotoring condition is known as inverted running.

Synchronous motor

Rated synchronous motors at 150KW (200 H.P) to 15 MW (20000H.P) - speeds-> 150 rpm to 1800

rpm.


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For synchronous motors, supply frequency in cycles per second = the rotor revolves at the same

cycles per second.

Synchronous motor is a doubly fed motor where torque is non-zero at only one speed called

synchronous speed.

If the mechanical angle between the stator and rotor poles increases, then stator current isincreased.

Use of Synchronous motors: Synchronous motors are rarely used below 50 H.P. because of their

much higher initial cost compared to 3-phase induction motors. However, their unique characteristics

of constant speed operation, power-factor control and high operating efficiency make them highly

suitable for heavy industry, particularly for applications requiring low speed and high horsepower.

1. A very common application is for driving air or gas compressors. It is especially desirable that

compressors be driven at constant speed since their output and efficiency vary considerably with the

speed at which they are operated.

2. Another important application is for driving large DC generators. The synchronous motor does the

additional job of improving the p.f of the system.

Note: In addition to the above mentioned cases, synchronous motors are also prominently found in

the following application:

1. Large, low head pumps

2. Flour-mill line shafts

3. Rubber mills and mixers

4. Crushers

5. Chippers

6. Centrifugal pumps

7. Rolling mills

8. Pulp grinders

9. Frequency changers etc.

Hunting of a synchronous motor- Although the speed of a synchronous motor is constant, it oftentends to oscillate or hunt around its synchronous speed. This hunting is caused due to (1) change in

load (2) change in excitation (3) change in some other condition in the system. Hunting results in

rapid change of mechanical angle between the rotor and stator poles and corresponding variations

of stator current. If hunting becomes too severe, either the circuit of the motor will be opened by the

circuit breaker or the motor will pull-out of synchronism and come to a halt. Hunting is reduced by

damper winding provided on the rotor of the motor.


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Transmission and Distribution

For very long high voltage transmission lines over current protection is distance protection.

For greater length and exposure to atmosphere overhead line have more chance of faultsoccurring.

Li2 = Cv2

To eliminate voltage surge required, R = (L/C)

% reactance at Base KVA = {(Base KVA) / (Rated KVA)} (% reactance of rated KVA)

Short circuit current, Isc = I {100 / (% reactance)}, I = Full load current

% resistance of line = (Base KVA Resistance of line) / {10 (KV)2}

Zero Phase Sequence Current in each Phase = 1/3 [current in neutral wire]

Positive Sequence Impedance = Negative Sequence Impedance

Zero Phase Sequence Component of Current (R-phase), IR0 = 1/3 [IR + IY + IB]

Positive Phase Sequence Component of Current (R-phase), IR1 = 1/3 [IR+ IY+ 2 IB]

Negative Phase Sequence Component of Current (R-phase), IR2 = 1/3 [IR+ 2 IY+ IB]

IR0 = IY0 = IB0

IR = IR0 + IR1 + IR2

IY = IR0 + 2IR1+ IR2

3-wire DC system makes 2 voltages, (1) Between the neutral and any of the two outers (2)

Between the outers.

In interconnected systems feeder ring is energized by more then 1 substation so it is the most

reliable.

Insulation resistance, R = (/2L) loge(r2/r1), r1 = inner radius, r2 = outer radius, L = length of thecable.

Thus R (1/L)

gmax/ gmin = D/d, g= stress, d = core diameter, D = internal sheath diameter

gmax = 2v / {d loge(D/d)}

Cable Capacitance, C = [rl/ {41.4 log10(D/d)}] 10-9 F


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Insulation capacitances are ineffective for DC, so string efficiency is 100%

Voltage across string = Phase Voltage

String Efficiency = {(Voltage across string) / (No. of insulators V3)} 100, V3 = Voltage across the

string nearest to the conductor.

For shunt capacitance decreases voltage across various discs tend to be uniform, thus

V3 decreases and string efficiency increases.

Sag, S = wl2/ 8T, w=weight per meter length of the conductor, l= length of span, T = tension

S 1/T, S is provided to reduce T

X1 = (l/ 2) (Th/wl), w = weight per meter, T = Tension in the conductor

X2 = (l/ 2) + (Th/wl)

Working tension, T = Working stress Area of conductor

Density of Ice = 0.9 103 Kg/m3

Volume of ice per meter length of conductor = /4 [(D + 2t) 2 D2] 1 = t (D + t)

t = thickness of ice on conductor

For transmission, area of cross-section of conductor 1/V2

For transmission, volume of conductor material (also cost of conductor material, c), v 1 /(V2cos2), for P.F unity, C orv 1 / V2

Energy lost per annum(annual) = 2I2Rt / 1000 KWh, R = Resistance of the wire, t = time in hours in

a year

Annual cost of energy lost = cost per KWh annual energy loss

Annual charge = annual interest and depreciation on capital cost of cable.

According to Kelvins law, for most economical cross-section of the conductor variable annualcharge = Annual cost of energy lost

The disadvantage of a radial feeder by using definite time relay is that if there are a number of

feeders in series, tripping time for faults near the supply end become high. This is undesirable

because the safety of the equipment and other components in line of the system will be endangered.

Symmetrical components do not have separate existence. They are only mathematical

components of unbalanced currents or voltages which do exist in the system.

The choice of transmission voltage mainly depends on the following factors:

1. Length of transmission line


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2. Amount of power to be transmitted

3. Percentage voltage regulation required

4. Cost of switchgear, terminal equipment and conductor material.

An electric supply system consists of three principle components viz the power station, thetransmission lines and the distribution system.

Switchgear

The operating time of relay should be less then 1 second.

Isolators open a circuit under no load condition.

Current chopping occurs at air-blast circuit breaker for low currents.

Air-blast circuit breaker- High pressure air-blast is used for extinguishing the arc. The contacts are

opened in a flow of air-blast established by the opening of blast valve. The air blast cools the arc and

sweeps away the arcing products to the atmosphere. This rapidly increased the dielectric strength of

the medium between contacts and prevents from reestablishing the arc. Consequently the arc is

extinguished and flow of current is interrupted.

The rated normal current = Current rating of the CB

Rated symmetrical breaking current of a CB = (VA rating)/(3 Voltage rating)

Breaking Capacity = VA rating of the CB

Rated making current = 2.55 Rated symmetrical breaking current

Low oil CB has small amount of oil only sufficient for arc extinction.

Breaking capacity of SF6 does not depend on weather and moisture, pollution and atmospheric

pressure etc.

Pick-up current = Rated secondary current of C.T Current Setting

Plug Setting Multiplier (P.S.M) = (Fault Current in relay coil) / (Pick-up current)

For fuse wire I2 r3

The operating time of a fuse = 0.002 sec. and for CB = 0.2 sec.

Peak Re-striking Voltage occurs at time, t = 1 / (2fn), t = time for 1st peak.

Average Rate of Rise of Re-striking Voltage,

(RRRV) = (Peak re-striking voltage) / (time to reach the peak)


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The fusing current is the minimum current at which the fuse element melts and thus disconnects

the circuit protected by it.

The fusing current depends upon the various factors such as:

1. Material of fuse element

2. Length-the smaller the length, the greater the current because a short fuse can easily conduct

away the heat.

3. Diameter

4. Size and location of terminals

5. Previous history

6. Type of enclosure used

Circuit Breaker => Circuit Breakers are designed to interrupt either normal or short-circuit currents.They behave like big switches that may be opened or closed either manually or automatically. A

circuit breaker will automatically open a circuit whenever the line current exceeds a preset value.

Digital logic

Boolean algebra- (1) X+X = 1 (2) X+1 = 1 (3) (X) = X (4) (X+Y) = X Y (5) X+XY = X (6) X.1 = X(7) X.X = 0 (8) X.X = X (9) X+YZ = (X+Y)(X+Z) (10) (XY) = X + Y (11) X(X+Y) = X

RS Flip-flop-

S R Q Q

0 0 1 0 (after s=1, R= 0)

0 1 (after s=0, R= 1)

0 1 0 1

1 0 1 0

1 1 0 0

Electronics

Transistor current gain, = IC / IB

= IC / IE

**Common collector connection is primarily used for impedance matching purpose. Because of its

high input impedance and low output impedance.


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In case of Darlington Pair, = 12

Induction motor

For induction motor, (rotor output) / (rotor input) = 1 S

Circuit

1 Coulomb = Charge on 625 1016 electrons.

Active element => Supplies electrical energy to the circuit.

Passive element => Receives electrical energy and then converts it into heat or store in an

electrical field or magnetic field.

Nodes => Two or more circuit elements are joined.

Junctions => Three or more circuit elements are joined.

Branch => Lies between two junction points.

Loop => Closed path of a network.

Mesh => Elementary form of a loop which cannot be further divided into other loops.

Reciprocity Theorem => In any linear bilateral network, if an emf E acting in a branch X causes a

current I in branch Y, then the same emf E located in branch Y will causes a current I in branch X.

1 H.P = 746 watts.

Work done per second = 2NT/60 watts, N=> R.P.M, T=> Torque

Charge on capacitor during charging, q = Q (1 e-t/RC), Q = Max. Charge

Voltage on capacitor during charging, v= V (1 - e-t/RC)

During discharging, q= Q e-t/RC

During discharging, I = I0 e-t/RC

Paramagnetic materials => Relative permeability slightly greater then 1 and much less then 1.1 =>

Aluminum and Platinum.

Diamagnetic Materials => Relative permeability slightly less then 1. => Carbon, Copper and Silveretc.

Ferromagnetic materials => Relative permeability much greater then 1.

Lenzs law => The induced current will flow in such a direction so as to oppose the cause thatproduced it.

Transformer


24/46

All day efficiency, all-day = (KWH output in 24 hours) / (KWH input in 24 hours)

Protection provided to a distribution transformer-

1. Air break switch (gang-operating switch) with a fuse is provided on the 11 KV side of the

transformer for isolation and protection respectively.

2. LT switch with a fuse is provided on the LT side of the transformer for isolation and protection

against feeder faults.

History of electricity in Bangladesh

1st electric lamp in Dhaka 7/12/1901

Electricity generation capacity of Bangladesh 5560mw

Electricity requirement in Bangladesh 6066mw

In 2008-2009 per head daily production of electricity is 172 KWH

In 2008-2009 Max. electricity production in Bangladesh is 4130 mw

Kaptai hydro electric power plant established in 1962

Ruppur nuclear power plant- 1961

Rural Electrification Board established in 1977

Khulna barge-mount power plant capacity 110mw

Bijoyer alo from Labuan iland of Malaysia 3/8/1999130mw

Bd system loss-6.62%

DESA system loss 14.26%

Meghna ghat elect. 450mw largest in bd

1st gas elect horipur, Shilet

1st coal elect bor pukuria, dinaj pur

New 4 elect- 1. Narayan gonj shiddirgonj 210mw 2. Shylhet shahji bajar 60mw 3. Gazipur tongi

80mw 4. Dinajpur borpukuria 250 mw

#Economizer

Economizers are mechanical devices intended to reduce energy consumption, or to perform another

useful function like preheating a fluid. The term economizer is used for other purposes as well. In

simple terms, an economizer is a heat exchanger.


25/46

#Solar cell working principle and raw materials

Traditional explanation

The photons of a light beam have a characteristic energy determined by the frequency of the light. In

the photoemission process, if an electron within some material absorbs the energy of one photon

and thus has more energy than the work function (the electron binding energy) of the material, it is

ejected. If the photon energy is too low, the electron is unable to escape the material. Increasing the

intensity of the light beam increases the number of photons in the light beam, and thus increases the

number of electrons emitted, but does not increase the energy that each electron possesses. Thus

the energy of the emitted electrons does not depend on the intensity of the incoming light, but only

on the energy of the individual photons.

Raw materials

The solar module consists of the silicon semiconductor surrounded by protective material in a metal

frame.

#Globalization

Globalization describes an ongoing process by which regional economies, societies, and cultures

have become integrated through a globe-spanning network of communication and trade. The term is

sometimes used to refer specifically to economic globalization: the integration of national economies

into the international economy through trade, foreign direct investment, capital flows, migration, and

the spread of technology.

#Global warming

1. Carbon dioxide and other air pollution that is collecting in the atmosphere like a thickening blanket,trapping the sun's heat and causing the planet to warm up.

2. Global warming doesn't create hurricanes, but it does make them stronger and more dangerous.Because the ocean is getting warmer, tropical storms can pick up more energy and become more

powerful. So global warming could turn, say, a category 3 storm into a much more dangerous

category 4 storm.

1. Melting glaciers, early snowmelt and severe droughts will cause more dramatic water shortages inthe American West.

2. Rising sea levels will lead to coastal flooding on the Eastern seaboard, in Florida, and in other areas,such as the Gulf of Mexico.

3. Warmer sea surface temperatures will fuel more intense hurricanes in the southeastern Atlantic andGulf coasts.

4. Forests, farms and cities will face troublesome new pests and more mosquito-borne diseases.5. Disruption of habitats such as coral reefs and alpine meadows could drive many plant and animal

species to extinction.

6. The United States produce 25 percent of the carbon dioxide pollution from fossil-fuel burning -- byfar the largest share of any country. In fact, the United States emits more carbon dioxide than China,

India and Japan, combined.

#Name of the liquid mixture in lead-acid battery


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Electrolytes

#Use of Capacitors

1. Energy storage

2. Pulsed power and weapons

3. Power conditioning

4. Power factor correction

5. Supression and coupling

6. Decoupling

7. Noise filters and snubbers--When an inductive circuit is opened, the current through the

inductance collapses quickly, creating a large voltage across the open circuit of the switch or relay. If

the inductance is large enough, the energy will generate a spark, causing the contact points tooxidize, deteriorate, or sometimes weld together, or destroying a solid-state switch.

Asnubbercapacitor across the newly opened circuit creates a path for this impulse to bypass the

contact points, thereby preserving their life; these were commonly found incontact breakerignition

systems, for instance. Similarly, in smaller scale circuits, the spark may not be enough to damage

the switch but will stillradiateundesirableradio frequency interference(RFI), which a filter

capacitor absorbs. Snubber capacitors are usually employed with a low-value resistor in series, to

dissipate energy and minimize RFI. Such resistor-capacitor combinations are available in a single

package.Capacitors are also used in parallel to interrupt units of a high-voltagecircuit breakerin

order to equally distribute the voltage between these units. In this case they are called grading

capacitors.

8. Motor starters

9. Signal processing-The energy stored in a capacitor can be used to representinformation,

either in binary form, as inDRAMs, or in analogue form, as inanalog sampled filtersandCCDs.

Capacitors can be used inanalog circuitsas components of integrators or more complex filters and

innegative feedbackloop stabilization. Signal processing circuits also use capacitors tointegratea

current signal.

10. Tuned circuits

11. Sensing

#BCD Code

1. To BCD-encode decimal number using the common encoding, each decimal digit is stored in a

four-bitnibble.

Decimal: 0 1 2 3 4 5 6 7 8 9

BCD: 0000 0001 0010 0011 0100 0101 0110 0111 1000 1001
http://en.wikipedia.org/wiki/Snubberhttp://en.wikipedia.org/wiki/Snubberhttp://en.wikipedia.org/wiki/Snubberhttp://en.wikipedia.org/wiki/Contact_breakerhttp://en.wikipedia.org/wiki/Contact_breakerhttp://en.wikipedia.org/wiki/Ignition_systemhttp://en.wikipedia.org/wiki/Ignition_systemhttp://en.wikipedia.org/wiki/Ignition_systemhttp://en.wikipedia.org/wiki/Ignition_systemhttp://en.wikipedia.org/wiki/Spark-gap_transmitterhttp://en.wikipedia.org/wiki/Spark-gap_transmitterhttp://en.wikipedia.org/wiki/Spark-gap_transmitterhttp://en.wikipedia.org/wiki/Radio_frequency_interferencehttp://en.wikipedia.org/wiki/Radio_frequency_interferencehttp://en.wikipedia.org/wiki/Radio_frequency_interferencehttp://en.wikipedia.org/wiki/Circuit_breakerhttp://en.wikipedia.org/wiki/Circuit_breakerhttp://en.wikipedia.org/wiki/Informationhttp://en.wikipedia.org/wiki/Informationhttp://en.wikipedia.org/wiki/Informationhttp://en.wikipedia.org/wiki/DRAMhttp://en.wikipedia.org/wiki/DRAMhttp://en.wikipedia.org/wiki/DRAMhttp://en.wikipedia.org/wiki/Analog_sampled_filterhttp://en.wikipedia.org/wiki/Analog_sampled_filterhttp://en.wikipedia.org/wiki/Analog_sampled_filterhttp://en.wikipedia.org/wiki/Charge-coupled_devicehttp://en.wikipedia.org/wiki/Charge-coupled_devicehttp://en.wikipedia.org/wiki/Charge-coupled_devicehttp://en.wikipedia.org/wiki/Analog_circuithttp://en.wikipedia.org/wiki/Analog_circuithttp://en.wikipedia.org/wiki/Analog_circuithttp://en.wikipedia.org/wiki/Negative_feedbackhttp://en.wikipedia.org/wiki/Negative_feedbackhttp://en.wikipedia.org/wiki/Negative_feedbackhttp://en.wikipedia.org/wiki/Integralhttp://en.wikipedia.org/wiki/Integralhttp://en.wikipedia.org/wiki/Integralhttp://en.wikipedia.org/wiki/Nibblehttp://en.wikipedia.org/wiki/Nibblehttp://en.wikipedia.org/wiki/Nibblehttp://en.wikipedia.org/wiki/Nibblehttp://en.wikipedia.org/wiki/Integralhttp://en.wikipedia.org/wiki/Negative_feedbackhttp://en.wikipedia.org/wiki/Analog_circuithttp://en.wikipedia.org/wiki/Charge-coupled_devicehttp://en.wikipedia.org/wiki/Analog_sampled_filterhttp://en.wikipedia.org/wiki/DRAMhttp://en.wikipedia.org/wiki/Informationhttp://en.wikipedia.org/wiki/Circuit_breakerhttp://en.wikipedia.org/wiki/Radio_frequency_interferencehttp://en.wikipedia.org/wiki/Spark-gap_transmitterhttp://en.wikipedia.org/wiki/Ignition_systemhttp://en.wikipedia.org/wiki/Ignition_systemhttp://en.wikipedia.org/wiki/Contact_breakerhttp://en.wikipedia.org/wiki/Snubber


27/46

2. Thus, the BCD encoding for the number 127 would be:

Decimal: 1 2 3

BCD: 0001 0010 0111

# Conditions for Parallel operation of transformer-

1. Primary windings of the transformers should be suitable for the supply system voltage andfrequency.

2. The transformers should be properly connected with regard to polarity.3. The voltage rating of both primaries and secondaries should be identical (same turn ratio or

transformation ratio).

4. The percentage impedance should be equal in magnitude and same X/R ratio in order to avoidcirculating currents and operation at different power factors.

5. With transformers having different KVA ratings the equivalent impedances should be inverselyproportional to the individual KVA rating if circulation currents to be avoided.

#Conditions for Parallel operation of Alternator-

1. The terminal voltage (effective) of the incoming alternator must be the same as the bus-bar voltage.2. The speed of the incoming machine must be such that its frequency (=120f/p) equals the bus-bar

frequency.

3. The phase of the alternator voltage must be identical with the phase of the bus-bar voltage.#Alternator Protection

The three main type protection systems are:

over current protection reverse power trip under voltage trip

The over current protection works by removing power supplies to non essential services on a

preferential basis. This is done with the help of preferential trips which decides the load that is to be

removed in case of overloading of the distribution system.

Reverse power trip and under voltage trip are required only if there is more than one alternator on

board which requires working of two alternators on load.

#Over-Current Protection

Every alternator has an over current protection. With the help of this trip, the alternator and

distribution system can be protected from various faults but the main thing to be considered in thismethod is to maintain power to the distribution system till the time the alternator trips on any other

protection devices.

For this reason, the protection device has been designed in such a way that in case the over current

is not high enough, a time delay provided by an inverse definite minimum time (IDMT) relay occurs,

which prevents the alternator from tripping in case the over current values reduces back to normal

within the IDMT characteristics


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But in case of a major fault such as short circuit, the alternator will trip instantaneously without any

delay, protecting all devices on the distribution system. Overload of alternator is caused either due to

increased switchboard load or serious fault causing very high current flow.

If sudden over load occurs then, the load is reduced with the help of preferential trips which removes

non essential load such as of air conditioning, ventilation fans etc., from the switchboard. Thesepreferential trips are operated by relays which are set to about 110% of the normal full load of

alternator.

#Inverse Definite Minimum Time (IDMT) Relay

The construction of this type of relay is similar to that of a household watt meter. The actual inverse

time delay characteristics are obtained with the help of an induction type relay.

#Reverse Power Protection

There is not much difference between an alternator andelectric motorsfrom the engineer's

perspective. They are both based on similar principles. So just imagine what would happen if analternator suddenly would act as a motor. This is only possible in systems where two or more

generators arerunning in parallel.

Hence this type of protection system is used only if there is more than one alternator on board a

ship. The system is designed in such a way that it will release the breaker and prevent motoring of

alternator if a reversal of power occurs. This protection device is also used to prevent damage to the

prime mover, which might be stopped due to some fault. Though it is extremely difficult to detect

reverse current with an alternating current system, reverse power can be detected and protection

can be provided by reverse power relay.

#Programmable logic controller

A programmable logic controller (PLC) or programmable controller is a digital computer used

for automation of electromechanical processes, such as control of machinery on factory assembly

lines, amusement rides, or lighting fixtures. PLCs are used in many industries and machines. Unlike

general-purpose computers, the PLC is designed for multiple inputs and output arrangements,

extended temperature ranges, immunity to electrical noise, and resistance to vibration and impact.

Programs to control machine operation are typically stored in battery-backed or non-volatile memory.

A PLC is an example of a real time system since output results must be produced in response to

input conditions within a bounded time, otherwise unintended operation will result.

The main difference from other computers is that PLCs are armored for severe conditions (such as

dust, moisture, heat, cold) and have the facility for extensiveinput/output(I/O) arrangements. Theseconnect the PLC tosensorsandactuators. PLCs read limitswitches, analog process variables (such

as temperature and pressure), and the positions of complex positioning systems. Some usemachine

vision. On the actuator side, PLCs operateelectric motors,pneumaticorhydrauliccylinders,

magneticrelays,solenoids, or analog outputs. The input/output arrangements may be built into a

simple PLC, or the PLC may have external I/O modules attached to a computer network that plugs

into the PLC.
http://www.brighthub.com/engineering/electrical/articles/42787.aspxhttp://www.brighthub.com/engineering/electrical/articles/42787.aspxhttp://www.brighthub.com/engineering/electrical/articles/42787.aspxhttp://www.brighthub.com/engineering/marine/articles/31782.aspxhttp://www.brighthub.com/engineering/marine/articles/31782.aspxhttp://www.brighthub.com/engineering/marine/articles/31782.aspxhttp://en.wikipedia.org/wiki/Input/outputhttp://en.wikipedia.org/wiki/Input/outputhttp://en.wikipedia.org/wiki/Input/outputhttp://en.wikipedia.org/wiki/Sensorhttp://en.wikipedia.org/wiki/Sensorhttp://en.wikipedia.org/wiki/Sensorhttp://en.wikipedia.org/wiki/Actuatorhttp://en.wikipedia.org/wiki/Actuatorhttp://en.wikipedia.org/wiki/Switchhttp://en.wikipedia.org/wiki/Switchhttp://en.wikipedia.org/wiki/Switchhttp://en.wikipedia.org/wiki/Machine_visionhttp://en.wikipedia.org/wiki/Machine_visionhttp://en.wikipedia.org/wiki/Machine_visionhttp://en.wikipedia.org/wiki/Machine_visionhttp://en.wikipedia.org/wiki/Electric_motorhttp://en.wikipedia.org/wiki/Electric_motorhttp://en.wikipedia.org/wiki/Electric_motorhttp://en.wikipedia.org/wiki/Pneumatichttp://en.wikipedia.org/wiki/Pneumatichttp://en.wikipedia.org/wiki/Hydraulichttp://en.wikipedia.org/wiki/Hydraulichttp://en.wikipedia.org/wiki/Hydraulichttp://en.wikipedia.org/wiki/Relayhttp://en.wikipedia.org/wiki/Relayhttp://en.wikipedia.org/wiki/Relayhttp://en.wikipedia.org/wiki/Solenoidhttp://en.wikipedia.org/wiki/Solenoidhttp://en.wikipedia.org/wiki/Solenoidhttp://en.wikipedia.org/wiki/Solenoidhttp://en.wikipedia.org/wiki/Relayhttp://en.wikipedia.org/wiki/Hydraulichttp://en.wikipedia.org/wiki/Pneumatichttp://en.wikipedia.org/wiki/Electric_motorhttp://en.wikipedia.org/wiki/Machine_visionhttp://en.wikipedia.org/wiki/Machine_visionhttp://en.wikipedia.org/wiki/Switchhttp://en.wikipedia.org/wiki/Actuatorhttp://en.wikipedia.org/wiki/Sensorhttp://en.wikipedia.org/wiki/Input/outputhttp://www.brighthub.com/engineering/marine/articles/31782.aspxhttp://www.brighthub.com/engineering/electrical/articles/42787.aspx


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#explain power factor in simple words.....

The beneficial part of the power supplied to the circuit that the circuit actually performs work i.e. ratio

of real power to apparent power is called power factor

#Which power plant has high load factor?

Nuclear power plant. This plant consumes less energy for generating high power.

#How many types of dc batteries standards are present?

1.2v (Ni-cad) 1.5 v (dry cell), 6v (lead-acid), 9v, 12v (lead-acid)

#How many methods for starting an induction motor?

1. DIRECT ON LINE STARTER (DOL)

2. STAR DELTA STARTER

3. VARIABLE FREQUENCY DRIVES

4. RESISTANCE STARTERS

5. AUTO TRANSFORMER STARTER

6. SERIES REACTOR STARTER

#An electric bulb is connected on a holder. One end of the wire is connected to the phase and

the remaining one connected to earth instead of neutral. the bulb will glow or not? if yes then

what is the need of neutral?

Yes the bulb will glow, if the earth is pakka. The neutral is earthed at each distribution transformers,

form this point only neutral is taken.

#If a capacitor connected to power supply will it consume power & will it going to get

recorded in energy meter

This consumption of energy is getting recorded in electronic energy meters but I dont know it iscorrect or not

#Can u start a tube light without a starter?

YES! Tube light starter is just like a push button. Not of use once the system is started. When u have

switched on the supply in the TL touch and remove the contacts of the 2 terminals that were to be

connected to the starter a several times so that u generate appropriate di/dt, until the required

voltage is induced in the choke coil that u'll see as light starting glowing

#What is meant by HRC fuses? In the distribution system different rating hrc fuses are used

for transformer (which is 3k, 6k, 10k, 15k etc).what is k?


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HRC stand for "high rupturing capacity" fuse & k stands for thousand, 3k means 3000 ohm, 5k

means 5000 ohm like wise.

#What is the difference between earth resistance and earth electrode resistance?

Only one of the terminals is evident in the earth resistance. In order to find the second terminal we

should recourse to its definition: Earth Resistance is the resistance existing between the electrically

accessible part of a buried electrode and another point of the earth, which is far away.

The resistance of the electrode has the following components:

(A) the resistance of the metal and that of the connection to it.

(B) the contact resistance of the surrounding earth to the electrode.

(C) the resistance in the surrounding earth to current flow or earth resistivity which is often the most

significant factor.

#Can anyone explain how the vector diagram of a transformer which doesn't have name platebe found whether it is Dy11/Dy5/Yd11 and so on?

The designation Yy indicates that the upper and under voltage windings have been star-connected.

Yd denotes upper voltage winding as star and under voltage winding as delta.

Vector group designation

Vector group = circuit + index

Example:

Yy0 =Y star connection of the upper voltage winding OS,

y star connection of the under voltage winding US,

030 degrees = 0 degrees phase displacement

Yah! can.

If Dy11

Primary winding having only 3 terminals with high resistance and secondary having 4 terminals with

low resistance.

If Yd11

Primary having 4 terminals with high resistance and

secondary having 3 terminals with low resistance.

If Dy5

This is not a verctor group.


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Eppdi???

Ya ! first of all you can decide Yd or Dy by seeing it primary & secondary connections and 1,2,3....11

we can decide it by doing vector group tests int that we can check phase shift of wave form & we

can conclude Whether Dy or Yd can be ascertained from the bushings.To find particular vector

group ,CRO can be used finding relationship between the current vectors.e.g. In Dy 11 I2R lags I1Rby 30 degrees Number 11 denotes position of minute- hand in a clock. It can also found out by

taking various voltage measurements and analyzing.

#Why we do occ & scc test in transformer?how we can do it ?

occ test is perfomed to find no load loss of transformer.

scc test is perfomed to find full load loss of transformer.

oc for iron loss

sc for copper loss

#What is Electricity?

FLOW OF ELECTRONS INA CONDUCTOR AGAINST A RESISTANCE

#What is meant by Floating Neutral? How to prevent floating neutral is one whose voltage is

not constant?

floating earthing means the neutral is not earth by any means like solid earthing, or impedance

earthing etc. due to that if any earth fault occur in any phase the other phase voltage increase root 3

times from the earth but faulty phase voltage become zero from the earth. But your line voltage

remains the same.

It is prevented by use of NGR

Floating neutral means due to if voltage in all three phase is not same due to asymmetrical load at

output then voltage drop through all phase are not same so neutral point is shifted from center is

prevent by maintain constant load at all phase

Floating neutral means ungrounded neutral

#What is Skin effect in Cable?

Whenever an alternating current pass through the conductor the current density is near the surface

of the conductor is greater than the core (middle of the conductor).

#What is cogging in motors?

Cogging phenomenon is seen in the case of 3 phase Induction Motor (SCIM-Squirrel Cage Induction

Motor).


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When number of stator and rotor slots are exactly equal and polarity of conductors facing each other

in slots is opposite, In such cases, there is a magnetic locking between facing conductors housed in

stator and rotor slots. Since the rotor inertia is large so motor does not start at all. This is called

Cogging.

This is avoided by skewing the rotor conductor slots over the surface by angle in the range of 5degree.

#What is relation between voltage and current and don't take resistance here. If we suppose

say when there is increase in voltage what happens to current? Is it decrease or increase?

It is further added to my answer that if a fixed amount of power is to be transmitted, higher the

voltage, lower will be the current.

For a particular power to be transmitted then for increase in voltage we can see decrement in

current....this principle is generally used in transmission lines....

#What is Modulation and need for Modulation?

Modulation is a scheme which alters some characteristics of the high frequency carrier in

accordance with the low frequency message signal called the modulation signal.

There are three types of modulation:

1.Amplitude modulation (AM): In this type of modulation frequency and phase remains same, only

the amplitude changes.

2. Frequency modulation (FM): In FM the peak amplitude and phase remains constant but as the

amplitude of the information changes, the frequency of the carrier changes correspondingly.

3. Phase Modulation (PM): PM is used in the same system as an alternative to frequency

modulation. In phase modulation, the phase of the carrier signal is modulated to follow the changing

voltage level of the modulating signal.

Need for Modulation:

1. Efficient Radiation

2. Frequency Translation

3. Multiplexing

4. Reduction of Noise

#Why transformer neutral earthing? What are advantages?

Transformer neutral in distribution transformer is earthed because:

1) Keep neutral at zero potential,

2) keep all three phase voltage balance even if load is unbalance,


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3) To provide return path for current in case of earth fault condition,

Neutral is grounded in transformer

1) Is for restricted earth fault protection (REF)

2) To eliminate the arching ground fault

#Why Flexi Links are used in between generator terminal output & bus duct conductor?

In output of Generator terminal and Bus duct, we use flexi links i.e. no hard connection because to

avoid the transfer of vibration of generator to bus duct.

That has also something to do with avoiding high tension on the generator terminals due to the

physical weight of the hard bus bar. Plus, limiting the transfer of the generator vibration to the bus.

#What will happen if DC supply is given on the primary of a transformer?

Mainly transformer has high inductance and low resistance. In case of DC supply inductance will notact; only resistance will act in the circuit. So high current will flow through primary side of the

transformer. For this reason coil and insulation will burn out.

Mathematical reason is that net impedance of a system is formed of

Z = sq.rt(X.sq + R.sq)

here X = 2*pi*f*l since f = 0 in DC,

X also becomes 0.

Hence Z also =R which is very small in this case. When voltage applied will be like short circuit

hence the winding will burn.

#What is various impedance in any machine?

In a synchronous machine, there are three kinds of impedances

1. Xd - synchronous reactance; (used in steady state modeling) - a special case is salient pole

machines where sync impedance is split into direct axis impedance and quadrature axis impedance ,

i.e. Xd and Xq . See double reactance theory for more details

2. Xd' - transient reactance ;( used in transient modeling)

3. Xd'' - subtransient reactance.

All other machines have three kinds of impedences which are used in power system modeling

1. positive sequence impedence

2. negative sequence impedence

3. zero sequence impedence


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#How do you convert the lt amps on a transformer to the ht amps?

The formula is given below by using which u can find out the secondary current

V1I1 (FOR LT) = V2I2 (FOR HT)

I2 = (V1I1)/V2

#What is the meaning of harmonic in AC waves and when which are produce and what is the

effects on line and give any other information about harmonics

The phenomenon of generation of signal which have multiple times of frequency of supply signal is

known as Harmonics of that applied signal....

Normally harmonics may generate in different way....

It affects the power quality of electric signal on line...

#How feedback transformer works to feedback power in Slip Power Recovery System?

Feed Back transformer is a just Step up transformer.

it voltage up, up to system voltage.

#I have one transformer 1000kva, 11/.433kv. we are using 200/5 amp. ct unit for measuring

purpose, & one 800 amp. V.C.B Unit for the same t/f operate, I wants to known the primary

current of T/F is only 53 amp. why we used 800 amp. V.C.B ? & 200/5 amp. CT Unit.

You are right, you should fight with your electricity board regarding the HT Metering CT ratio. You

should actually required 100/5 ratio. But while deasigning VCB it is always prfarable to go on higher

side because it can sustain the heavy short circuit current. One thing should be noticed that theprotection CTs should be as per the Primary current of the ransformer.

#What is the full form of MCCB

Molded Case Circuit Breaker

#Is it possible to increasing the real power generation by reducing the reactive power in

power generation plants?

When we bring down the reactive power, there will be some decrease in the load current of the

machine. Hence we get more allowance to get additional load on it which eventually results in more

power output from the machine.

#What is the need of motor starter?

In almost all the motors, in starting condition, Armature current will be very high. To control that

current, starters will be used.

To control the starting current of the motor thus avoiding overheating of the windings.


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To avoid under voltage hence tripping of other equipments on the same line i.e. for big motors

Able to vary the speed of the motor .Provides different starting/stopping points of the same motor.

Provides control measures of over current, earth fault i.e. overload relays, circuit breakers, fuses

Provides power isolation mechanism when one works on the motor i.e. fuses, breakers, isolators

Able to know the current taken by the motor i.e. ammeters

#How is working the auto transformer?

If you dont get confused with the name (auto-transformer) and its internal construction, it is same asnormal transformer as far as its working is concerned.

The construction is little different. Primary and secondary windings are physically same i.e. only one

winding is there. This winding has multiple taps. The winding between common tap and one tap is

Primary winding. The winding between common tap and some other tap is Secondary winding.

#What is difference between A.C & D.C Resistance?

d.c resistance is only resistance due to the virtue of

conductor property only, a.c resistance is due to both

virtue of conductor property and skin effect.

#Describe the difference between a microcontroller and a general purpose computer.

Microcontrollers are nothing but regulators based on microprocessors. Microcontrollers are used for

certain predefined operations like in robotics, temperature sensor devices etc.

Computers are used to design microcontrollers.

Micro controllers are low level controllers designed for

specific functions. on the other hand the general purpose

computer has a high level computing compared to micro

controllers.

Micro controllers have small memory like RAM ROM compared

to GP comuters

Also the type of OS porting varies in both the controllers

#What is the relation of current with torque when motor started in Y-connection???

Both current & torque are high in star connection.


36/46

#IN 3 phase induction motor where we give i/p supply & how its working?

The input supply is given to the 3 phase stator windings. Due to this the rotating magnetic field is

created. This rotating magnetic field cuts the rotor bars. Due to this emf is developed in the rotor

bars according to Faraday's law. This emf causes current to flow in rotor bars which interacts with

rotating magnetic field of stator. Hence force is experienced

by rotor and it rotates.

#What is battery Amphere-hour? How the battery back up time period is calculated?

The battery is rated in ampere-hour. It means either "X" NOS OF AMPERE CURRENT WILL BE

DELIVERED FOR "Y" NUMBER OF HOURS BEFORE IT GETS DISCHARGED , so the ampere-

hours will be XxY.

The back up time will depend on the load applied to the battery in terms of watts/kws ....... amp-hrs

divided by watts

Ampere Hour=(Watts/DC Voltage)*(1/Inv.Efficiency)*

(1/Batt.Efficiency)*No.of Hours

Ex:W=500W V=12V Inv eff.=0.9 Batt.eff=0.7 Time=1hour

Then Ah=(500/12)*(1/0.9)*(1/0.7)*1

=66.13

It means for 1 hour backup we can use 65Ah Battery for 500W/12V Inverter.

Similarly,for 2 hours Ah=65*2=130Ah

#What is the formula to calculate capacitor bank rating for a certain load?

Calculation of Required Kvar to Raise Power Factor

A negative power factor is interpreted as a leading power factor. A positive power factor is

considered a lagging power factor. A negative kvar value should be interpreted as capacitive kvar

that shall be removed. A positive kvar value should be interpreted as capacitive kvar that shall be

added.

Calculation of Required Kvar

Known Variables: Initial or Present Power Factor, Desired Power Factor, and Real Power of Load

KVAR = KW (Tan i - Tan d)

Where,

i = cos-1 Pfi = Initial Power Factor Angle


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d = cos-1 Pfd = Finial Power Factor Angle

Top of Form

Bottom of Form

#What is AVR in Generators and How is it working.

AVR MEANS AUTOMATIC VOLTAGE REGULATOR.AVR IS HAVING TWO INPUT TERMINALS

AND TWO OUT PUT TERMINALS OUT PUT IS GIVING DC VOLTAGE .IT IS CLOSED LOOP

CONTROL SYSTEM. I WILL GIVE ONE EXAMPLE . AVR HAVING TERMINALS LIKE S1, S2, AND

F1, F2. S1 AND S2 ARE INPUT TERMINALS THAT IS CONNECTED TO OUT PUT OF THE

ALTERNATOR VOLTAGE. F1 AN F2 IS CONNEDCTED TO FIELD OF THE EXITER TERMINALS. I

WILL EXPLAIN HOW IT IS

WORKS. EXITER IS COUPLED WITH ALTERNATOR ROTOR. EXITER FIELD IS HAVING OWN

RESIDUAL MAGNITISUM DUE THIS MAGNETISUM ALTERNATOR GETS SMALL VOLTAGE

THAT IS AROUND 10-20VOLTS AT OUT PUT VOLTAGE. U WANT SEE THIS VOLTAGEREMOVE F1 AND F2 CONNETIONS CHECK VOLTAGE AT OUT PUT TERMINALS. THESE OUT

VOLTAGE IS SENDING TO AVR INPUT TERMINALS THAT IS S1 & S2 THEN AVR GIVES SAME

VOLTAGE TO EXITER FIELD THEN EXITER FIELD GIVES MORE MAGNETISUM DUE TO THIS

ALTERNATOR GIVES MORE VOLTAGE AGAIN THIS VOLTAGE GIVEN TO AVR LIKE THIS

THESE CYCLES GOIES ON UP REACH REQUIRED VOLTAGE OF THE ALTERNATOR. AT

REACHING OUT PUT REQUIRED VOLTAGE AVR IS NOT SENDING MORE VOLTAGE TO

EXITER THERE IS CUTT OF SET POINT . WHOLE THESE CYCLE TAKEN VERY LESS TIME

MEANS MAY BE 1 SEC. AVR OUTPUT VOLTAGE IS DEPENDS UPON THE COMPANY OF THE

AVR AND REQUIREMTNT OF THE ALTERNATOR.

AVR is heart of genset .its developing & maintain the genset voltage as constant .its building voltageby reference of voltage produce by genset .for Ex: if genset rated voltage is 11kv means intially

genset produce some minimum voltage (7- 14v)ac only is called resuidual voltage .that voltage is

input of AVR .avr developing the voltage upto rated Kv by some programmable circuit in AVr .that

O/p voltage is DC only its called as Excitation voltage. we fed the excitation voltage to Excitor in

genset.after genset reached rated KV the AVr continusly maintain the voltage as constant

#What happens when the field of an alternator connected to infinite bus is suddenly removed?

if suddenly remove field from generator it will take reactive power from infinite bus ,but it will

continuously supplies active power ,that means it act as induction Generator.

#What should be the voltage between neutral and earth of a UPS?

ideally u can says 0 volts, but practically it is not possible to attain 0V due to soil or environmental

conditions. it might be as low as possible near about 1-2V Max. Otherwise put water ,check earthling

wire....etc.

#what is the MNA in case of d.c. machin? what is the importance of it? how it affect the brush

position?


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MNA stands for Magnetic neutral axis. It is an imaginary line perpendicular to magnetic lines and

passing exactly at the centre of magnetic field. The emf induced at MNA will be zero as the

conductors will not cut any flux at MNA. At MNA the conductors will be parallel to magnetic lines.

Hence brushes are located at MNA to collect current. During armature reaction the MNA shifts in the

direction of rotation of generator. So the brushes should be shifted to new position of MNA to avoid

sparking.

#If we convert watt into KVA, which one formula use by us----

watt/p.f=kVA*1000

kW=kVA*p.f

#Where r use in Dc motor in power plant?

we use the DC motors for the fuel handling system and for ID and PD fans.

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