electrical engineering interview Interview Questions

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electrical engineering interview Interview Questions

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what is the difference b/w bus reactor and line reactor? BUS REACTOR:-(Electricity) An air-core inductor connected between two buses or two sections of the same bus in order to limit the effects of voltage transients on either bus. Reduce fault current.Bus reactors a devise installed in a bus to maintain system voltage when the load on the bus changes by releasing reactive power as usable power to bolster the voltage. LINE REACTOR:- A line reactor is placed in line at point of use or just after a transformer to maintain a stable amperage to the user, capacitor. why power factor is only cosine angle not sine angle? P.f = cosø bcz, p.f=true powr/ apparent powr; i.e, true powr=VI Cosø &apprnt powr=VI. So, = VI COSø/VI. I.e, p.f=cosø. Its only cosin angle The product of VI COSØ is the true powr & is measured in watts(using wattmeters). So, the product VI is apprnt powr measured in volt-ampere., Like V*I in watts , (using voltmeter&ammeter) if a bird sits on the service wire it didn't get shock then why the human get shock when he touches it(assume that human is not on the ground and any object i.e. in space)?

birds also get shok when they are wet and it is not possible to get shock to human when he has no grounding because current will not flow through it
why ac ratings are in tons? the ac ratings are tons, this is nothing but the amount of ice which is gonna produce by the frozen.. for exmpl 4 tons of ac will romove the heat upto 48000 btu(british thermal unit

Why electrons flow only one direction in semiconductors. why can not flow both directions?

electron doesnot flows they only drifts because semiconductors are basically combination of trivalent(acceptors/p-type)and pentavalent(donor/n-type), on applying thermal pressure generation of holes and recombination of electron ion pair takes place, this led to drifting of electrons in one direction resulting in potential barrier

what is mean by harmonics?Type of harmonics? Ans1:

Harmonic is presence of unwanted frequencies. Few types of harmonics are listed below. these frequencies mainly gets generated due to modern electronic equipment e.g Computer, inverters etc. 3rd level harmonics 5th level harmonic 7th Level Harmonic 11th level harmonics 13th level harmonics 3rd level harmonic is normal, however when it goes up to 5th or 7th it's considered a problem. 11th and 13th level harmonics usually found where rectifiers and DC loads are connected.

Ans2: presence of unwanted frequency component in current or voltage other than fundamental frequency is known as harmonics . basically harmonics are ac and dc harmonics . we can also classified them as 1.even harmonics (2nd ,4th,6th )frequency component 2.odd harmonics (3,5,7) generally lower harmonics are more dangerous than higher order harmonics. why a bulb glow when connected with two end phase and neutral respectively.? Phase has positive charge and earth containneutral charge.so to bulbglows when it contain positive and negative charge .whenever phase is positive automatically neutralbecomes negative so bu Bulb glows . how an electron flow in a wire. can it flow like a car running on the road? electrons does not flow in wire they only drifts . Means they tranfer their charge to another electron bounded tightly with. so we can say it cannot flow like car running on road. You can say they moves like human standing in overloaded bus when brake of bus apply What is a charge.? charge means here Electric charge. It is the physical property of matter that causes it to experience a force when close to other electrically charged matter. WHAT MEANT BY XLPE CABLE? Cross-linked polyethylene what is FET? field-effect transistor

Why we use two wattmeter during block rotor test and no load test of 3phase induction motor? And how can we understand the neat power requirement of from this test? Ans1: As the pf is very low, one of the 3 watt meters will show negative reading Ans2: In this test, we use two single phase wattmeters or one 3phase wattmeter, during test one of the 1phase wattmeter reads positive value and other one reads negative value of 3 phase power. but in usage of 3 phase wattmeter gives directly the whole power of 3 phase supply.
During speed control of dc shunt motor with flux control methode,when armature voltage is constant, How can it possible to increasing the armature current?

Ans:speed get reduced, as E=V-Ir and E=K*SPEED*FIELD speed= V-Ir/K*FLUX
Why a dc generator gives result during two different condition,1)separately exited gen.Eg=110 V, field current=0.25 A. 2) self exited gen.Eg=30 V, field current=0.025 A.

It may be for large value of flux in case of separately exited gen. But this large difference is not aspectable. whether energy meter is connected in series or parallel.why? ans1: To measure the amount of energy consumed ih home/industry or any where we connect the CT in serise In measuring & protection circuit we connect the CT,s are in series and VTs are connected in parallel due to Ans2:

energy meter is connected in series with the line,because it reads the amount of power consumed for a particular time,the amount power proportional to the load current,so the energy meter is on series. What is diffrence bitween electrical and electronics ? Ans1:current flow through conductor is electrical and current flowing through semiconductor is electronics. E.g. fan is electrical and fan regulator is electronics what is the rating of Insulation tester for measure 15 KW, 3 phase 415V 50 HZ induction motor? Ans1: 500 volts For LV it is usually 2 times the phase to earth voltage Ans2: Phase to Phase = 500 volt Phase to Earth = 500 volt IR Value should be greater then 200 Mega Ohm.
what is the formula of high voltage testing of three phase induction motor?

2 times the rated volts in KV plus 1 KV (Ex: for 415 V=2*0.415+1=1.8 KV)
while selecting transformers normally Ans: we ask for 33 kva feeder why not 33/415?

for 33/433 kv

Appliances rated voltage is 415V. If Trasformer is specied at 415V, it will further drop down at receving end and appliances may not work To compensate Voltage drop at receiving end
Which generation plant has instant start??

Diesel power plant as it works like a generator in stantly starts. what diff. between earth nd neutral?

Ans1: Neutral point is achieved by connecting common point of either two OR three diffrent phases of supply. However, earth is a zero potential point which is having solid connection with ground. Ther can be potential in neutral but earth point will always be at zero potential. Ans2: The value of potential(volts) for a balanced polyphase(eg. 3 phase system) is zero volts.But for unbalanced polyphase system the value depends on relative phase shift angle between phases.For single phase system neutral is the normal return path for the load current.But earth is supposed accidental bypassing path for the fault currents(eg. shortckt. current,leakage current,etc. etc..)It might be might not be a p.d b/w neutral and earth depends on the type of distribution system (single phase or poly phase),type of earthing system ie, TN,TNCS,TNS,IT,TT and its connections.

what is the difference between ups & inverter??? 1)inverter is when EB power is there what is input votlage same out put voltage (eg: input 210v = output also 210v) when EB power failure it will goes inverter mode (battery mode )and voltage also increase as per inverter voltage setting. 2)UPS is What ever input Eb voltage(eg:210v)output voltage is as per the voltage setting (eg:230v) same when Eb power failer same (battery mode) voltage is same eg:(230v) any cost of time out put voltages is same . Ans2: both are ac to dc to ac converters only difference is switching speed based on use of switches like mosfet gto etc.ups operates faster than inverter. Ans3:

UPS - (Un interupted power supply) It is used when power shut down it will gives the power as same as 23o volts upto the EB power comes . INVERTER- It is device which converts the AC supply to DC supply and stores in the battery when EB supply is on and when EB power shut down it will convert again the DC power to AC power from the battery to switch boards through the UPS.
what is the acb tripping & closing voltage???

normally used 110v dc for tripping and closing coils .
what is difference b/w bus duct & bus bar???

Busbar is a conductor of electrical power. It is usually made of copper but sometimes also of aluminium. It is usually rectangular in cross section. Busbars are used inside switchboards where it is normally not insulated but protected from access by panels. The busbars are mounted on insulating blocks. A bus duct is an enclosure which contains a number of busbars (phases, neutral and earth). It is used for connection between switchboards. Here the enclosure us used to protect the busbars from being touched. the busbars are also held in insulating blocks. Ans2: Bus bar means it is a current carrying device.Bus duct it is a bus bar enclosing equipment WHAT IS M.P.C.B??? Ans1: MPCB means Motor Protection circuit breaker.it is used to protect the running motor from faults. Ans2: M.P.C.B=Motor protection circuit breaker. it is a electrical device,it is used to isolate

(protect)the fault part of the powersystem at abnormal condition,motor protection means isolate the motar fault at running condition.......... Why transformer rating in KVA? describe. Ans1 : kva is active power+ reactive power the really used power is active power only. which is v*i*cos pi. but we dont know user's power factor so ratings are kva for t/f. Ans2: transformer rating in kva because power factor of transformer is always depended on other load. Ans3: Trafo losses depends on voltage and current only. voltage influences the core loss or no load loss and current or load influences the copper loss. no loss depends on power factor of the load. thus efficiency is mainly governed by v and I only. hence o/p whis is the ratio of (i/p - losses)/ i/p is controlled by v and I. Hence trafo rating is defined in KVA no KW. Ans 4 : Transformers Are Rated In Kva Because Generally There Are Two Losses In A Transformer I.e Core/iron Loss & copper Loss Due To Transformer Is A Stready Device So Their Is No Loss Of Mechanical . Due To the Core Losses Depends upon Voltage(v) & copper Losses Depends Upon Current(A) So The total Losses Depends Upon VA I.e KVA . Ans5 : due to the losses of transformer dependas on the current and voltage.
what is the disadvantage of lagging power factor & why ?

the disadv of lag pf is that it causes the voltage regulation to increase towards maximum value due to which the o/p volt decreases. volt reg= (V no load - V full load) = IR cos phy - IX sin phy in case of lag pf the current is lagging i.e I<-phy so , the volt reg = IR cos phy + IX sin phy is more ans2: Power factor is nothing but angle between voltage and current. As the angle increases means pf lags result high requirement of currents which creates voltage drops. Therefore it can damage the equipments as equipments are designed as per their voltages it possible to say motor horsepower number without seeing name plate(when name plate damaged) ???? if you find in motor full load current and multiplication in voltage you will find out motor KW after 1KW= 0.742 hp What is sequence network? Discuss all three types of sequence network.? Sequence networks are to network representation of a system in terms of its sequence components. Any 3 Phase system can be studied for its performance in terms of its 3 sequence components. The three sequence components are Zero Sequence component, Positive sequence component and negative sequence component. CAN WE GENERATE AC WITHOUT DC EXCITATION? Yes By replacing the field winding with permanent magnets. The problem with doing this is that the output voltage is controlled only by the speed of the alternator. This means that at constant speed (frequency)the voltage will fall as the load is increased. If the voltage must be constant the

speed must be increased with load. So the frequency will not be constant. Maximum efficiency occurs in electrical machines is at i)Full load,ii)50% of full load,iii)Near full load,iv)None of these?? Full Load Which loss in DC machine is maximum? in dc machines the losses are core loss,cu loss,stray loss, mechanical loss, brush loss.... in which mechanical loss is maximum funtion of choke??? It works as ballast (gives high voltage) at start and controls current by dropping voltage during running. We have 2 11kv/433v transformers for 2saperate acb for mv panels and 1 bus coupler acb. Suppose I’ll close bus coupler what will happen or it will be share the load. Please suggestion and any modification required.???

Any one incomer has to be tripped according to the plc logic implemented or according to schematic / control such that only one incomer will feed entire bus with bus coupler on . why transformers are put with supply for 24 hours before giving supplying to a locality?

to avoid the harmonics mainly as during the starting due to the inrush currents "nd harmonic component is generated so to avoid it and give us the perfect supply we are doing this.. mainly to avoid the harmonics and different errors. Ans2:

it is to allowed the rush current to induce maximum flush in to the field for stability so that there will be not fluctuation in supply voltage to the locality due to in rush currents. 10 hp motor ,7.5 kw ,960 rpm, full load current 15.5, delta connection so how much take current no load trial time. any formula please give..... second question :- what is different between brekar control wiring and motor starter control wiring??? There is no separate formula for calculating no load current, It depends upon it internal losses like Wearing & Tearing, Bearing condition and IR losses etc. Secondly, We can see that, In Breaker circuit only NO is used for Closing, Opening and Triping Purpose, where as with out NC circuit it wouldnt be complete. To trip the motor we should break the NC element. In India normally 110 Vdc for breaker and 230 Vac supply for Starter control wiring. Why should not we run the IM in low voltage, if runs means what will happen?(for example instead of 400V we give 250V supply)??

for low voltage(1 phase - 230 volt) operation! IM needs some mechanical machine to start the motor but while in high voltage(3 phase - 440 volt) their is no any requirement like in low voltage.....so........because in high voltage rmf is generate to start the motor

A 3-phase IM is supplied from 50 Hz system. It runs at speed of 1440 rpm and slip equal to 4 %. Calculate rotor frequency. A 50 Hz B 0 Hz C 4 Hz D 6 Hz?? ANS: 2Hz

Rotorfrequency = s×frequency of applied voltage to stator = sf SO, ROTOR F = 0.04*50=2Hz In 3 phase induction motor even with low Voltage it reaches nearly it's rated speed why? (for example instead of 400V it reaches with 250V) Ans1: due to high starting torque In 3 phase induction motor even with low Voltage it reaches nearly it's rated speed why? (for example instead of 400V it reaches with 250V)??? With an induction motor the speed is dependant on the frequency and is the same as the frequency minus slip. For a 4 pole motor this is about 1440 to 1460 RPM. It does not depend on the voltage. The torque is dependant on the voltage. A 400 volt motor supplied at 250 volts would not have anywhere the power output. why the AC or DC motor takes high starting current? because at the starts dc motor do not have the back emf and due to that applied voltage is not oppose by back emf and winding have small amount of resistance so according to ohm's law current increse to 50-60% of it's rated current. and in case of ac motor as the at the start there is no any rotation of the rotor wrt stator winding and the emf induced in the rotor winding depens upon the flux link with it and it's relative speed as the we apply full rated voltage at first

it cause max flux link with rotor as it remain stationary and max emf induce in rotor winding as rotor winding is short circuited in induction motor it cause high flow of current in rotor . that's bot ac and dc takes high current @ starts . which motors is used in mixies?? Ans1:universal motors Ans2: ac motors Are we do it in single transformer with dual primary as 11 KV and 33 KV and constant output as 415 Volts? As per mine it is possible. As load in a transformer increases, mutual flux linkage in core= A increases B decreases C not change D first increase then decrease??? Ans1: As load increases in a transformer , mutual flux linkage in core decreases. Ans 2: see in a transformer for producing flux only no load current(magnetizing current)is required.now as load is put on secondary side by lenz's law flux in core decreases which is compensated by drawing greate current from primary.thus flux linkage remains constant. Transfer of electric power from primary to secondary in a transformer takes place A electrically B magnetically C electro-magnetically D conductively??

i feel it ll be electromagnetically.......coz the basic principal of transformer is electromagnetic induction How Many Brick in One Cubic Meter Standard(Actual) size of Modular Brick = 190mm x 90mm x 90mm Nominal(with mortar) size of Modular Brick = 200mm x 100mm x 100mm

One Cubic Meter Brick work = 1cum = 1m x 1m x 1m = 1000mm x 1000mm x 1000mm Now if we Divide 1cum Brick Masonary work by one Brick volume then we get number of bricks in 1cum. So number of bricks in one cubic meter "Brick Masonary Work"= 1cum/1Brick volume = 1000mm*1000mm*1000mm/200mm*100mm*100mm = 500 So number of Bricks in one cubic meter "brick masonary work" = 500 Nos. what is active and passive components? ans.- active devices are those which have self source of energy and passive devices are those have not self source of energy... example-: active device : ideal voltage & current source, bipolar transistors,FETs etc. passive device : resistor, capacitor,inductor. What is the use of induction motor? as a prime mover why magnetic flow meter is not used in thermel power plant operation ans1: because magnetic flowmeter works in conductive liquid. in power plant we use non conductive water. Ans2: Generally we use to measure the feed water ,Steam ,FD Air,Make up water and re circulation cooling water in the power plants apart from the re circulation cooling water all are non conductive liquids and gasses so we could not use magnetic flow meter. We can use it in cooling water re circulation line of cooling tower but it is quite expensive than Annubar flow meter. If we impress a dc voltage of 230 V on an unloaded 230 V, 50 Hz transformer it wi A give high secondary voltage B burn out C give secondary voltage as per K D none of these??

no it ll burn out............ Material commonly used in making transformer core is A cast iron B mild steel C high silicom steel D high carbon steel?? high silicon steel Which of the following connection is best suited for 3-ph, 4 wire system A delta-delta B star-star C delta-star D star-delta?? (C)In a distribution system, delta-star is best suited Short circuit test of induction motor is carried out to compute= A stator resistance B rotor resistance C magnetizing reactance D stator reactance?? rotor resistance which of the following loss varies with the load a)hystresis loss b)eddy current loss c)copper loss d)core loss?? The answer is (c) copper losses. The copper losses I*I*R losses, they varies with square of load current how would you determin the horsepower of a motor without a nameplate?? Ans1:you measure motor full load current with meter than you will multiply with voltage you find motor power in watts than again you will multiply 0.742 its value for 1hp equal 0.742 watts . Ans2: CONDUCT HEAT RUN TEST, PLOT TEMPERATURE TIME CURVE AND THEN USE RMS POWER METHOD FIND ITS POWER RATING. THEREAFTER YOU CAN COMPUTE THE HP BY USING THE RELATION 1 HP = 735.5 WATTS what is the difference between single core,three core,three and half core and four core, and where it can be used ??

These types of core constructions are employed in single and three phase transformer construction. Based on magnitude of the flux carried by the magnetic path in the core section, the classification of type of core varies. 33/11kv stepdown transfomer, transformer will stepdown to 11kv & remaining 22kv where it will go?? Ans1: Step down transformer is high voltage low current in primary and low voltage high current in secondary so the remaining 22kv converted to a current according to transformer parameters. Ans2: Transformer transfers the power from one circuit (primary) to another circuit (secondary) at constant frequency. If voltage at circuit is higher then current will be lower in magnitude whereas another side, the voltage will be lower and current will be more. Squirrel cage induction motor has constant A slip B stator current C rotor resistance D speed at all load?? C rotor resistance it is fixed at design stage only and we cant change it bcoz it is not accessible In winding design, the angle of chording (&#946;) is nearly equal to A 45 degree B 60 degree C 30 degree D 90 degree?? C 30 degrees because by using 36 degrees we can eliminate harmonics but its difficult to design for 36 degrees hence in practical it is designed for 30 degrees . In scott connected transformer number of turns in the primary winding of teaser transformer is % of number of primary turns of main transformer A 86.6 B 57.7 C 14.4 D 33.3??? 86.6

kVA rating of open delta connected transformer is _____% kVA rating of delta/delta connection A 50 B 70.7 C 57.7 D 75.7?? 57.7 In auto transformer if power transferred inductively and conductively are same, the ratio of output/input voltage is A 1 B 0.7 C 0.5 D none of above?? so, 1-k=k=> k=0.5 v1/v2=0.5 ans is 0.5 No load efficiency of transformer is ___________ than full load efficiency?? GREATER Why the transformers are rated in KVA. ? in new view.?? Power given to the transformer is Apparent power (K) it is the supply power of the power house and the power given by the transformer to the Load is the true power or Active power which is actual used by the load and rest is the reactive power i.e. square of apparent power including iron loss & copper loss in KVA = square of true power in KW + square of reactive power in KVAR. A sound transformer efficiency is 98% (input power- Iron loss (k) & copper loss = output power). For full resistive load (at unity pf ) a 100 KVA transformer gives 100 KW (approx.) true power, and for inductive power at .8 pf the same will able to give 80 KW (approx.) true power, but the types of load (its pf) is not know before which to be connected across the transformer, this is only why the transformers are rated in KVA and it is meaningless to discuss about the iron loss and copper loss because the Inventor and UPS are also rated in KVA. In other hand or in simply we can

say due to unknowing power factor the transformer, generator etc are rated in KVA. In heavy size transformer copper loss is more and it vary with load and Climate i.e. transformer oil condition but it is not main reason for rating the transformer in KVA. why we take transformer reading in kva?? Ans1: Copper loss & iron loss ,Cu Loss its depends upon current (A),Iron loss depends upon Voltage (V) ,Thats y trafo rating in KVA Ans2: Kilo volt ampier Ans3: The transformer will transfer both real and reactive power . That is the apparent power in KVA . That'y it is rated in KVA. How many types of DC current?? no dc current and ac current only voltage different ac voltage and dc voltage. current mean move electron through the conductor it not a different and type too. why two point starter,three point starter, four point starter cannot be used for A.C machines? Ac is portus a alternating fulx these starters are not used What is sinusodial & non sinsodial current?? Ac is synosaital, dc is non synosaital why we use crushed stone in switch yard? Ans1: to protect the land , and to avoid the stay of water and to avoid the grow of the grass Ans2: To avoid the rodent,snake movement and grass developing Page10 completed

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