Current Electricity Interview Questions & answers

Best Current Electricity Interview Questions and Answers

Dear Readers, Welcome to Current Electricity Interview Questions and Answers have been designed specially to get you acquainted with the nature of questions you may encounter during your Job interview for the subject of Current Electricity. These Current Electricity Questions are very important for campus placement test and job interviews. As per my experience good interviewers hardly plan to ask any particular questions during your Job interview and these model questions are asked in the online technical test and interview of many IT & Non IT Industries.

 

1. What is electric traction?

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 days, magnetic traction is also used for bullet trains.and basically dc motors are used for electric traction systems.

2. How can you start-up the 40w tube lite with 230v AC/DC without using any choke/Coil?

It’s possible by means of Electronic chokes,otherwise it’s not possible to ionize the particles in tube light with normal voltage.

 

3. What is “pu” in electrical engineering?

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).

 

4. Operation carried out in Thermal power stations?

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.

 

5. Why link is provided in neutral of an ac circuit and fuse in phase of ac circuit?

Link is provided at a Neutral common point in the circuit from which various connection 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 happen the fuse connected in the particular control circuit alone will blow off.

 

6. How tube light circuit is connected and how it works?

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.

 

7. whats is MARX CIRCUIT?

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.

 

8. What is encoder, how it function?

An encoder is a device used to change a signal (such as a bitstream) 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 to another. 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.

 

9. What are the advantages of speed control using thyristor?

Advantages :

Fast Switching Characteristics than Mosfet, BJT, IGBT

Low cost

Higher Accurate.

 

10. Why Human body feel Electric shock ?? n in an Electric train during running , We did nt feel any Shock ? why?

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.

 

11. what is the principle of motor?

Whenever a current carrying conductor is placed in an magnetic field it produce turning or twisting movement is called as torque.

 

12. Why, when birds sit on transmission lines or current wires doesn’t get shock?

Its true that if birds touch the single one line (phase or neutral) they don’t get electrical shock… if birds 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.

 

13. what is meant by armature reaction?

The effect of armature flu to main flux is called armature reaction. The armature flux may support main flux or opposes main flux.

 

14. what happen if we give 220 volts dc supply to d bulb r tube light?

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.

 

15. Which motor has high Starting Torque and Staring current DC motor, Induction motor or Synchronous motor?

DC Series motor has high starting torque. We can not start the Induction motor and Synchronous motors on load, but can not start the DC series motor without load.

 

16. what is ACSR cable and where we use it?

ACSR means Aluminium conductor steel reinforced, this conductor is used in transmission & distribution.

 

17. What is vacuum circuit breaker.define with cause and where be use it Device?

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 ionised 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 kv.

 

18. What will happen when power factor is leading in distribution of power?

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

losses in form of heat will be reduced,

cable becomes less bulky and easy to carry, and very cheap to afford, &

it also reduces over heating of transformers.

 

19. whats the one main difference between UPS & inverter ? And electrical engineering & electronics engineering ?

uninterrupt 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 with high dc voltage.but ups start with 2v dc with 7 amp. but inverter is startwith 2v,24,dc to 36v dc and 20amp to 80amp battery with long time backup.

 

20. What is 2 phase motor?

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.

 

21. Advantages of vvvf drives over non vvvf drives for EOT cranes?

smooth start and stop.

no jerking of load.

exact positioning

better protection for motor.

high/low speed selection.

reliability of break shoe.

programmable break control.

easy circutry

reduction in controls

increases motor life

 

22. What is the significance of vector grouping in Power Transformers?

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 phace difference between the current and voltage. EG. DYN means Delta primary, Wye Secondry and the current is at o clock reffered to the voltage.

 

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

Its Single Phase induction motor which mostly squirrel cage rotor and are capacitor start capacitor run.

 

24. Give two basic speed control scheme of DC shunt motor?

By using flux control method:in this method a rheostat is connected across the field winding to control 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 controlled .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.

 

25. what is the principle of motor?

Whenever a current carrying conductor is placed in an magnetic field it produce turning or twisting movement is called as torque.

 

26. what is meant by armature reaction?

The effect of armature flu to main flux is called armature reaction. The armature flux may support main flux or opposes main flux.

 

27. Give two basic speed control scheme of DC shunt motor?

By using flux control method:in this method a rheostat is connected across the field winding to control 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 controlled .armature control method:in this method a rheostat is connected across armature wdg.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.

 

28. what is the difference between synchronous generator & asynchronous generator?

In simple, synchronous generator supply’s both active and reactive power but asynchronous generator(induction generator) supply’s only active power and observe reactive power for magnetizing.This type of generators are used in windmills.

 

29. What is the Polarization index value ? (pi value)and simple definition of polarization index ?

Its ratio between insulation resistance(IR)i.e meggar value for 0min to insulation resistance for min. It ranges from 5-7 for new motors & normally for motor to be in good condition it should be Greater than .5 .

 

30. Why syn. generators are used for the production of electricity?

synchronous machines have capability to work on different power factor(or say different imaginary power varying the field emf. Hence syn. generators r used for the production of electricity.

 

31. What is the difference between synchronous generator & asynchronous generator?

In simple, synchronous generator supply’s both active and reactive power but asynchronous generator(induction generator) supply’s only active power and observe reactive power for magnetizing.This type of generators are used in windmills.

 

32. 1 ton is equal to how many watts?

1 ton = 12000 BTU/hr and to convert BTU/hr to horsepower, 12,000 * 0.000929 = 4.715 hp therefore 1 ton = 4.715*.746 = .5 KW.

 

33. why syn. generators r used for the production of electricity?

synchronous machines have capability to work on different power factor(or say different imaginary pow varying the field emf. Hence syn. generators r used for the production of electricity.

 

34. Enlist types of dc generator?

D.C.Generators are classified into two types:

separatly exicted d.c.generator

self exicted d.c.generator,

which is further classified into;1)series 2)shunt and compound(which is further classified into cumulative and differential).

35. What is Automatic Voltage regulator(AVR)?

AVR is an abbreviation for Automatic Voltage Regulator.It is important part in Synchronous Generators, it controls theoutput voltage of the generator by controlling its excitation current. Thus it can control the output Reactive Power of the Generator.

 

36. What is an exciter and how does it work?

There are two types of exciters, static exciter and rotory exciter.purpose of excitor is to supply the excitation dc voltage to the fixed poles of generator.Rotory excitor is an additional small generator mounted on the shaft of main generator. if it is dc generator, it will supply dc to the rotory poles through slip ring and brushes( conventional alternator). if it is an ac excitor, out put of ac excitor is rectified by rotating diodes and supply dc to main fixed poles.ac excitor is the ac generator whose field winding are stationary and armature rotates. initial voltage is built up by residual magnetism.It gives the starting torque to the generator.

 

37. Difference between a four point starter and three point starter?

The shunt connection in four point stater is provided separately form the line where as in three point stater it is connected with line which is the drawback in three point stater

 

38. Why use the VCB at High Transmission System ? Why can’t use ACB?

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 . That y always vacuum used as inHT breaker and air used as in LT .

 

39. What is the difference between surge arrestor and lightning arrestor?

LA is installed outside and the effect of lightning is grounded,where as surge arrestor installed inside panels comprising of resistors which consumes the energy and nullify the effect of surge.

 

40. What happens if i connect a capacitor to a generator load?

Connecting a capacitor across a generator always improves power factor,but it will help depends up on the engine capacity of the alternator,other wise 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.

 

41. Why the capacitors works on ac only?

Generally capacitor gives infinite resistance to dc components(i.e., block the dc components). it allows the ac

components to pass through.

 

42. Explain the working principal of the circuit breaker?

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 circuitArc is extinguished by the Arc Quenching media like air, oil, vacuum etc.

 

43. How many types of colling system it transformers?

ONAN (oil natural,air natural)

ONAF (oil natural,air forced)

OFAF (oil forced,air forced)

ODWF (oil direct,water forced)

OFAN (oil forced,air forced)

 

44. Define What is the function of anti-pumping in circuit breaker?

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.

 

45. What is stepper motor.what is its uses?

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.

 

46. How to calculate capacitor bank value to maintain unity power factor with some suitable example?

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

#e= EXISTING P.F.

#d= DESIRED P.F.

 

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

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.Particularly when 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 electromagneticflux produced by primary with secondary.They work on the ratio to they are designed.E.g if CTis of ratio 50005A and it has to measure secondary current of 8000A.then ANS=8000*55000=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.

 

48. 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?

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 the load current in the transformer is because of core loss so it is minimum.

 

49. What is power factor? whether it should be high or low? why?

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).

 

50. What is the difference between Isolator and Circuit Breaker?

Isolator is a 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 the load on it must be made zero and then it can safely operated. its specification only rated current is given.But circuit breaker is

onload 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.

 

51. what is boucholz relay and the significance of it in to the transformer?

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 other wise it trips the circuit by its own tripping mechanism.

 

52. What is SF6 Circuit Breaker?

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

 

53. What is frantic effect?

Output voltage is greater than the input voltage or receiving end voltage is greater than the sending end voltage.

 

54. What is meant by insulation voltage in cables? explain it?

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.

 

55. Why we do 2 types of earthing on transformer (ie:)body earthing & neutral earthing , what is function. i am going to install a oo kva transformer & 380 kva DG set what should the earthing value?

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.

 

56. What is the difference between MCB & MCCB, Where it can be used?

MCB is miniature circuit breaker which is thermal operated and use for short circuit protection in small 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.

 

57. Where should the lighting arrestor be placed in distribution lines?

Near distribution transformers and out going feeders of 11kv and incomming feeder of 33kv and near power transformers in sub-stations.

 

58. Define IDMT relay?

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.

 

59. What are the transformer losses?

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.

 

60. What is the count of hvdc transmission lines in India?

Resolution:At present there are three hvdc transmission lines in india

 

chandrapur to padghe(mumbai)–(100 MW at ±00 kV DC)

rehand to delhi (100 MW at ±00 kV DC)

talchal to kolar (200 MW)

61. 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”.

 

62. 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.

 

63. 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.

 

64. 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 current unbalance in the primary we use delta winding in the primary. So delta / star transformer is used for lighting loads.

 

65. 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 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.

 

66. 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.

 

67. 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 difference in current.

 

68. How electrical power is generated by an A.C Generator?

For the generation of elect power we need a prime mover which supplies mechanical power input to the

alternator, can be steam turbines,or hydro turbines .When poles of the rotor moves under the armature

conductors which are placed on the stator ,field flux cut the armature conductor ,therefore voltage is generated

and is of sinusoidal in nature…due to polarity change of rotor poles(i,e) N-S-N-S.

 

69. Why an ac solenoid valve attract the plunger even though we interchanges the terminal? Will the poles changes?

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.

 

70. What is derating?, why it is necessary, it is same for all means for drives, motors,and cables.

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.

 

71. Why temperature rise is conducted in bus bars and isolators?

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.

 

72. 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?

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. Over voltage 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.

 

73. If one lamp connects between two phases it will glow or not?

If the voltage between the two phase is equal to the lamp voltage then the lamp will glow. When the voltage difference is big it will damage the lamp and when the difference is smaller the lamp will glow depending on the type of lamp.

 

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

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.

 

75. What are HRC fuses and where it is used?

HRC stand for “high rupturing capacity” fuse and it is used in distribution system for electrical transformers.

 

76. Which power plant has high load factor?

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.

 

77. Mention the methods for starting an induction motor?

The different methods of starting an induction motor

 

DOL:direct online starter

Star delta starter

Auto transformer starter

Resistance starter

Series reactor starter

78. 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.

 

79. Explain What is use of lockout relay in ht voltage?

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

 

80. What is the power factor of an alternator at no load?

At no load Synchronous Impedance of the alternator is responsible for creating angle difference. So it should be zero lagging like inductor.

 

81. Explain 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.

 

82. 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. The reason 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.

 

83. 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.

 

84. 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).

 

85. 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.

 

86. 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.

 

87. What is the difference between isolators and –>electrical circuit breakers? What is bus-bar?

Resolution:Isolators are mainly for switching purpose under normal conditions but they cannot operate in fault conditions .Actually they used for isolating the CBs for maintenance. Whereas CB gets activated under fault conditions according to the fault detected.Bus bar is nothing but a junction where the power is getting distributed for independent loads.

 

88. What are the advantage of free wheeling diode in a Full Wave rectifier?

Resolution:It reduces the harmonics and it also reduces sparking and arching across the mechanical switch so that it reduces the voltage spike seen in a inductive load

 

89. What is the function of interposing current transformer?

Resolution:The main function of an interposing current transformer is to balance the currents supplied to the relay where there would otherwise be an imbalance due to the ratios of the main current transformers.

Interposing current transformer are equipped with a wide range of taps that can be selected by the user to achieve the balance required.

 

90. What are Motor Generator Sets and explain the different ways the motor generator set can be used ?

Resolution:Motor Generator Sets are a combination of an electrical generator and an engine mounted together to form a single piece of equipment. Motor generator set is also referred to as a genset, or more commonly, a generator.

The motor generator set can used in the following different ways:

 

Alternating current (AC) to direct current (DC)

DC to AC

DC at one voltage to DC at another voltage

AC at one frequency to AC at another harmonically-related frequency

91. Define what is power quality meter ?

Power Quality meters are common in many industrial environment. Small units are now available for home use as well. They give operators the ability to monitor the both perturbations on the power supply, as well as power used within a building, or by a single machine or appliance. In some situations, equipment function and operation is monitored and controlled from a remote location where communication is via modem, or highspeed

communication lines.So we can understand the importance of power measurement through power quality meters.

 

92. What is the different between digital phase converter and ordinary phase converter?

Digital phase converter are a recent development in phase converter technology that utilizes proprietary software in a powerful microprocessor to control solid state power switching components. This microprocessor, called a digital signal processor (DSP), monitors the phase conversion process, continually adjusting the input and output modules of the converter to maintain perfectly balanced three-phase power under all load conditions.

 

93. Explain the operation of variable frequency transformer?

A variable frequency transformer is used to transmit electricity between two asynchronous alternating current domains. A variable frequency transformer is a doubly-fed electric machine resembling a vertical shaft hydroelectric generator with a three-phase wound rotor, connected by slip rings to one external ac power circuit.

A direct-current torque motor is mounted on the same shaft. Changing the direction of torque applied to the shaft changes the direction of power flow; with no applied torque, the shaft rotates due to the difference in frequency between the networks connected to the rotor and stator.The variable frequency transformer behaves as a continuously adjustable phase-shifting transformer. It allows control of the power flow between two

networks .

 

94. What is the main use of rotary phase converter ?

Rotary phase converter will be converting single phase power into true balanced 3 phase power,so it is often called as single phase to three phase converter . Often the advantages of 3 phase motors, and other 3 phase equipment, make it worthwhile to convert single phase to 3 phase so that small and large consumers need not want to pay for the extra cost of a 3 phase service but may still wish to use 3 phase equipment.

 

95. Use of switch mode power converter in real-time basis?

Switch mode power converter can be used in the following 5 different ways

 

step down an unregulated dc input voltage to produce a regulated dc output voltage using a circuit known as Buck Converter or Step-Down SMPS,

step up an unregulated dc input voltage to produce a regulated dc output voltage using a circuit known as Boost Converter or Step-Up SMPS,

step up or step down an unregulated dc input voltage to produce a regulated dc output voltage,

invert the input dc voltage using usually a circuit such as the Cuk converter, and

produce multiple dc outputs using a circuit such as the fly-back converter.

96. Which type of oil is used as a transformer oil?

Transformer oil, or insulating oil, is usually a highly-refined mineral oil that is stable at high temperatures and has excellent electrical insulating properties. It is used in oil filled transformers, some types of high voltage capacitors, fluorescent lamp ballasts, and some types of high voltage switches and circuit breakers. Its functions are to insulate, suppress corona and arcing, and to serve as a coolant.

Well into the 170s, polychlorinated biphenyls (PCB)s were often used as a dielectric fluid since they are not flammable. They are toxic, and under incomplete combustion, can form highly toxic products such as furan.

Starting in the early 170s, concerns about the toxicity of PCBs have led to their banning in many countries.

Today, non-toxic, stable silicon-based or fluoridated hydrocarbons are used, where the added expense of a fireresistant liquid offsets additional building cost for a transformer vault. Combustion-resistant vegetable oil-based dielectric coolants and synthetic pentaerythritol tetra fatty acid (C7, C8) esters are also becoming increasingly common as alternatives to naphthenic mineral oil. Esters are non-toxic to aquatic life, readily biodegradable, and have a lower volatility and higher flash points than mineral oil.

 

97. If we give 2334 A, 540V on Primary side of 1.125 MVA step up transformer, then what will be the Secondary Current, If Secondary Voltage=11 KV?

As we know the Voltage & current relation for transformer-V1/V2 = I2/I1

We Know, VI= 540 V; V2=11KV or 11000 V; I1= 2334 Amps.

By putting these value on Relation-

540/11000= I2/2334

So,I2 = 114.5 Amps

 

98. what are the points to be consider for MCB(miniature circuit breaker selection?

I(L)*1.25=I(MAX) maximum current. Mcb specification are done on maximum current flow in circuit.

 

99. What is the full form of KVAR?

We know there are three types of power in Electrical as Active, apparent & reactive. So KVAR is stand for “Kilo

Volt Amps with Reactive component.

 

100. What is excitation?

Excitation is applying an external voltage to DC shunt coil in DC motors.

 

ELECTRICAL Interview Questions ::

101. In three pin plug 6 Amp. 220v AC rating. why earth pin diameter is higher than other two pin? what its purpose ?

Because Current flow in the conductor is inversely proportional to the conductor diameter. So if any short circuits occur in the system first high currents bypassed in the Earthling terminal.( R=Pl/a area of the conductor increases resistance value decreases)

 

102. Difference between megger test equipment and contact resistance meter test instruments?

Megger test equipment used to measure cable electric resistance, conductor continuity, phase identification where as contact resistance meter test instruments used to measure low resistance like relays ,contactors.

 

103. When we connect the large capacitor bank in series ?

we connect large capacitor bank in series to improve the voltage power supply at the load end in balanced transmission line when there is considerable voltage drop along the balanced transmission line due to high impedance of the line.So in order to bring the voltage at the load terminals within its limits (i.e (+ or – %6 )of the rated high terminal voltage )the large capacitor bank is used in series.

 

104. What is electrical diversity factor in electrical installations?

Electrical diversity factor is the ratio of the sum of the individual maximum demands of the various subdivisions of a system, or part of a system, to the maximum demand of the whole system, or part of the system, under consideration. Electrical diversity factor is usually more than one.

 

105. Why field rheostat is kept in minimum position while armature rheostat at maximum position?

In motors at the time of starting the armature resistance is introduced to reduce the high starting current and the field resistance is kept minimum to have high starting torque.

 

106. Why computer humming sound occurred in HT transmission line?

This computer humming sound is coming due to ionization (breakdown of air into charged particles) of air around transmission conductor. This effect is called as Corona effect, and it is considered as power loss.

 

107. Explain What is rated speed?

At the time of motor taking normal current (rated current)the speed of the motor is called rated speed. It is a speed at which any system take small current and give maximum efficiency.

 

108. What is different between resistance grounding system and resistance earthing system?

Resistance grounding system means connecting the neutral point of the load to the ground to carry the residual current in case of unbalanced conditions through the neutral to the ground whereas resistance earthing system is done in an electric equipment in order to protect he equipment in occurrence of fault in the system.

 

109. Why should be the frequency 50 Hz 60Hz only why not others like 45, 95 56 or anything , why should we maintain the frequency constant if so why it is only 50 Hz 60Hz?

We can have the frequency at any frequency you like, but than you must also make your own motors,high voltage transformers or any other equipment you want to use.We maintain the frequency at 50hz or 60hz because the world maintains a standard at 50 /60hz and the equipments are are made to operate at these frequency.

 

110. How to determine alternating current frequency?

Zero crossings of the sine wave to trigger a monostable (pulse generator) is a way to determine alternating current frequency. A fixed width pulse is generated for each cycle. Thus there are “n” pulses per second, each with with a constant energy. The more pulses there are per second, the more the energy. The pulses are integrated (filtered or averaged) to get a steady DC voltage which is proportional to frequency. This voltage can then be displayed on an analogue or digital voltmeter, indicating frequency. This method is more suitable than a direct counter, as it can get good accuracy in a second or so.

 

111. Why electricity in India is in the multiples of 11 like 11kv, 22kv, 33kv ?

Transformer Induced voltage equation contains 4.44 factor.

E=4.44*f*T*phi

E -Induced emf per phase

T -number of turns

f -frequency

phi -maximum flux per pole

From the equation we see that E is proportional to 4.4 and it is in turn multiple of 11.

So always transmission voltage is multiple of 11

 

112. Why we use ac system in India why not dc ?

Firstly, the output of power stations comes from a rotary turbine, which by it’s nature is AC and therefore requires no power electronics to convert to DC. Secondly it is much easier to change the voltage of AC electricity for transmission and distribution. thirdly the cost of plant associated with AC transmission (circuit breakers, transformers etc) is much lower than the equivilant of DC transmission AC transmission provides a number of technical advantages. When a fault on the network occurs, a large fault current occurs. In an AC system this becomes much easier to interupt, as the sine wave current will naturally tend to zero at some point making the current easier to interrupt.

 

113. Which type of motor is used in trains, what is the rating of supply used explain Working principal?

Dc series is in the trains to get high starting torque while starting of the trains and operating voltage is 1500v dc.

 

114. Battery banks are in connected in series or parallel and why?

Battery banks are always connected in series in order to get a multiplied voltage where the AH or current capacity remaining same. Ex : 24 nos. 2V,200Ah batteries connected in series will give 48V,200Ah output (AH = Ampere hours)

 

115. What is inrush current?

Inrush current is the current drawn by a piece of electrically operated equipment when power is first applied. It can occur with AC or DC powered equipment, and can happen even with low supply voltages.

 

116. In a Tap changing transformer where is the tap connected, is it connected in the primary side or secondary side?

Tapings are connected to high voltage winding side, because of low current. If we connect tapings to low voltage side, sparks will produce while tap changing operation due to high current.

 

117. Why transformer ratings are in kva?

Since the power factor of transformer is dependent on load we only define VA rating and does not include power factor .In case of motors, power factor depend on construction and hence rating of motors is in KWatts and include power factor.

 

118. Define what is difference between fuse and breaker?

Fuses are burned at the time of over current flows in the circuit but breakers are just open(not burn) at the time of over current flow. Fuses are used in only one time but breakers are used by multiple number of times.

 

119. What is the difference between delta-delta, delta-star transformer?

Delta-delta transformer is used at generating station or a receiving station for Change of Voltage (i,e) generally it is used where the Voltage is high & Current is low.Delta-star is a distribution kind of transformer where from secondary star neutral is taken as a return path and this configuration is used for Step down voltage phenomena.

 

120. Capacitor is load free component but why ampere meter shows current when capacitor bank breaker close?

As we know that Electrical is having two type of load, Active and Reactive .Capacitor is a reactive load which is not considering as a load,& its factor is Isin@ .Meter is design based on Current RMS value because of it meter is showing the current RMS value.

 

121. What’s electric traction?

Traction implies with the electric power for traction system i. e. for railways, trams, trolleys etc. electric traction implies use of the electricity for all these. Now a day, magnetic traction is also utilised for bullet trains. Essentially dc motors are utilized for electric traction systems.

 

122. What is “pu” in EE?

Pu stands for per unit in power system. (pu = actual value/ base value)

 

123. Define stepper motor. What is the use of stepper motor?

The motor which work or act on the applied input pulse in it, is called as stepper motor.This stepper motor is under the category of synchronous motor, which often does not fully depend of complete cycle. It likes to works in either direction related to steps. for this purpose it mainly used in automation parts.

 

124. What is a differential amplifier? Also, explain CMRR.

Differential Amplifier: The amplifier, which is used to amplify the voltage difference between two input-lines neither of which is grounded, is called differential amplifier. This reduces the amount of noise which is injected into the amplifier, because any noise appearing simultaneously on both the input-terminals as the amplifying circuitry rejects it being a common mode signal.

CMRR: It can be defined as the ratio of differential voltage-gain to common made voltage gain. If a differential amplifier is perfect, CMRR will be infinite because in that case common mode voltage gain would be zero.

 

125. What is use of lockout relay in ht voltage?

A lock-out relay is generally placed in line before or after the e-stop switch so the power can be shut off at one central location. This relay is powered by the same electrical source as the control power which 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.

 

126. How can you start-up the 40w tube lite with 230v AC/DC without using any choke/Coil?

It’s possible with Electronic choke. otherwise it’s not possible to ionize the particles in tube. light, with normal voltage.

 

127. What types domain of Laplace transforms? What behavior can Laplace transform predict how the system work?

Types domain of Laplace transforms is s-domain, Laplace transforms provide a method to find position, acceleration or voltage the system will have.

 

128. In the magnetic fluxes, what is the role of armature reaction?

The armature flux has an important role for the running condition. This armature flux can oppose the main flux or it may support the main flux for better running condition. This effect of supporting and opposing of main flux to armature flux is called armature reaction.

 

129. Explain thin film resistors and wire-wound resistors

Thin film resistors- It is constructed as a thin film of resistive material is deposited on an insulating substrate.

Desired results are obtained by either trimming the layer thickness or by cutting helical grooves of suitable pitch along its length. During this process, the value of the resistance is monitored closely and cutting of grooves is stopped as soon as the desired value of resistance is obtained.

Wire wound resistors – length of wire wound around an insulating cylindrical core are known as wire wound resistors. These wires are made of materials such as Constantan and Manganin because of their high resistivity, and low temperature coefficients. The complete wire wound resistor is coated with an insulating material such as baked enamel

 

130. whats the one main difference between UPS & inverter ? And electrical engineering & electronics engineering ?

uninterrupt 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 with high dc voltage.but ups start with v dc with 7 amp. but inverter is startwith v,24,dc to 36v dc and 0amp to 180amp battery with long time backup.

 

131. What are the operation carried out in Thermal power station?

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

 

132. What is the difference between Electronic regulator and ordinary rheostat regulator for fans?

The difference between the electronic and ordinary regulator is the fact that in electronic reg. power losses tend to be less because as we minimize the speed the electronic reg. give the power necessary for that particular speed but in case of ordinary rheostat type reg. 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 is controlled but in rheostatic control resistance is decreased by steps to achievespeed control.

 

133. What is 2 phase motor?

A two phase motor is often 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.

 

134. What does quality factor depend on in resonance?

Quality factor q depends on frequency and bandwidth.

 

135. What are the types of power in electrical power?

There are normally three types of power are counted in electrical power. They are,

 

Apparent power

Active power

Reactive power

136. What are the advantages of VSCF wind electrical system?

Advantages of VSCF wind electrical system are:

 

No complex pitch changing mechanism is needed.

Aero turbine always keeps going at maximum efficiency point.

Extra energy in the high wind speed region of the speed – duration curve can be extracted

Significant reduction in aerodynamic stresses, which are associated with constant – speed operation.

137. What is slip in an induction motor?

Slip can be defined as the distinction between the flux speed (Ns) and the rotor speed (N). Speed of the rotor of

an induction motor is always less than its synchronous speed. It is usually expressed as a percentage of

synchronous speed (Ns) and represented by the symbol ‘S’.

 

138. Why link is provided in neutral of an ac circuit and fuse in phase of ac circuit?

Link is provided at a Neutral common point in the circuit from which various connection 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 happen the fuse connected in the particular control circuit alone will blow off.

 

139. State the difference between generator and alternator?

Generator and alternator are two devices, which converts mechanical energy into electrical energy. Both have the same principle of electromagnetic induction, the only difference is that their construction. Generator persists stationary magnetic field and rotating conductor which rolls on the armature with slip rings and brushes riding against each other, hence it converts the induced emf into dc current for external load whereas an alternator has a stationary armature and rotating magnetic field for high voltages but for low voltage output rotating armature and stationary magnetic field is used.

 

140. What is ACSR cable and where we use it?

ACSR means Aluminium conductor steel reinforced, this conductor is used in transmission & distribution.

 

141. Why star delta starter is preferred with induction motor?

Star delta starter is preferred with induction motor due to following reasons:

 

Starting current is reduced 3-4 times of the direct current due to which voltage drops and hence it causes less losses.

Star delta starter circuit comes in circuit first during starting of motor, which reduces voltage 3 times, that is why current also reduces up to 3 times and hence less motor burning is caused.

In addition, starting torque is increased and it prevents the damage of motor winding.

142. State the difference between generator and alternator

Generator and alternator are two devices, which converts mechanical energy into electrical energy. Both have the same principle of electromagnetic induction, the only difference is that their construction. Generator persists stationary magnetic field and rotating conductor which rolls on the armature with slip rings and brushes riding against each other, hence it converts the induced emf into dc current for external load whereas an alternator has a stationary armature and rotating magnetic field for high voltages but for low voltage output rotating armature and stationary magnetic field is used.

 

143. Why AC systems are preferred over DC systems?

Due to following reasons, AC systems are preferred over DC systems:

a. It is easy to maintain and change the voltage of AC electricity for transmission and distribution.

b. Plant cost for AC transmission (circuit breakers, transformers etc) is much lower than the equivalent DC transmission

c. From power stations, AC is produced so it is better to use AC then DC instead of converting it.

d. When a large fault occurs in a network, it is easier to interrupt in an AC system, as the sine wave current will naturally tend to zero at some point making the current easier to interrupt.

 

144. How can you relate power engineering with electrical engineering?

Power engineering is a sub division of electrical engineering. It deals with generation, transmission and distribution of energy in electrical form. Design of all power equipments also comes under power engineering. Power engineers may work on the design and maintenance of the power grid i.e. called on grid systems and they might work on off grid systems that are not connected to the system.

 

145. What are the various kind of cables used for transmission?

Cables, which are used for transmitting power, can be categorized in three forms:

 

Low-tension cables, which can transmit voltage upto 1000 volts.

High-tension cables can transmit voltage upto 23000 volts.

Super tension cables can transmit voltage 66 kV to 132 kV.

146. Why back emf used for a dc motor? highlight its significance.

The induced emf developed when the rotating conductors of the armature between the poles of magnet, in a DC motor, cut the magnetic flux, opposes the current flowing through the conductor, when the armature rotates, is called back emf. Its value depends upon the speed of rotation of the armature conductors. In starting, the value of back emf is zero.

 

147. What is slip in an induction motor?

Slip can be defined as the difference between the flux speed (Ns) and the rotor speed (N). Speed of the rotor of an induction motor is always less than its synchronous speed. It is usually expressed as a percentage of synchronous speed (Ns) and represented by the symbol ‘S’.

 148. Explain the application of storage batteries.

Storage batteries are used for various purposes, some of the applications are mentioned below:

 

For the operation of protective devices and for emergency lighting at generating stations and substations.

For starting, ignition and lighting of automobiles, aircrafts etc.

For lighting on steam and diesel railways trains.

As a supply power source in telephone exchange, laboratories and broad casting stations.

For emergency lighting at hospitals, banks, rural areas where electricity supplies are not possible.

149. Explain advantages of storage batteries

Few advantages of storage batteries are mentioned below:

 

Most efficient form of storing energy portably.

Stored energy is available immediately because there is no lag of time for delivering the stored energy.

Reliable source for supply of energy.

The energy can be drawn at a fairly constant rate.

150. What are the different methods for the starting of a synchronous motor.

Starting methods: Synchronous motor can be started by the following two methods:

 

By means of an auxiliary motor: The rotor of a synchronous motor is rotated by auxiliary motor. Then rotor poles are excited due to which the rotor field is locked with the stator-revolving field and continuous rotation is obtained.

By providing damper winding: Here, bar conductors are embedded in the outer periphery of the rotor poles and are short-circuited with the short-circuiting rings at both sides. The machine is started as a squirrel cage induction motor first. When it picks up speed, excitation is given to the rotor and the rotor starts rotating continuously as the rotor field is locked with stator revolving field.

151. Name the types of motors used in vacuum cleaners, phonographic appliances, vending machines,refrigerators, rolling mills, lathes, power factor improvement and cranes.

Following motors are used: –

 

Vacuum cleaners- Universal motor.

Phonographic appliances – Hysteresis motor.

Vending machines – Shaded pole motor.

Refrigerators – Capacitor split phase motors.

Rolling mills – Cumulative motors.

Lathes – DC shunt motors.

Power factor improvement – Synchronous motors.

152. State Thevenin’s Theorem:

According to thevenin’s theorem, the current flowing through a load resistance Connected across any two terminals of a linear active bilateral network is the ratio open circuit voltage (i.e. the voltage across the two terminals when RL is removed) and sum of load resistance and internal resistance of the network. It is given by Voc / (Ri + RL).

 

153. State Norton’s Theorem

The Norton’s theorem explains the fact that there are two terminals and they are as follows:

 

One is terminal active network containing voltage sources

Another is the resistance that is viewed from the output terminals. The output terminals are equivalent to the constant source of current and it allows giving the parallel resistance.

The Norton’s theorem also explains about the constant current that is equal to the current of the short circuit placed across the terminals. The parallel resistance of the network can be viewed from the open circuit terminals when all the voltage and current sources are removed and replaced by the internal resistance.

154. State Maximum power transfer theorem

The Maximum power transfer theorem explains about the load that a resistance will extract from the network.

This includes the maximum power from the network and in this case the load resistance is being is equal to the resistance of the network and it also allows the resistance to be equal to the resistance of the network. This resistance can be viewed by the output terminals and the energy sources can be removed by leaving the internal resistance behind.

 

155. Explain different losses in a transformer.

There are two types of losses occurring in transformer:

 

Constant losses or Iron losses: The losses that occur in the core are known as core losses or iron losses.

Two types of iron losses are:

1. eddy current loss

2. Hysteresis loss.

These losses depend upon the supply voltage, frequency, core material and its construction. As long as supply voltage and frequency is constant, these losses remain the same whether the transformer is loaded or not. These are also known as constant losses.

Variable losses or copper losses: when the transformer is loaded, current flows in primary and secondary windings, there is loss of electrical energy due to the resistance of the primary winding, and secondary winding and they are called variable losses. These losses depend upon the loading conditions of the transformers.

Therefore, these losses are also called as variable losses.

156. Explain different types of D.C motors? Give their applications

Different type of DC motors and their applications are as follows:-

 

Shunt motors: It has a constant speed though its starting torque is not very high. Therefore, it is suitable for constant speed drive, where high starting torque is not required such as pumps, blowers, fan, lathe machines, tools, belt or chain conveyor etc.

Service motors: It has high starting torque & its speed is inversely proportional to the loading conditions i.e. when lightly loaded, the speed is high and when heavily loaded, it is low. Therefore, motor is used in lifts, cranes, traction work, coal loader and coal cutter in coalmines etc.

Compound motors: It also has high starting torque and variable speed. Its advantage is, it can run at NIL loads without any danger. This motor will therefore find its application in loads having high inertia load or requiring high intermittent torque such as elevators, conveyor, rolling mill, planes, presses, shears and punches, coal cutter and winding machines etc.

157. Explain the process of commutation in a dc machine. Explain what are inter-poles and why they are required in a dc machine.

Commutation: It is phenomenon when an armature coil moves under the influence of one pole- pair; it carries constant current in one direction. As the coil moves into the influence of the next pole- pair, the current in it must reverse. This reversal of current in a coil is called commutation. Several coils undergo commutation simultaneously. The reversal of current is opposed by the static coil emf and therefore must be aided in some

fashion for smooth current reversal, which otherwise would result in sparking at the brushes.

 

The aiding emf is dynamically induced into the coils undergoing commutation by means of compoles or interpoles, which are series excited by the armature current. These are located in the interpolar region of the main poles and therefore influence the armature coils only when these undergo commutation.

 

158. Comment on the working principle of operation of a single-phase transformer.

Working principle of operation of a single-phase transformer can be explained as An AC supply passes through the primary winding, a current will start flowing in the primary winding. As a result, the flux is set. This flux is linked with primary and secondary windings. Hence, voltage is induced in both the windings. Now, when the load is connected to the secondary side, the current will start flowing in the load in the secondary winding, resulting in the flow of additional current in the secondary winding. Hence, according to Faraday’s laws of electromagnetic induction, emf will be induced in both the windings. The voltage induced in the primary winding is due to its self inductance and known as self induced emf and

according to Lenze’s law it will oppose the cause i.e. supply voltage hence called as back emf. The voltage induced in secondary coil is known as mutually induced voltage. Hence, transformer works on the principle of electromagnetic induction.

 

159. Define the following terms:-

• Reliability,

• Maximum demand,

• Reserve-generating capacity,

• Availability (operational).

Reliability: It is the capacity of the power system to serve all power demands without failure over long periods.

Maximum Demand: It is maximum load demand required in a power station during a given period.

Reserve generating capacity: Extra generation capacity installed to meet the need of scheduled downtimes for preventive maintenance is called reserve-generating capacity.

Availability: As the percentage of the time a unit is available to produce power whether needed by the system or not.

 

160. Mention the disadvantages of low power factor? How can it be improved?

Disadvantages of low power factor:

 

Line losses are 1.57 times unity power factor.

Larger generators and transformers are required.

Low lagging power factor causes a large voltage drop, hence extra regulation equipment is required to keep voltage drop within prescribed limits.

Greater conductor size: To transmit or distribute a fixed amount of power at fixed voltage, the conductors will have to carry more current at low power factor. This requires a large conductor size.

161. State the methods of improving power factor?

Methods of improving power factor:

 

By connecting static capacitors in parallel with the load operating at lagging power factor.

A synchronous motor takes a leading current when over excited and therefore behaves like a capacitor.

By using phase advancers to improve the power factor of induction motors. It provides exciting ampere turns to the rotor circuit of the motor. By providing more ampere-turns than required, the induction motor can be made to operate on leading power factor like an overexcited synchronous motor.

162. State the factors, for the choice of electrical system for an aero turbine.

The choice of electrical system for an aero turbine is guided by three factors:

 

Type of electrical output: dc, variable- frequency ac, and constant- frequency ac.

Aero turbine rotational speed: constant speed with variable blade pitch, nearly constant speed with simpler pitch- changing mechanism or variable speed with fixed pitch blades.

Utilization of electrical energy output: in conjunction with battery or other form of storage, or interconnection with power grid.

163. What are the advantages of VSCF wind electrical system?

Advantages of VSCF wind electrical system are:

 

No complex pitch changing mechanism is needed.

Aero turbine always operates at maximum efficiency point.

Extra energy in the high wind speed region of the speed – duration curve can be extracted

Significant reduction in aerodynamic stresses, which are associated with constant – speed operation.

164. Explain the terms real power, apparent power and reactive power for ac circuits and also the units used.

 

Real Power: It is the product of voltage, current and power factor i.e. P = V I cos j and basic unit of real power is watt. i.e. Expressed as W or kW.

Apparent power: It is the product of voltage and current. Apparent power = V I and basic unit of apparent power is volt- ampere. Expressed as VA or KVA.

Reactive Power: It is the product of voltage, current and sine of angle between the voltage and current i.e.

Reactive power = voltage X current X sinj or Reactive power = V I sin j and has no other unit but expressed in VAR or KVAR.

165. Define the following: Average demand, Maximum demand, Demand factor, Load factor.

 

Average Demand: the average power requirement during some specified period of time of considerable duration is called the average demand of installation.

Maximum Demand: The maximum demand of an installation is defined as the greatest of all the demand, which have occurred during a given period. It is measured accordingly to specifications, over a prescribed time interval during a certain period.

Demand Factor: It is defined as the ratio of actual maximum demand made by the load to the rating of the connected load.

Load Factor: It is defined as the ratio of the average power to the maximum demand.

166. Explain forward resistance, static resistance and dynamic resistance of a pn junction diode.

 

Forward Resistance: Resistance offered in a diode circuit, when it is forward biased, is called forward resistance.

DC or Static Resistance: DC resistance can be explained as the ratio of the dc-voltage across the diode to the direct current flowing through it.

AC or Dynamic Resistance: It can be defined as the reciprocal of the slope of the forward characteristic of the diode. It is the resistance offered by a diode to the changing forward current.

167. How does Zener phenomenon differ from Avalanche breakdown?

The phenomenon when the depletion region expands and the potential barrier increases leading to a very high electric field across the junction, due to which suddenly the reverse current increases under a very high reverse voltage is called Zener effect. Zener-breakdown or Avalanche breakdown may occur independently or both of these may occur simultaneously. Diode junctions that breakdown below 5v are caused by Zener Effect.

Junctions that experience breakdown above 5v are caused by avalanche-effect. The Zener-breakdown occurs in heavily doped junctions, which produce narrow depletion layers. The avalanche breakdown occurs in lightly doped junctions, which produce wide depletion layers.

 

168. Compare JFET’s and MOSFET’s.

Comparison of JFET’s and MOSFET’s:

 

JFET’s can only be operated in the depletion mode whereas MOSFET’s can be operated in either depletion or in enhancement mode. In a JFET, if the gate is forward-biased, excess-carrier injunction occurs and the gatecurrent is substantial.

MOSFET’s have input impedance much higher than that of JFET’s. Thus is due to negligible small leakage current.

JFET’s have characteristic curves more flat than that of MOSFET is indicating a higher drain resistance.

When JFET is operated with a reverse-bias on the junction, the gate-current IG is larger than it would be in a comparable MOSFET.

169. Explain thin film resistors and wire-wound resistors

a. Thin film resistors- It is constructed as a thin film of resistive material is deposited on an insulating substrate.

Desired results are obtained by either trimming the layer thickness or by cutting helical grooves of suitable pitch along its length. During this process, the value of the resistance is monitored closely and cutting of grooves is stopped as soon as the desired value of resistance is obtained.

b. Wire wound resistors – length of wire wound around an insulating cylindrical core are known as wire wound resistors. These wires are made of materials such as Constantan and Manganin because of their high resistivity, and low temperature coefficients. The complete wire wound resistor is coated with an insulating material such as baked enamel

 

170. What is a differential amplifier? Also, explain CMRR.

Differential Amplifier: The amplifier, which is used to amplify the voltage difference between two input-lines neither of which is grounded, is called differential amplifier. This reduces the amount of noise injected into the amplifier, because any noise appearing simultaneously on both the input-terminals as the amplifying circuitry rejects it being a common mode signal.

CMRR: It can be defined as the ratio of differential voltage-gain to common made voltage gain. If a differential amplifier is perfect, CMRR would be infinite because in that case common mode voltage gain would be zero.

 

171. What is the difference between electronic regulator and ordinary electrical rheostat regulator for fans?

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.

 

172. What is the voltage gain or transfer function of amplifier?

Vout/Vin

 

173. What does the KVAR means?

The KVAR indicates the electrical power. KVAR means “Kilo Volt Amperes with Reactive components”

 

174. Why use the VCB at High Transmission System ? Why can’t use ACB?

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 . That y always vacuum used as inHT breaker and air used as in LT .

 

175. What is the difference between MCB & MCCB, Where it can be used?

MCB is miniature circuit breaker which is thermal operated and use for short circuit protection in small 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.