ELECTRICAL Engineering Interview Questions With Answers
1. What is electric traction?
A:Electric traction means using the electric power for traction system (i.e. for railways,trams, trolleys etc).
Electric traction means use of the electricity for all the above machines. Now a 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?
A: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?
A:Pu stands for per unit and this will be used in single line diagram of power distribution and it is like a huge
electrical circuit with no of components (generators, transformers, loads) with different ratings (in MVA and
KV). To bring all the ratings into common platform we use pu concept in which, in general largest MVA and
KV ratings of the component is considered as base values, then all other component ratings will get back into
this basis.Those values are called as pu values. (p.u=actual value/base value).
4. Operation carried out in Thermal power stations?
A:The water is obtained in the boiler and the coal is burnt so that steam is obtained this steam is allowed to hit
the turbine , the turbine which is coupled with the generator generates the electricity.
5. Why link is provided in neutral of an ac circuit and fuse in phase of ac circuit?
A:Link is provided at a Neutral common point in the circuit from which various 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.
5. What is the difference between electronic regulator and ordinary electrical rheostat regulator for fans?
A:The difference between the electronic and ordinary electrical regulator is that in electronic regulator power
losses are less because as we decrease the speed the electronic regulator gives the power needed for that
particular speed but in case of ordinary rheostat type regulator, the power wastage is same for every speed and
no power is saved.In electronic regulator, triac is employed for speed control by varying the firing angle speed
and it is controlled but in rheostatic ,control resistance is decreased by steps to achieve speed control.
6. How tube light circuit is connected and how it works?
A:A choke is connected in one end of the tube light and a starter is in series with the circuit. When supply is
provided ,the starter will interrupt the supply cycle of AC. Due to the sudden change of supply the chock will
generate around 1000volts . This volt will capable of to break the electrons inside the tube to make electron
flow. once the current passes through the tube the starter circuit will be out of part. now there is no change of
supply causes choke voltage normalized and act as minimize the current.
7. whats is MARX CIRCUIT?
A:It is used with generators for charging a number of capacitor in parallel and discharging them in series.It is
used when voltage required for testing is higher than the available.
8. What is encoder, how it function?
A: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?
A:Advantages :1. Fast Switching Characterstics than Mosfet, BJT, IGBT 2. Low cost 3. Higher Accurate.
10. Why Human body feel Electric shock ?? n in an Electric train during running , We did nt feel any Shock ?
why?
A:Unfortunately our body is a pretty good conductor of electricity, The golden rule is Current takes the lowest
resistant path if you have insulation to our feet as the circuit is not complete (wearing rubber footwear which
doing some repairs is advisable as our footwear is a high resistance path not much current flows through our
body).The electric train is well insulated from its electrical system.
11. what is the principle of motor?
Whenever a current carrying conductor is placed in an magnetic field it produce turning or twisting movemnt
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 vaccum currcuit breaker.define with cause and where be use it Device?
A breaker is normally used to break a ciruit. while breaking the circuit, the contact terminals will be separated.
At the time of seperation 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 vaccum gas. since the air in the CB is having vaccum pressure the arc formation is
interrupted. VCB’s can be used upto 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:
1.losses in form of heat will be reduced,
2.cable becomes less bulky and easy to carry, and very
cheap to afford, &
3. it also reduces over heating of tranformers.
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?
1.smooth start and stop.
2.no jerking of load.
3.exact posiitoning
4.better protection for motor.
5.high/low speed selection.
6.reliability of break shoe.
7.programmable break control.
8.easy circutry
9.reduction in controls
0.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 betweent 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:
1)separatly exicted d.c.generator
2)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 queching property compare to air because in VCB ,the die electric
strengths equal to 8 times of air . That y always vaccum 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.
1. 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.
2. 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, vaccum etc.
3. How many types of colling system it transformers?
1. ONAN (oil natural,air natural)
2. ONAF (oil natural,air forced)
3. OFAF (oil forced,air forced)
4. ODWF (oil direct,water forced)
5. OFAN (oil forced,air forced)
4. 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.
5. 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.
6. 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.
7. 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.
8. 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
9. 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).
10. 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.
1. 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.
2. 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.
3. What is frantic effect?
Output voltage is greater than the input voltage or receiving end voltage is greater than the sending end
voltage.
4. 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.
5. 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.
6. 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.
7. 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.
8. 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.
9. 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
1)chandrapur to padghe(mumbai)–(100 MW at ±00 kV DC)
2)rehand to delhi (100 MW at ±00 kV DC)
3)talchal to kolar (200 MW)
1. 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”.
2. 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.
3. 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.
4. 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.
5. 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.
6. 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.
7. 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.
8. 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.
9. 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.
10. 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.