Electrical interview question:What is the function of
anti-pumping in circuit breaker?
Answer: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.
Electrical interview question:what is stepper
motor.what is its uses?
Answer: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.
Electrical interview question:how to calculate
capacitor bank value to maintain unity power factor with some suitable example?
Answer:KVAR= KW(TAN(COS(-1)#e)- TAN(COS(-1)#d) )
#e= EXISTING P.F.
#d= DESIRED P.F.
Electrical interview question:Tell me in detail about
c.t. and p.t. ?(Company:reliance)
Answer: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 5000\5A and it has to measure secondary
current of
8000A.then ANS=8000*5\5000=8Aand this result will be given to ammeter .and
after measuring 8A we can calculate the primary current.same is
the operation of PT but measuring voltage.
Electrical interview question: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?
Answer: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.
Electrical interview question:what is power factor?
whether it should be high or low? why?
Answer: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 apperent 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 lossesif pf is good ex: 0.95 to meet
actual power load has to draw less current(V constant),result in less losses).
Electrical interview question:What is
the difference between Isolator and Circuit Breaker?
Answer: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.
Electrical interview question:what is boucholz relay
and the significance of it in to the transformer?
Answer: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.
Electrical interview question:What is SF6 Circuit
Breaker?
Answer:SF6 is Sulpher hexa Flouride gas.. if this gas is
used as arc quenching medium in a Circuitbreaker means SF6 CB.
Electrical interview question:what is ferrantic
effect?
Answer:Output voltage is greater than the input voltage or
receiving end voltage is greater than the sending end voltage.
The Ferranti effect is a rise in voltage
occurring at the receiving end of a long transmission line, relative to
the voltage at the sending end, which occurs when the line is energized but
there is a very light load or the load is disconnected.
This effect is due to the voltage drop across the line inductance (due to
charging current) being in phase with the sending end voltages. Therefore both
capacitance and inductance are responsible for producing this phenomenon.[1]
The Ferranti Effect will be more pronounced the longer the line and the
higher the voltage applied.[2] The relative voltage rise is proportional
to the square of the line length.[3]
Due to high capacitance, the Ferranti effect is much more pronounced in
underground cables, even in short lengths.
Electrical interview question: what is meant by
insulation voltage in cables? explain it?
Answer: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.
Electrical interview question: Why we do 2 types of
earthing on transformer (ie:)body earthing & neutral earthing , what is
function. i am going to install a 5oo kva transformer & 380 kva DG set what
should the earthing value?
Answer: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.
Electrical interview question:What is
the difference between MCB & MCCB, Where it can be used?
Answer: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.
Electrical interview question:Where should the
lighting arrestor be placed in distribution lines?
Answer:Near distribution transformers and out going feeders
of 11kv and incomming feeder of 33kv and near power transformers in
sub-stations.
Electrical interview question:Define IDMT relay?
Answer: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.
Electrical interview question:What are the transformer
losses?
Answer: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.
Electrical interview question:What is meant by regenerative
braking?
Answer: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 applyacross
the resistor and due to load the motor stops quickly.This type of breaking is
called as "Regenerative Breaking".
Electrical interview question:Why is the starting current
high in a DC motor?
Answer: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
Electrical interview question:What are the advantages of
star-delta starter with induction motor?
Answer:(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.
Electrical interview question:Why Delta Star Transformers
are used for Lighting Loads?
Answer: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.
Electrical interview question:Why in a three pin plug the
earth pin is thicker and longer than the other pins?
Answer: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.
Electrical interview question:Why series motor cannot be
started on no-load?
Answer:Series motor cannot be started without load because of high starting
torque. Series motor are used in Trains, Crane etc.
Electrical interview question:Why ELCB can't work if N
input of ELCB do not connect to ground?
Answer: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.
Electrical interview question:How electrical power is
generated by an A.C Generator?
Answer: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.
Electrical interview question:Why an ac solenoid valve
attract the plunger even though we interchanges the terminal? Will the poles
changes?
Answer:Yes because the poles changes for every half-cycle
of ac voltageso 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.
Electrical interview question:What is derating?, why it is
necessary, it is same for all means for drives, motors,and cables.
Answer: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.
Electrical interview question:Why temperature rise is
conducted in bus bars and isolators?
Answer: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.
Electrical interview question: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?
Answer: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.
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