Electrical Interview questions
Q: What is inrush current?
A:
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.
Q: In a Tap changing transformer
where is the tap connected, is it connected in the primary side or secondary
side?
A: 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.
Q:Why transformer ratings are in
kva?
A: 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.
Q:What is the difference between
delta-delta, delta-star transformer?
A: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.
Q: Capacitor is load free
component but why ampere meter shows current when capacitor bank breaker closes?
A: 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.
Q: How to determine
alternating current frequency?
A: Zero crossings of the sine wave to trigger a constable (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.
Q: Why electricity in India is in the multiples of 11 like 11kv,
22kv, 33kv?
A:
Transformer Induced voltage equation contains 4.44 factors.
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
Q:Why we
use ac system in India why not dc ?
A:
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 equivalent
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 interrupt, as the sine wave current will
naturally tend to zero at some point making the current easier to interrupt.
Q: Which
type of motor is used in trains, what is the rating of supply used explain
Working principal?
A: Dc series is in the trains to get high starting torque while
starting of the trains and operating voltage is 1500v dc.
Q:
Battery banks are in connected in series or parallel and why?
A: 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)
Query:Difference between megger
test equipment and contact resistance meter test instruments?
Resolution: Megger test equipment used to measure cable electric
resistance, conductor continuity, and phase identification where as contact
resistance meter test instruments used to measure low resistance like relays,
contactors.
Query: When we connect the large capacitor bank in series?
Resolution: 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.
Query: What is electrical diversity factor in
electrical installations?
Resolution: 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.
Query: Why field rheostat is kept in minimum position
while armature rheostat at maximum position?
Resolution :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.
Query:Why computer humming sound occurred in HT
transmission line?
Resolution :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.
Query:What is rated speed?
Resolution :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.
Query: What is different between resistance grounding
system and resistance earthing system?
Resolution :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.
Query: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?
Resolution :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 made
to operate at these frequency.
Query:
Use of switch mode power converter in real-time basis?
Resolution: Switch mode power converter can be used in the
following 5 different ways
1) 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,
2)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,
3)step up or step down an unregulated dc input voltage to produce a regulated
dc output voltage,
4)invert the input dc voltage using usually a circuit such as the Cuk
converter, and
5)produce multiple dc outputs using a circuit such as the fly-back converter.
Query: Which type of oil is used as transformer oil?
Resolution: 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 1970s, 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 1970s, 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 fire-resistant 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.
Query: 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?
Resolution: 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
Query: what are the points to be consider for MCB(miniature circuit
breaker selection?
Resolution: I (L)*1.25=I(MAX) maximum current. Mcb specification
is done on maximum current flow in circuit.
Query:what is the full form of KVAR?
Resolution: 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.
Query:What is excitation?
Resolution: Excitation is applying an external voltage to DC
shunt coil
in DC motors.
Query: In three pin plug 6 Amp. 220v AC rating. Why earth pin
diameter is higher than other two pin? What its purpose?
Resolution: 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)
Query:
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.
Query: 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:
1. Alternating current (AC) to direct current (DC)
2.DC to AC
3.DC at one voltage to DC at another voltage
4.AC at one frequency to AC at another harmonically-related frequency
Query: What is power quality meter?
Resolution: Power Quality meters are common in many industrial environments.
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 high-speed communication
lines.So we can understand the importance of power measurement through power
quality meters.
Query: What is the different between digital phase converter and
ordinary phase converter?
Resolution: Digital phase converter is 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.
Query: Explain the operation of variable frequency transformer?
Resolution: 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.
Query: What is the main use of rotary phase converter?
Resolution: 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.
Query:
How to determine capacitor tolerance codes?
Resolution: In electronic circuits, the capacitor tolerance can
be determined by a code that appears on the casing. The code is a letter that
often follows a three-digit number (such as 130Z).The first two are the 1st and
2nd significant digits and the third is a multiplier code. Most of the time the
last digit tells you how many zeros to write after the first two digits and
these are read as Pico-Farads.
Query: Why most of analog o/p devices having o/p range 4 to 20 mA and
not 0 to 20 mA?
Resolution:4-20 mA is a standard range used to
indicate measured values for any process. 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.
Query: Two bulbs of 100w and 40w respectively connected in series across
a 230v supply which bulb will glow bright and why?
Resolution: Since two bulbs are in series they
will get equal amount of electrical current but as the supply voltage is
constant across the bulb (P=V^2/R).So the resistance of 40W bulb is greater and
voltage across 40W is more (V=IR) so 40W bulb will glow brighter.
Query:
What is meant by knee point voltage?
Resolution: Knee point voltage is calculated for
electrical Current transformers and is very important factor to choose a CT. It
is the voltage at which a CT gets saturated.(CT-current transformer).
Query:
What is reverse power relay?
Resolution: Reverse Power flow relay are used in
generating stations’ protection. A generating stations is supposed to fed power
to the grid and in case generating units are off, there is no generation in the
plant then plant may take power from grid. To stop the flow of power from grid
to generator we use reverse power relay.
Query: What will happen if DC supply is given on the primary of a
transformer?
Resolution: Mainly transformer has high inductance and low
resistance.In case of DC supply there is no inductance ,only resistance will
act in the electrical circuit. So high electrical current will flow
through primary side of the transformer.So for this reason coil and insulation
will burn out.
Query: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.
Query:What are the advantage of freewheeling 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
Q:How do you select a cable size (Cu
& Al) for a particular load?
A:At first calculate the electrical current of the load, after that
derate the electrical current considering derating factor(depending on site
condition and laying of cable) after choose the cable size from cable catalog
considering derating electrical current.After that measure the length of cable
required from supply point of load to load poin. Calculate the voltage drop
which will max 3% (resistance and reactance of cable found from cable catalog
of selecting cable) if voltage drop>3%
then choose next higher size of cable.
Q: What are HRC fuses and where it is used?
A: HRC stand for "high rupturing capacity" fuse and it is used
in distribution system for electrical transformers.
Q: Which power plant has high load factor?
A: All base load power plants have a high load factor. If we use high
efficiency power plants to supply the base load, we can reduce the cost of
generation.Hydel power plants have a higher efficiency than thermal &
nuclear power plants.
Q: Mention the methods for starting an induction motor?
A: The different methods of starting an induction motor
DOL:direct online starter
Star delta starter
Auto transformer starter
Resistance starter
Series reactor starter
Q: What is the difference between earth resistance and earth electrode
resistance?
A: Only one of the terminals is evident in the earth resistance. In
order to find the second terminal we should recourse to its definition: Earth
Resistance is the resistance existing between the 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.
Q: What is use of lockout relay in HT voltage?
A: A lock-out relay is generally placed in line before or after the
e-stop switch so the power can be shut off at one central location. This relay
is powered by the same electrical source as the control power and is operated
by a key lock switch. The relay itself may have up to 24 contact points within
the unit itself. This allows the control power for multiple machines to be
locked out by the turn of a single key switch.
Q: What is the power factor of an alternator at no load?
A:At no load Synchronous Impedance of the alternator is responsible for
creating angle difference. So it should be zero lagging like inductor.
Q: How electrical power is generated by
an A.C Generator?
A: 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.
Q: Why ac solenoid valves attract the plunger even though we interchange the
terminal? Will the poles changes?
A: Yes because the poles change for every half-cycle of ac voltage so
the polarity of AC voltage is continuously changing for every half cycle. so,
interchanging of terminals in ac system does not show any difference. That's
why the ac solenoid attract the plunger even though it's terminals are interchanged.
Q: What is derating?, why it is
necessary, it is same for all means for drives, motors,and cables.
A: The
current currying of cables will change depending upon the site temperature
(location of site), type of run (it will run through duct, trench, buried
etc.), number of tray, depth of trench, distance between cables. Considering
this condition actual current currying capacity of cable reduce than current
currying capacity (which given to cable Catalogue) this is called derating.
Q:Why temperature rise is conducted in bus bars and isolators?
A: Bus bars
and isolators are rated for continuous power flow, that means they carry heavy
currents which rises their temperature. so it is necessary to test this devices
for temperature rise.
Q:When voltage increases then current also increases then what is the need
of over voltage relay and over current relay? Can we measure over voltage and
over current by measuring current only?
A:No.We can't sense the over voltage by just measuring the current only
because the current increases not only for over voltages but also for under
voltage(As most of the loads are non-linear in nature).So,the over voltage
protection & over current protection are completely different. 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.
Q:If one lamp connects between two phases it will glow or not?
A: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.
Query: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)--(1500 MW at ±500 kV DC)
2)rehand to delhi (1500 MW at ±500 kV DC)
3)talchal to kolar (2500 MW)
Query:What is meant by regenerative braking?
Resolution:When the supply is cutt off for a
running motor, it still continue running due to inertia. In order to stop it
quickly we place a load(resitor) across the armature winding and the motor
should have maintained continuous field supply. so that back e.m.f voltage is
made to apply across the resistor and due to load the motor stops quickly.This
type of breaking is called as "Regenerative Breaking".
Query:Why is the starting current high in a DC motor?
Resolution:In DC motors, Voltage equation is
V=Eb-IaRa (V = Terminal voltage,Eb = Back emf in Motor,Ia = Armature current,Ra
= Aramture resistance).At starting, Eb is zero. Therefore, V=IaRa, Ia = V/Ra
,where Ra is very less like 0.01ohm.i.e, Ia will become enormously increased.
Query:What
are the advantages of star-delta starter with induction motor?
Resolution:(1). The main advantage of using the
star delta starter is reduction of current during the starting of the
motor.Starting current is reduced to 3-4 times Of current of Direct online
starting.(2). Hence the starting current is reduced , the voltage drops during
the starting of motor in systems are reduced.
Query:Why
Delta Star Transformers are used for Lighting Loads?
Resolution:For lighting loads, neutral conductor
is must and hence the secondary must be star winding. and this lighting load is
always unbalanced in all three phases. To minimize the current unbalance in the
primary we use delta winding in the primary. So delta / star transformer is
used for lighting loads.
Query:Why
in a three pin plug the earth pin is thicker and longer than the other pins?
Resolution:It depends upon R=rho l/a where area(a)
is inversely proportional to resistance (R), so if (a) increases, R decreases
& if R is less the leakage current will take low resistance path so the
earth pin should be thicker. It is longer because the The First to make the
connection and Last to disconnnect should be earth Pin. This assures Safety for
the person who uses the electrical instrument.
Query:Why
series motor cannot be started on no-load?
Resolution:Series motor cannot be started without
load because of high starting torque. Series motor are used in Trains, Crane
etc.
Query:Why
ELCB can't work if N input of ELCB do not connect to ground?
Resolution:ELCB is used to detect earth leakage
fault. Once the phase and neutral are connected in an ELCB, the current will
flow through phase and that much current will have to return neutral so
resultant current is zero. Once there is a ground fault in the load side,
current from phase will directly pass through earth and it will not return
through neutral through ELCB. That means once side current is going and not
returning and hence because of this difference in current ELCB wil trip and it
will safe guard the other circuits from faulty loads. If the neutral is not
grounded, fault current will definitely high and that full fault current will
come back through ELCB, and there will be no difference in current
Q: Why we do 2 types of earthing on
transformer (ie:)body earthing & neutral earthing , what is function. i am
going to install a 5oo kva transformer & 380 kva DG set what should the
earthing value?
A:The two
types of earthing are Familiar as Equipment earthing and system earthing. In
Equipment earthing: body ( non conducting part)of the equipment shouldd be
earthed to safegaurd the human beings.system Earthing : In this neutral of the
supply source ( Transformer or Generator) should be grounded. With this,in case
of unbalanced loading neutral will not be shifted.so that unbalanced voltages
will not arise. We can protect the equipment also. With size of the equipment(
transformer or alternator)and selection of relying system earthing will be
further classified into directly earthed,Impedance earthing, resistive (NGRs)
earthing.
Q:What is the difference between MCB
& MCCB, Where it can be used?
A:MCB is
miniature circuit breaker which is thermal operated and use for short circuit
protection 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.
Q:Where should the lighting arrestor be
placed in distribution lines?
A:Near
distribution transformers and out going feeders of 11kv and incomming feeder of
33kv and near power transformers in sub-stations.
Q:Define IDMT relay?
A:It is an
inverse definite minimum time relay.In IDMT relay its operating is inversely
proportional and also a characteristic of minimum time after which this relay
operates.It is inverse in the sense ,the tripping time will decrease as the
magnitude of fault current increase.
Q:What are the transformer losses?
A:TRANSFORMER
LOSSES - Transformer losses have two sources-copper loss and magnetic loss.
Copper losses are caused by the resistance of the wire (I2R). Magnetic losses
are caused by eddy currents and hysteresis in the core. Copper loss is a
constant after the coil has been wound and therefore a measurable loss.
Hysteresis loss is constant for a particular voltage and current. Eddy-current
loss, however, is different for each frequency passed through the transformer.
Q:What is the difference between
Isolator and Circuit Breaker?
A: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.
Q:what is boucholz relay and the
significance of it in to the transformer?
A:Boucholz
relay is a device which is used for the protection of transformer from its
internal faults, it is a gas based relay. whenever any internal fault occurs in
a transformer, the boucholz relay at once gives a horn for some time, if the
transformer is isolated from the circuit then it stop its sound itself other
wise it trips the circuit by its own tripping mechanism.
Q:What is SF6 Circuit Breaker?
A:SF6 is
Sulpher hexa Flouride gas.. if this gas is used as arc quenching medium in a
Circuitbreaker means SF6 CB.
Q:what is ferrantic effect?
A:Output
voltage is greater than the input voltage or receiving end voltage is greater
than the sending end voltage.
Q: what is meant by insulation voltage in
cables? explain it?
A:It is the
property of a cable by virtue of it can withstand the applied voltage without
rupturing it is known as insulation level of the cable.
Q:How to
calculate capacitor bank value to maintain unity power factor with some suitable
example?
A:KVAR=
KW(TAN(COS(-1)#e)- TAN(COS(-1)#d) )
#e= EXISTING P.F.
#d= DESIRED P.F.
Q:Tell me in
detail about c.t. and p.t. ?(Company:reliance)
A:The term
C.T means current transformer,and the term P.T means potential transformer.In
circuit where measurements of high voltage and high current is involved they
are used there.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.
Q:There are
a Transformer and an induction machine. Those two have the same supply. For
which device the load current will be maximum? And why?
A:The motor
has max load current compare to that of transformer because the motor consumes
real power.. and the transformer is only producing the working flux and its not
consuming.. hence the load current in the transformer is because of core loss
so it is minimum.
Q:what is
power factor? whether it should be high or low? why?
A:Power
factor should be high in order to get smooth operation of the system.Low power
factor means losses will be more.it is the ratio of true power to 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).
:What happens if i connect a capacitor
to a generator load?
A:Connecting
a capacitor across a generator always improves powerfactor,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.
Q:Why the capacitors works on ac only?
A:Generally
capacitor gives infinite resistance to dc components(i.e., block the dc
components). it allows the ac components to pass through.
Q:Explain the working principal of the
circuit breaker?
A:Circuit
Breaker is one which makes or breaks the circuit.It has two contacts namely
fixed contact & moving contact.under normal condition the moving contact
comes in contact with fixed contact thereby forming the closed contact for the
flow of current. During abnormal & faulty conditions(when current exceeds
the rated value) an arc is produced between the fixed & moving contacts
& thereby it forms the open circuitArc is extinguished by the Arc Quenching
media like air, oil, vaccum etc.
Q:How many types of colling system it
transformers?
A:1. ONAN
(oil natural,air natural)
2. ONAF (oil natural,air forced)
3. OFAF (oil forced,air forced)
4. ODWF (oil direct,water forced)
5. OFAN (oil forced,air forced)
Q:What is the function of anti-pumping
in circuit breaker?
A:when
breaker is close at one time by close push button,the anti pumping contactor
prevent re close the breaker by close push button after if it already close.
Q:what is stepper motor.what is its
uses?
A:Stepper
motor is the electrical machine which act upon input pulse applied to it. it is
one type of synchronous motor which runs in steps in either direction instead
of running in complete cycle.so, in automation parts it is used.
Q:What is Automatic Voltage
regulator(AVR)?
A: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.
Q:What is an exciter and how does it work?
A: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.
Q:Difference between a four point
starter and three point starter?
A: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
Q:Why use the VCB at High Transmission System ? Why can't use ACB?
A: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 .
Q:What is the difference between surge arrestor and lightning arrestor?
A: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.
Q:Why syn. generators r used for the
production of electricity?
A:synchronous
machines have capability to work on different power factor(or say
different imaginary power varying the field
emf. Hence syn. generators r used for the production of electricity.
Q:What is the difference between
synchronous generator & asynchronous generator?
A: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.
Q:1 ton is equal to how many watts?
A:1 ton =
12000 BTU/hr and to convert BTU/hr to horsepower,
12,000 * 0.0003929 = 4.715 hp therefore 1
ton = 4.715*.746 = 3.5 KW.
Q:why syn. generators r used for the
production of electricity?
A:synchronous
machines have capability to work on differentpower factor(or say
different imaginary pow varying the field
emf. Hence syn. generators r used for the production of electricity.
Q:Enlist types of dc generator?
A: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
3)compound(which is further classified into
cumulative and differential).
Q:Give two basic speed control scheme of
DC shunt motor?
A:1. By
using flux control method:in this method a rheostat is connected across the
field winding 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 2.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.
Q:what is the principle of motor?
A:Whenever a
current carrying conductor is placed in an magnetic field it produce turning or
twisting movement is called as torque.
Q:what is meant by armature reaction?
A:The effect
of armature flu to main flux is called armature reaction. The armature flux may
support main flux or opposes main flux.
Q:Give two basic speed control scheme of
DC shunt motor?
A:1. By
using flux control method:in this method a rheostat is connected across the
field winding 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 2.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.
Q:what is the difference between
synchronous generator & asynchronous generator?
A: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.
Q:What is the Polarization index value ?
(pi value)and simple definition of polarization index ?
A:Its ratio
between insulation resistance(IR)i.e meggar value
for 10min to insulation resistance for 1
min. It ranges from 5-7 for new motors & normally for motor to be in good
condition it should be Greater than 2.5 .
Q:What will happen when power factor is
leading in distribution of power?
A: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.
Q:whats the one main difference between
UPS & inverter ? And electrical engineering & electronics engineering ?
A: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
12v dc with 7 amp. but inverter is startwith 12v,24,dc to 36v dc and 120amp to
180amp battery with long time backup.
Q:What is 2 phase motor?
A:A two
phase motor is a motor with the the starting winding and the running winding
have a phase split. e.g;ac servo motor.where the auxiliary winding and the control
winding have a phase split of 90 degree.
Q:Advantages of vvvf drives over non
vvvf drives for EOT cranes?
A: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
10.increases motor life
Q:What is the significance of vector
grouping in Power Transformers?
A:Every
power transformer has a vector group listed by its manufacturer. Fundamentally
it tells you the information about how the windings are connected (delta or
wye) and the phace difference betweent the current and voltage. EG. DYN11 means
Delta primary, Wye Secondry and the current is at 11 o clock reffered to the
voltage.
Q: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 ?
A:Its Single
Phase induction motor which mostly squirrel cage rotor and are capacitor start
capacitor run.
Q:Why, when birds sit on transmission
lines or current wires doesn't get shock?
A:Its true that if birds touch the single one line (phase or neutral)
they don't get electrical shock... 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.
Q:what is meant by armature reaction?
A:The effect
of armature flu to main flux is called armature reaction. The armature flux may
support main flux or opposes main flux.
Q:what happen if we give 220 volts dc supply to d bulb r tube light?
A:Bulbs
[devices] for AC are designed to operate such that it offers high impedance to
AC supply. Normally they have low resistance. When DC supply is applied, due to
low resistance, the current through lamp would be so high that it may damage
the bulb element.
Q:Which motor has high Starting Torque and Staring current DC motor,
Induction motor or Synchronous motor?
A:DC Series
motor has high starting torque. We can not start the Induction motor and
Synchronous motors on load, but can not start the DC series motor without load.
Q:what is ACSR cable and where we use it?
A:ACSR means
Aluminium conductor steel reinforced, this conductor is used in transmission
& distribution.
Q:What is vaccum currcuit breaker.define
with cause and where be use it Device?
A: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 11kv.
Q: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.
Q: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.
Q: 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.
Q: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.
Q: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.
Q:what is the principle of motor?
A:Whenever a
current carrying conductor is placed in an magnetic field it produce turning or
twisting movemnt is called as torque.
Q: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.
Q: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.
Q: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).
Q: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.
Q: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.
Q: 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.
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