ELECTRICAL Engineering Interview Questions and Answers
1. 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.
2. 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.
3. 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.
4. 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.
5. 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.
6. 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.
7. 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
8. 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.
9. 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.
10. 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.
1. 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.
2. 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.
3. 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.
4. 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).
5. 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.
6. 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.
7. 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.
8. 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
9. 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.
10. 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
1. 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 high-
speed communication lines.So we can understand the importance of power measurement through power quality
meters.
2. 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.
3. 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 .
4. 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.
5. Use of switch mode power converter in real-time basis?
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.
6. 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 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.
7. 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
8. 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.
9. 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.
10. What is excitation?
Excitation is applying an external voltage to DC shunt coil in DC motors.
1. 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)
2. 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.
3. 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.
4. 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.
5. 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.
6. 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.
7. 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.
8. 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.
9. 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.
10. 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.
1. 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
2. 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.
3. 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.
4. 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)
5. 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.
6. 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.
7. 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.
8. 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.
9. 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.
10. 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.
1. 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.
2. What is “pu” in EE?
Pu stands for per unit in power system. (pu = actual value/ base value)
3. 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.
4. 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.
5. 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.
6. 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.
7. 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.
8. 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.
9. 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
10. 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.
1. 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
2. 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.
3. 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.
4. What does quality factor depend on in resonance?
Quality factor q depends on frequency and bandwidth.
5. 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
6. 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.
7. 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’.
8. 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.
9. 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.
10. What is ACSR cable and where we use it?
ACSR means Aluminium conductor steel reinforced, this conductor is used in transmission & distribution.
197. What is the voltage gain or transfer function of amplifier?
Vout/Vin
198. What does the KVAR means?
The KVAR indicates the electrical power. KVAR means “Kilo Volt Amperes with Reactive components”
199. 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 .
200. 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.