Most important interview answer and quistion for EEE Part-1

140. What is ACSR cable and where we use it?
ACSR means Aluminium conductor steel reinforced,
this conductor is used in transmission & distribution.
141. Why star delta starter is preferred with
induction motor?
Star delta starter is preferred with induction motor
due to following reasons:
• Starting current is reduced 3-4 times of the direct
current due to which voltage drops and hence it
causes less losses.
• Star delta starter circuit comes in circuit first
during starting of motor, which reduces voltage 3
times, that is why current also reduces up to 3 times
and hence less motor burning is caused.
• In addition, starting torque is increased and it
prevents the damage of motor winding.
142. State the difference between generator and
alternator
Generator and alternator are two devices, which
converts mechanical energy into electrical energy.
Both have the same principle of electromagnetic
induction, the only difference is that their
construction. Generator persists stationary magnetic
field and rotating conductor which rolls on the
armature with slip rings and brushes riding against
each other, hence it converts the induced emf into dc
current for external load whereas an alternator has
a stationary armature and rotating magnetic field for
high voltages but for low voltage output rotating
armature and stationary magnetic field is used.
143. Why AC systems are preferred over DC
systems?
Due to following reasons, AC systems are preferred
over DC systems:
a. It is easy to maintain and change the voltage of AC
electricity for transmission and distribution.
b. Plant cost for AC transmission (circuit breakers,
transformers etc) is much lower than the equivalent
DC transmission
c. From power stations, AC is produced so it is better
to use AC then DC instead of converting it.
d. When a large fault occurs in a network, it is easier
to interrupt in an AC system, as the sine wave
current will naturally tend to zero at some point
making the current easier to interrupt.
144. How can you relate power engineering with
electrical engineering?
Power engineering is a sub division of electrical
engineering. It deals with generation, transmission
and distribution of energy in electrical form. Design
of all power equipments also comes under power
engineering. Power engineers may work on the
design and maintenance of the power grid i.e. called
on grid systems and they might work on off grid
systems that are not connected to the system.
145. What are the various kind of cables used for
transmission?
Cables, which are used for transmitting power, can
be categorized in three forms:
• Low-tension cables, which can transmit voltage
upto 1000 volts.
• High-tension cables can transmit voltage upto
23000 volts.
• Super tension cables can transmit voltage 66 kV to
132 kV.
146. Why back emf used for a dc motor? highlight
its significance.
The induced emf developed when the rotating
conductors of the armature between the poles of
magnet, in a DC motor, cut the magnetic flux,
opposes the current flowing through the conductor,
when the armature rotates, is called back emf. Its
value depends upon the speed of rotation of the
armature conductors. In starting, the value of back
emf is zero.
147. What is slip in an induction motor?
Slip can be defined as the difference between the
flux speed (Ns) and the rotor speed (N). Speed of the
rotor of an induction motor is always less than its
synchronous speed. It is usually expressed as a
percentage of synchronous speed (Ns) and
represented by the symbol ‘S’.
148. Explain the application of storage batteries.
Storage batteries are used for various purposes,
some of the applications are mentioned below:
• For the operation of protective devices and for
emergency lighting at generating stations and
substations.
• For starting, ignition and lighting of automobiles,
aircrafts etc.
• For lighting on steam and diesel railways trains.
• As a supply power source in telephone exchange,
laboratories and broad casting stations.
• For emergency lighting at hospitals, banks, rural
areas where electricity supplies are not possible.
149. Explain advantages of storage batteries
Few advantages of storage batteries are mentioned
below:
• Most efficient form of storing energy portably.
• Stored energy is available immediately because
there is no lag of time for delivering the stored
energy.
• Reliable source for supply of energy.
• The energy can be drawn at a fairly constant rate.
160. What are the different methods for the
starting of a synchronous motor.
Starting methods: Synchronous motor can be started
by the following two methods:
• By means of an auxiliary motor: The rotor of a
synchronous motor is rotated by auxiliary motor.
Then rotor poles are excited due to which the rotor
field is locked with the stator-revolving field and
continuous rotation is obtained.
• By providing damper winding: Here, bar
conductors are embedded in the outer periphery of
the rotor poles and are short-circuited with the
short-circuiting rings at both sides. The machine is
started as a squirrel cage induction motor first.
When it picks up speed, excitation is given to the
rotor and the rotor starts rotating continuously as
the rotor field is locked with stator revolving field.
161. Name the types of motors used in vacuum
cleaners, phonographic appliances, vending
machines,refrigerators, rolling mills, lathes,
power factor improvement and cranes.
Following motors are used: –
• Vacuum cleaners- Universal motor.
• Phonographic appliances – Hysteresis motor.
• Vending machines – Shaded pole motor.
• Refrigerators – Capacitor split phase motors.
• Rolling mills – Cumulative motors.
• Lathes – DC shunt motors.
• Power factor improvement – Synchronous motors.
162. State Thevenin’s Theorem:
According to thevenin’s theorem, the current flowing
through a load resistance Connected across any two
terminals of a linear active bilateral network is the
ratio open circuit voltage (i.e. the voltage across the
two terminals when RL is removed) and sum of load
resistance and internal resistance of the network. It
is given by Voc / (Ri + RL).
163. State Norton’s Theorem
The Norton’s theorem explains the fact that there are
two terminals and they are as follows:
• One is terminal active network containing voltage
sources
• Another is the resistance that is viewed from the
output terminals. The output terminals are
equivalent to the constant source of current and it
allows giving the parallel resistance.
The Norton’s theorem also explains about the
constant current that is equal to the current of the
short circuit placed across the terminals. The parallel
resistance of the network can be viewed from the
open circuit terminals when all the voltage and
current sources are removed and replaced by the
internal resistance.
164. State Maximum power transfer theorem
The Maximum power transfer theorem explains
about the load that a resistance will extract from the
network.
This includes the maximum power from the network
and in this case the load resistance is being is equal
to the resistance of the network and it also allows the
resistance to be equal to the resistance of the
network. This resistance can be viewed by the output
terminals and the energy sources can be removed by
leaving the internal resistance behind.
165. Explain different losses in a transformer.
There are two types of losses occurring in
transformer:
• Constant losses or Iron losses: The losses that occur
in the core are known as core losses or iron losses.
Two types of iron losses are:
eddy current loss
Hysteresis loss.
These losses depend upon the supply voltage,
frequency, core material and its construction. As
long as supply voltage and frequency is constant,
these losses remain the same whether the
transformer is loaded or not. These are also known
as constant losses.
• Variable losses or copper losses: when the
transformer is loaded, current flows in primary and
secondary windings, there is loss of electrical energy
due to the resistance of the primary winding, and
secondary winding and they are called variable
losses. These losses depend upon the loading
conditions of the transformers.
Therefore, these losses are also called as variable
losses.
176. Explain different types of D.C motors? Give
their applications
Different type of DC motors and their applications
are as follows:-
• Shunt motors: It has a constant speed though its
starting torque is not very high. Therefore, it is
suitable for constant speed drive, where high
starting torque is not required such as pumps,
blowers, fan, lathe machines, tools, belt or chain
conveyor etc.
• Service motors: It has high starting torque & its
speed is inversely proportional to the loading
conditions i.e. when lightly loaded, the speed is high
and when heavily loaded, it is low. Therefore, motor
is used in lifts, cranes, traction work, coal loader and
coal cutter in coalmines etc.
• Compound motors: It also has high starting torque
and variable speed. Its advantage is, it can run at NIL
loads without any danger. This motor will therefore
find its application in loads having high inertia load
or requiring high intermittent torque such as
elevators, conveyor, rolling mill, planes, presses,
shears and punches, coal cutter and winding
machines etc.
177. Explain the process of commutation in a dc
machine. Explain what are inter-poles and why
they are required in a dc machine.
Commutation: It is phenomenon when an armature
coil moves under the influence of one pole- pair; it
carries constant current in one direction. As the coil
moves into the influence of the next pole- pair, the
current in it must reverse. This reversal of current in
a coil is called commutation. Several coils undergo
commutation simultaneously. The reversal of
current is opposed by the static coil emf and
therefore must be aided in some
fashion for smooth current reversal, which
otherwise would result in sparking at the brushes.
The aiding emf is dynamically induced into the coils
undergoing commutation by means of compoles or
interpoles, which are series excited by the armature
current. These are located in the interpolar region of
the main poles and therefore influence the armature
coils only when these undergo commutation.
178. Comment on the working principle of
operation of a single-phase transformer.
Working principle of operation of a single-phase
transformer can be explained as An AC supply passes
through the primary winding, a current will start
flowing in the primary winding. As a result, the flux
is set. This flux is linked with primary and secondary
windings. Hence, voltage is induced in both the
windings. Now, when the load is connected to the
secondary side, the current will start flowing in the
load in the secondary winding, resulting in the flow
of additional current in the secondary winding.
Hence, according to Faraday’s laws of
electromagnetic induction, emf will be induced in
both the windings. The voltage induced in the
primary winding is due to its self inductance and
known as self induced emf and
according to Lenze’s law it will oppose the cause i.e.
supply voltage hence called as back emf. The voltage
induced in secondary coil is known as mutually
induced voltage. Hence, transformer works on the
principle of electromagnetic induction.
179. Define the following terms:-
• Reliability,
• Maximum demand,
• Reserve-generating capacity,
• Availability (operational).
Reliability: It is the capacity of the power system to
serve all power demands without failure over long
periods.
Maximum Demand: It is maximum load demand
required in a power station during a given period.
Reserve generating capacity: Extra generation
capacity installed to meet the need of scheduled
downtimes for preventive maintenance is called
reserve-generating capacity.
Availability: As the percentage of the time a unit is
available to produce power whether needed by the
system or not.
180. Mention the disadvantages of low power
factor? How can it be improved?
Disadvantages of low power factor:
• Line losses are 1.57 times unity power factor.
• Larger generators and transformers are required.
• Low lagging power factor causes a large voltage
drop, hence extra regulation equipment is required
to keep voltage drop within prescribed limits.
• Greater conductor size: To transmit or distribute a
fixed amount of power at fixed voltage, the
conductors will have to carry more current at low
power factor. This requires a large conductor size.
181. State the methods of improving power
factor?
Methods of improving power factor:
• By connecting static capacitors in parallel with the
load operating at lagging power factor.
• A synchronous motor takes a leading current when
over excited and therefore behaves like a capacitor.
• By using phase advancers to improve the power
factor of induction motors. It provides exciting
ampere turns to the rotor circuit of the motor. By
providing more ampere-turns than required, the
induction motor can be made to operate on leading
power factor like an overexcited synchronous motor.
182. State the factors, for the choice of electrical
system for an aero turbine.
The choice of electrical system for an aero turbine is
guided by three factors:
• Type of electrical output: dc, variable- frequency ac,
and constant- frequency ac.
• Aero turbine rotational speed: constant speed with
variable blade pitch, nearly constant speed with
simpler pitch- changing mechanism or variable
speed with fixed pitch blades.
• Utilization of electrical energy output: in
conjunction with battery or other form of storage, or
interconnection with power grid.
183. What are the advantages of VSCF wind
electrical system?
Advantages of VSCF wind electrical system are:
• No complex pitch changing mechanism is needed.
• Aero turbine always operates at maximum
efficiency point.
• Extra energy in the high wind speed region of the
speed – duration curve can be extracted
• Significant reduction in aerodynamic stresses,
which are associated with constant – speed
operation.
184. Explain the terms real power, apparent
power and reactive power for ac circuits and also
the units used.
• Real Power: It is the product of voltage, current and
power factor i.e. P = V I cos j and basic unit of real
power is watt. i.e. Expressed as W or kW.
• Apparent power: It is the product of voltage and
current. Apparent power = V I and basic unit of
apparent power is volt- ampere. Expressed as VA or
KVA.
• Reactive Power: It is the product of voltage, current
and sine of angle between the voltage and current
i.e.
Reactive power = voltage X current X sinj or Reactive
power = V I sin j and has no other unit but expressed
in VAR or KVAR.
185. Define the following: Average demand,
Maximum demand, Demand factor, Load factor.
• Average Demand: the average power requirement
during some specified period of time of considerable
duration is called the average demand of installation.
• Maximum Demand: The maximum demand of an
installation is defined as the greatest of all the
demand, which have occurred during a given period.
It is measured accordingly to specifications, over a
prescribed time interval during a certain period.
• Demand Factor: It is defined as the ratio of actual
maximum demand made by the load to the rating of
the connected load.
• Load Factor: It is defined as the ratio of the average
power to the maximum demand.
186. Explain forward resistance, static resistance
and dynamic resistance of a pn junction diode.
• Forward Resistance: Resistance offered in a diode
circuit, when it is forward biased, is called
forwardresistance.
• DC or Static Resistance: DC resistance can be
explained as the ratio of the dc-voltage across the
diode to the direct current flowing through it.
• AC or Dynamic Resistance: It can be defined as the
reciprocal of the slope of the forward characteristic
of the diode. It is the resistance offered by a diode to
the changing forward current.
187. How does Zener phenomenon differ from
Avalanche breakdown?
The phenomenon when the depletion region
expands and the potential barrier increases leading
to a very high electric field across the junction, due
to which suddenly the reverse current increases
under a very high reverse voltage is called Zener
effect. Zener-breakdown or Avalanche breakdown
may occur independently or both of these may occur
simultaneously. Diode junctions that breakdown
below 5v are caused by Zener Effect.
Junctions that experience breakdown above 5v are
caused by avalanche-effect. The Zener-breakdown
occurs in heavily doped junctions, which produce
narrow depletion layers. The avalanche breakdown
occurs in lightly doped junctions, which produce
wide depletion layers.
191. Compare JFET’s and MOSFET’s.
Comparison of JFET’s and MOSFET’s:
• JFET’s can only be operated in the depletion mode
whereas MOSFET’s can be operated in either
depletion or in enhancement mode. In a JFET, if the
gate is forward-biased, excess-carrier injunction
occurs and the gatecurrent is substantial.
• MOSFET’s have input impedance much higher than
that of JFET’s. Thus is due to negligible small leakage
current.
• JFET’s have characteristic curves more flat than
that of MOSFET is indicating a higher drain
resistance.
• When JFET is operated with a reverse-bias on the
junction, the gate-current IG is larger than it would
be in a comparable MOSFET.
192. Explain thin film resistors and wire-wound
resistors
a. Thin film resistors- It is constructed as a thin film
of resistive material is deposited on an insulating
substrate.
Desired results are obtained by either trimming the
layer thickness or by cutting helical grooves of
suitable pitch along its length. During this process,
the value of the resistance is monitored closely and
cutting of grooves is stopped as soon as the desired
value of resistance is obtained.
b. Wire wound resistors – length of wire wound
around an insulating cylindrical core are known as
wire wound resistors. These wires are made of
materials such as Constantan and Manganin because
of their high resistivity, and low temperature
coefficients. The complete wire wound resistor is
coated with an insulating material such as baked
enamel
193. What is a differential amplifier? Also,
explain CMRR.
Differential Amplifier: The amplifier, which is used
to amplify the voltage difference between two inputlines
neither of which is grounded, is called
differential amplifier. This reduces the amount of
noise injected into the amplifier, because any noise
appearing simultaneously on both the inputterminals
as the amplifying circuitry rejects it being
a common mode signal.
CMRR: It can be defined as the ratio of differential
voltage-gain to common made voltage gain. If a
differential amplifier is perfect, CMRR would be
infinite because in that case common mode voltage
gain would be zero.
196. What is the difference between electronic
regulator and ordinary electrical rheostat
regulator for fans?
The difference between the electronic and ordinary
electrical regulator is that in electronic regulator
power losses are less because as we decrease the
speed the electronic regulator gives the power
needed for that particular speed but in case of
ordinary rheostat type regulator, the power wastage
is same for every speed and no power is saved.In
electronic regulator, triac is employed for speed
control by varying the firing angle speed and it is
controlled but in rheostatic ,control resistance is
decreased by steps to achieve speed control.
197. What is the voltage gain or transfer function
of amplifier?
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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.