Electrical Engineering 31 Important Interview Questions With Answers PDF

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1.  
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.
2.  
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.
3.  
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.
4.  
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.
5.  
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.
6.  
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.


7.  
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’.
8.  
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.
9.  
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.
10.   
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.
11.   
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.
12.   
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).


13.   
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.
14.   
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.
INDIAN HISTORY-FAMOUS BATTLES
QUIZ OF DAY-16/05/2017 GK & CURRENT AFFAIRS

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