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.
MAIN BUS TO Breaker INCOMING connection arrangement
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).
CT back end & Outgoing cable connection from CTs:
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.
GAP for Cable Laying:
15. 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:
o eddy current loss
o 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.
16. 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.
17. 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.
18. 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.
CT back end & Out going cable connection from CTs:
19. 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.
20. 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.
21. 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.
22. 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.
23. 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.
24. 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.
25. 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.
26. 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 forward-resistance.
• 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.
27. 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.
28. 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 gate-current 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.
30. 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
31. 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 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 would be
infinite because in that case common mode voltage gain would be zero.
SOME MORE QUESTIONS TO BE ANSWERED FOR BETTER ELECTRICAL KNOWLEDGE
1. How grounding is different than earthings?
2. What is diversity factor in electric installations?
3. Explain Marx circuit.
4. Why human body feel electric shock?
5. What is the principal of motor?
6. What is power factor?
7. Should power factor be low or high?
8. Difference between field rheostat and armature rheostat.
9. Why field rheostat is kept in minimum position?
10. Why armature rheostat is kept in maximum position?
11. What is meant by derating factor?
12. What is stiffness factor?
13. What is the dependency of stiffness on load angle??
14. What is 100% protection of generator? Why 100% is not used but 95% is
generally used?
15. What is the difference between a Verilog task and Verilog function?
16. What is the unit of magnetic flux density?
17. Why increase in current leads to increase in conductor temperature?
18. How can a equal potential zone be carried out in conductors?
19. What is essential to prove safe isolation of electrical circuit?
20. What is the ratio of true power to apparent power in an AC circuit?
21. What is power relay?
22. Differentiate between power relay and reverse power relay.
23. What is the suitable transmission voltage?
24. What is the maximum operating temperature for a thermoplastic insulated
cable?
25. Explain the working of variable frequency transformer?
26. Which oil can be used in transformer?
27. What is excitation in case of DC motor?
28. Why is the starting current high in DC motor?
29. What is rotary phase converter?
30. Differentiate between digital phase converter and ordinary phase converter.
31. What is knee point voltage?
32. List the advantages of star-delta starter with induction motor?
33. Which type of transformer is used for lighting loads?
34. What is star-delta transformer?
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