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.
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.
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.
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.
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.
Why back emf used for a dc motor? highlight its significance.

2. 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.
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’.
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 statorrevolving 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.

3. 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.
5. 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

4. 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 polepair; 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.

5. 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.

6. 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.

7. 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.

8. 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?