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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.
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.
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