# Electrical Eng 4th sem . Question Bank VENUS
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Q1.What is reactance voltage?Explain how interpoles
help in reducing effect of armature reaction?
Ans.Reactance Voltage:The voltage rise in the short cir-cuit coil due to inductive property
of thecoil, which
opposes the current reversal in it during the commuta-tion period, is called the reactance voltage.
Interpoles:Interpoles are similar to the main field poles
and located on the yoke between the main field
poles.They have winding in series with the armature
winding.
Interpoles have the function of reducing the armature
reaction effect in the commutating zone. They elimi-
nate the need to shift the brush assembly.
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Q2.Explain with neat sketches the speed-armature cur-
rent characteristics of D.C.shunt motor.
Ans.Refers to Chapter 2
Q3.Write the uses of:
(a) Potential transformer
(b) Welding transformer
Ans.(a) Potential transformer
(i) It is used for a metering purpose.
(ii) For the protection of the feeders.
(iii)For protecting the impedance of the generators.
(iv) For synchronising the generators and feeders.
Ans.(b)Welding transformer:Welding Transformers are
used in AC machines to change alternating current from
the power line into a low-voltage, high amperage cur-
rent in the secondary winding.
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Q3.Explain the advantages of using graphite brushes in-
stead of copper brushes in D.C. machines?
Ans.The advantages of using graphite brushes instead
of copper brushes in D.C. machines are :
1. High Melting Point:The carbon metal is generally hav-
ing high melting point compared to copper as well as
other metal. The carbon brush is having almost 3550
degree C melting temperature and copper having 1080
degree C only. We all know the brushes in dc motor
linked to nearly to the commutator. So whenever motor
running it will create the spark and friction amid brushes
and commutator. If temperature is increase then copper
brush cannot with stable that’s why we are using these
brushes.
2. Soft metal:The carbon brush is usually very soft metal
compared to copper. So whenever the motor running
the friction produced amid brush and commutator. As
per the softness of the brush will be not damage the
commutator. Incase if we are using copper brush it will
spoil the commutator because of its hardness.
3. Negative temperature co-efficient:The carbon brush
is having negative temperature co-efficient. It means
whenever the temperature increases then resistance re-
peatedly reduced. We know motor running time fric-
tion and heat produced and it will increase the tempera-
ture of the motor. So, carbon brush reduced resistance
due to mounting temperature and current surge will in-
creases.
4. Shape adopting:The carbon brush usually coming with
rectangular shape but it’s having elasticity nature. If we
will fix the new the brush near to the commutator as
well as once the motor starts to running its automati-
cally adopt the circular shape of commutator.
5. Self lubricating:The carbon brush as well as Evolution
Brushes is having self lubricating. So it’s not required
any other Greece or lubricating oil for cooling purpose.
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Q4.Write the applications of:
(a) D.C. series motor
(b) D.C.shunt motor
(c) D.C. differentially compounded motor
Ans.(a) D.C. series motor:These motors are very useful for
the applications requiring high starting torque like trac-
tion, lifts,hoists, cranes.They are also commonly used
for conveyors and in rolling mills.
Ans.(b) D.C.shunt motor:Being constant-speed motors,
they are commonly employed for the following appli-
cations:
Lathes, driling machines,grinders, shapers, planers,
milling machines, metal cutting machines, wood work-
ing machines, fans, blowers, compressors, conveyors,
vaccum cleaners, centrifugal and reciprocating pumps,
etc.
Ans.(c) D.C. differentially compound motor:Cumulative
compound motors are suitable for applications requir-
ing a high starting torque and particularly under fluctu-
ating load conditions.They have the advantage over the
series motors of a stable no load speed.Some of their
applications are:
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Q5.What will happen if D.C. series motor run on No-
load?Explain armature control method of speed con-
trol of D.C. series motor.
Ans. If the DC Series Motor is started at No Load then the
motor will speed up to its no-load speed, which may
damage the motor. At no-load, the load is actually what-
ever friction is in the motor such as from bearings,
brushes, and air resistance. This friction mostly heats
the motor. The motor will speed up to its no-load speed,
which may damage the motor.
Armature control method of speed control of D.C.
series motor
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Q6.Describe the advantage and disadvantages of AC and
DC transmission system.
Ans.The advantages of D.C transmission are:
(i) Only two conductors are required as compared to A.C
system which requires three conductors.
(ii) Inductances, capacitance, surge problems are not
present.
(iii)Economical use of underground cables is possible.
The disadvantages of D.C transmission are:
(i) D.C power cannot be generated at high voltage due to
commutation problems.
(ii) Steeping up and down D.C voltage is a complicated
process.
(iii)Cost is high.
The advantages of A.C transmission are:
(i) High voltage can be generated upto 33kV compared to
D.C system(11kv).
(ii) Voltage level can be easily stepped up/down depending
upon the requirement which is difficult with D.C.
(iii)A.C substations are more efficient than D.C substations.
(iv) Maintenance of A.C system is easier and cheaper
compared to D.C.
The disadvantages are:
(i) Inductance, capacitance effects are present which affect
the performance of line.
(ii) Volume of conductor material required is more than
D.C.
(iii)Skin effect increases line resistance.
(iv) Carona effect is seviour than D.C
(v) If cables are used, sheath than D.C.
(vi) Construction of transmission system is complicated.
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