76. A shunt generator running at 1000 r.p.m. has generated
e.m.f. as 200 V. If the speed
increases
to 1200 r.p.m., the generated e.m.f. will be nearly
(A) 150 V
(B) 175 V
(C) 240 V
(D) 290 V
Ans: C
77. The purpose of providing dummy coils in a generator is
(A) to reduce eddy current losses
(B) to enhance flux density
(C) to amplify voltage
(D) to provide mechanical balance for the
rotor
Ans: D
78. In a shunt generator the voltage build up is generally
restricted by
(A) speed limitation
(B) armature heating
(C) insulation restrictions
(D) saturation of iron
Ans: D
79. If a D.C. generator fails to build up the probable cause
could not be
(A) imperfect brush contact
(B) field resistance less than the
critical resistance
(C) no residual magnetism in the
generator
(D) faulty shunt connections tending to
reduce the residual magnetism
Ans: B
80. Flashing the field of D.C. generator means
(A) neutralising residual magnetism
(B) creating residual magnetism by a D.C.
source
(C) making the magnetic losses of forces
parallel
(D) increasing flux density by adding
extra turns of windings on poles
Ans: B
81. The e.m.f. induced in the armature of a shunt generator
is 600 V. The armature resistance is 0.1 ohm. If the armature current is 200 A,
the terminal voltage will be
(A) 640 V
(B) 620 V
(C) 600 V
(D) 580 V
Ans: D
82. In a D.C. generator the critical resistance refers to
the resistance of
(A) brushes
(B) field
(C) armature
(D) load
Ans: B
83. To achieve sparkless commutation brushes of a D.C. generator
are rockedm ahead so as to bring them
(A) just ahead of magnetic neutral axis
(B) in magnetic neutral axis
(C) just behind the magnetic neutral axis
Ans: A
84. Armature coil is short circuited by brushes when it lies
(A) along neutral axis
(B) along field axis
(C) in any of the above positions
(D) in none of the above positions
Ans: A
85. A cumulatively compounded long shunt generator when
operating as a motor would be
(A) cumulatively compounded long shunt
(B) differentially compounded long shunt
(C) cumulatively compounded short shunt
(D) differentially compounded short shunt
Ans: B
86. To avoid formation of grooves in the commutator of a
D.C. machine
(A) the brushes of opposite polarity
should track each other
(B) the brushes of same polarity should
track each other
(C) brush position has no effect on the
commutator grooving
(D)
inter-poles are provided
Ans: A
87. The following constitute short-circuit in the armature
winding.
(A) Insulation failure between two
com-mutator bars
(B) Insulation failure between two turns
of a coil
(C) Two of more turns of the same coil
getting grounded
(D) All of the above
Ans: D
88. The rapid wear of brushes takes place due to
(A) abrasion from dust
(B) excessive spring pressure
(C) rough commutator bars
(D) high mica insulation between
com-mutation bars
(E)
all of the above factors
Ans: E
89. Number of tappings for each equilizer ring is equal to
(A) number of pole pairs
(B) number of poles
(C) number of parallel paths
(D) number of commutator segments
Ans: A
90. A D.C. generator can be considered as
(A) rectifier
(B) prime mover
(C) rotating amplifier
(D) power pump
