The Hopkinson test is used for

 Q51. The armature reaction in a DC machine causes

A) Distortion of main flux
B) Demagnetization
C) Cross magnetization
D) All of the above
Ans: D

Q52. The neutral plane of a DC generator is shifted in the direction of
A) Rotation
B) Opposite to rotation
C) Perpendicular to field axis
D) Along the shaft
Ans: A

Q53. In a lap-wound DC armature, the number of parallel paths (A) equals
A) Number of poles (P)
B) 2
C) P/2
D) Depends on turns
Ans: A

Q54. In a wave-wound armature, the number of parallel paths (A) equals
A) 2
B) P
C) 4
D) Depends on winding type
Ans: A

Q55. The direction of rotation of a DC motor is determined by
A) Field polarity
B) Current direction
C) Both A and B
D) None of these
Ans: C

Q56. The main reason for sparking at brushes is
A) Poor commutation
B) Excessive load
C) Weak field
D) All of these
Ans: A

Q57. The commutator converts
A) AC to DC
B) DC to AC
C) DC to DC
D) AC to AC
Ans: A

Q58. Armature resistance drop in a DC machine is usually
A) Very low
B) Very high
C) Moderate
D) Infinite
Ans: A

Q59. The magnetizing ampere-turns per pole are proportional to
A) Armature current
B) Load current
C) Field current
D) Speed
Ans: C

Q60. The emf equation of a DC generator is
A) E = (PΦZN)/60A
B) E = 4.44 f N Φ
C) E = IR
D) E = VIt
Ans: A

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Q61. The armature current in a DC generator is equal to
A) Load current + Field current
B) Load current − Field current
C) Load current
D) Field current
Ans: A

Q62. For the same flux, generated emf is proportional to
A) Speed
B) Current
C) Torque
D) Armature resistance
Ans: A

Q63. If the field of a DC shunt motor opens, its speed will
A) Increase rapidly
B) Decrease
C) Remain constant
D) Reverse
Ans: A

Q64. The back emf of a DC motor is given by
A) Eb = (PΦZN)/60A
B) Eb = V + IaRa
C) Eb = IaRa
D) Eb = Torque × Speed
Ans: A

Q65. The speed control of a DC motor by field flux variation is known as
A) Flux control method
B) Armature control method
C) Voltage control method
D) Rheostatic control
Ans: A

Q66. In a DC shunt generator, if load current increases, terminal voltage
A) Increases
B) Decreases
C) Remains same
D) Oscillates
Ans: B

Q67. The speed of a DC shunt motor is approximately proportional to
A) Eb / Φ
B) Ia × Φ
C) Φ / Eb
D) Eb × Φ
Ans: A

Q68. The ratio of back emf to applied voltage in a DC motor is always
A) < 1
B) > 1
C) = 1
D) Zero
Ans: A

Q69. The condition for maximum efficiency in a DC generator is
A) Variable loss = Constant loss
B) Iron loss = Copper loss
C) Armature Cu loss = Constant losses
D) Mechanical loss = Iron loss
Ans: C

Q70. The load current in a DC series generator is equal to
A) Armature current
B) Field current
C) Sum of both
D) Difference of both
Ans: A

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Q71. Differential compound generators are used where
A) Constant voltage is required
B) Load current fluctuates widely
C) Rising voltage is needed with load
D) Voltage should drop rapidly with load
Ans: D

Q72. The function of equalizer rings in DC machines is
A) Equalize current in parallel paths
B) Increase efficiency
C) Reduce flux leakage
D) Reduce armature reaction
Ans: A

Q73. The condition for sparkless commutation is
A) Current reversal in short-circuited coil at zero emf
B) Reversal at maximum emf
C) Reversal at minimum emf
D) None
Ans: A

Q74. Commutation can be improved by
A) Using interpoles
B) Using compensating windings
C) Reducing brush shift
D) All of the above
Ans: D

Q75. The no-load characteristic of a DC generator gives relation between
A) Field current and emf
B) Speed and torque
C) Current and voltage
D) Voltage and load
Ans: A

Q76. The effect of armature reaction can be neutralized by
A) Compensating winding
B) Interpoles
C) Brush shift
D) All of the above
Ans: D

Q77. The field winding in a DC shunt generator has
A) Large number of turns, small current
B) Few turns, large current
C) Moderate turns, moderate current
D) Depends on machine rating
Ans: A

Q78. A short shunt compound generator means
A) Shunt field connected across armature only
B) Shunt field connected across series field
C) Shunt field connected across both
D) None
Ans: A

Q79. The emf induced in the armature coil of DC generator is
A) Alternating
B) Direct
C) Pulsating
D) Zero
Ans: A

Q80. The brushes are shifted from geometrical neutral axis because of
A) Armature reaction
B) Commutation
C) Both A and B
D) None
Ans: C

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Q81. The commutator segments are insulated by
A) Mica
B) Cotton
C) Paper
D) Rubber
Ans: A

Q82. For a 4-pole, lap-wound armature with 600 conductors, the number of parallel paths is
A) 4
B) 2
C) 8
D) 12
Ans: A

Q83. In a DC generator, if field resistance line does not cut OCC, the generator will
A) Not build up voltage
B) Build up normally
C) Overvolt
D) Reverse polarity
Ans: A

Q84. Critical resistance is defined as
A) Maximum field resistance for voltage build-up
B) Minimum field resistance
C) Ratio of voltage to current
D) None
Ans: A

Q85. The critical speed of a DC generator is the speed
A) At which field resistance equals critical resistance
B) At which flux becomes zero
C) At which current is maximum
D) None
Ans: A

Q86. The speed of a DC motor varies
A) Directly with back emf
B) Inversely with flux
C) Both A and B
D) None
Ans: C

Q87. A DC series motor develops high torque because
A) Field current increases with armature current
B) Field flux constant
C) Armature current small
D) High resistance
Ans: A

Q88. DC series motors are preferred for
A) Traction
B) Fans
C) Lathes
D) Pumps
Ans: A

Q89. The direction of rotation of DC motor can be reversed by reversing
A) Supply to armature
B) Field supply
C) Either A or B
D) Both simultaneously
Ans: C

Q90. In DC machines, commutation is improved by
A) Reducing brush width
B) Using carbon brushes
C) Using interpoles
D) All of the above
Ans: D

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Q91. The brush contact loss is proportional to
A) Brush current
B) Speed
C) Area of contact
D) Pressure
Ans: A

Q92. Armature copper loss is given by
A) Ia²Ra
B) I²R
C) EbIa
D) VIt
Ans: A

Q93. Stray losses in DC machines include
A) Eddy current loss
B) Hysteresis loss
C) Windage and friction loss
D) All of these
Ans: D

Q94. In a DC motor, mechanical losses are nearly
A) Constant
B) Proportional to speed
C) Proportional to current
D) Proportional to flux
Ans: A

Q95. The main component contributing to voltage drop in armature is
A) Armature resistance
B) Brush contact drop
C) Commutator resistance
D) All of these
Ans: D

Q96. Field control of speed in DC motors is limited by
A) Weakening of field
B) Armature reaction
C) Commutation
D) All of these
Ans: D

Q97. Regenerative braking in a DC motor occurs when
A) Back emf > Supply voltage
B) Back emf < Supply voltage
C) Both equal
D) Motor is switched off
Ans: A

Q98. The dynamic braking of a DC motor is done by
A) Connecting resistor across armature
B) Reversing supply
C) Reducing field
D) Shorting armature
Ans: A

Q99. The Hopkinson test is used for
A) Efficiency of DC machines
B) Loss measurement
C) Both A and B
D) None
Ans: C

Q100. Swinburne’s test is applicable to
A) Shunt machines
B) Series machines
C) Compound machines
D) Universal motors
Ans: A