41. The commutator in a DC generator is made of—
A) Copper segments insulated by mica
B) Aluminum segments insulated by paper
C) Iron segments insulated by varnish
D) Carbon blocks joined together
✅ Answer: A
Explanation: Commutator consists of copper segments separated by thin mica insulation.
42. The function of the armature winding is—
A) To produce magnetic flux
B) To carry the current where emf is induced
C) To hold the shaft
D) To cool the machine
✅ Answer: B
Explanation: The armature winding is where emf is induced and current is carried.
43. The brushes in a DC generator are placed—
A) Along the neutral axis
B) Along the main field axis
C) Between pole shoes
D) At random
✅ Answer: A
Explanation: Brushes are located on the magnetic neutral axis (MNA) to minimize sparking.
44. The load characteristic of a DC series generator is—
A) Rising initially then dropping
B) Constant
C) Linear
D) Drooping
✅ Answer: A
Explanation: Voltage first rises due to field strengthening, then drops due to saturation and losses.
45. The voltage regulation of a DC generator is—
A) Always positive
B) Always negative
C) Zero
D) Can be positive or negative
✅ Answer: A
Explanation: Terminal voltage decreases with load; hence regulation is positive.
46. The main function of pole shoe is—
A) To support the pole core
B) To spread magnetic flux uniformly
C) To increase speed
D) To collect current
✅ Answer: B
Explanation: Pole shoes spread the magnetic flux uniformly and reduce reluctance.
47. For a given flux, if the speed of a DC generator is doubled, the generated emf—
A) Halves
B) Doubles
C) Becomes four times
D) Remains same
✅ Answer: B
Explanation: ; if speed doubles, emf doubles.
48. When the field current increases, generated emf—
A) Decreases
B) Increases
C) Remains constant
D) Becomes zero
✅ Answer: B
Explanation: More field current → more flux → higher emf.
49. In a shunt generator, when load increases, terminal voltage—
A) Increases
B) Decreases
C) Remains same
D) First increases then decreases
✅ Answer: B
Explanation: Increased load causes more voltage drop in armature and field weakening.
50. The copper losses in a DC generator vary as—
A) Constant
B) ∝
C) ∝ I
D) ∝ speed
✅ Answer: B
Explanation: Copper loss , depends on current squared.
51. The iron losses in a DC generator are—
A) Constant for all loads
B) Proportional to current
C) Proportional to load
D) Negligible
✅ Answer: A
Explanation: Iron losses depend on flux and speed, nearly constant for a given speed.
52. Mechanical losses include—
A) Bearing friction and windage losses
B) Hysteresis losses
C) Copper losses
D) Eddy current losses
✅ Answer: A
Explanation: Friction and windage constitute mechanical losses.
53. The overall efficiency of a DC generator is—
A) Electrical output / Mechanical input
B) Mechanical output / Electrical input
C) Input / Output
D) None
✅ Answer: A
Explanation:
54. If field resistance is greater than critical resistance—
A) Generator will not excite
B) Generator will excite faster
C) Terminal voltage will increase
D) Armature current will increase
✅ Answer: A
Explanation: Self-excitation fails if field resistance > critical resistance.
55. Critical speed of a DC generator is—
A) Minimum speed for self-excitation
B) Maximum safe speed
C) Speed at which copper loss = iron loss
D) Rated speed
✅ Answer: A
Explanation: Critical speed is the minimum speed at which emf just builds up.
56. The main cause of failure to build up voltage in a DC generator is—
A) Open field circuit
B) Wrong field connection
C) Loss of residual magnetism
D) Any of these
✅ Answer: D
Explanation: All above conditions can prevent voltage build-up.
57. Residual magnetism in a generator can be restored by—
A) Flashing the field with a DC supply
B) Running at higher speed
C) Reducing load
D) Shorting field winding
✅ Answer: A
Explanation: Residual magnetism is restored by field flashing.
58. The direction of rotation of a DC generator is reversed. The voltage polarity will—
A) Reverse
B) Remain same
C) Become zero
D) Fluctuate
✅ Answer: A
Explanation: Both rotation and field direction affect emf polarity (Fleming’s rule).
59. The maximum efficiency occurs when—
A) Variable losses = constant losses
B) Armature current is maximum
C) Speed is maximum
D) Field current is minimum
✅ Answer: A
Explanation: For max efficiency, variable (Cu) losses equal constant (core + mech) losses.
60. The armature reaction reduces—
A) Terminal voltage
B) Generated emf
C) Flux per pole
D) All of these
✅ Answer: D
Explanation: Armature reaction weakens flux and causes voltage drop.
61. The brushes are made of carbon because—
A) Low friction, good conductivity
B) High friction
C) Light weight only
D) Insulating property
✅ Answer: A
Explanation: Carbon provides low friction and self-lubrication.
62. In a DC generator, the direction of induced emf is given by—
A) Fleming’s right-hand rule
B) Fleming’s left-hand rule
C) Lenz’s law
D) Ampere’s rule
✅ Answer: A
Explanation: Right-hand rule gives direction of emf.
63. Commutation means—
A) Current reversal in armature coil
B) Voltage reversal
C) Load change
D) Flux change
✅ Answer: A
Explanation: Commutation is the process of current reversal in armature coil.
64. The sparkless commutation can be achieved by—
A) Interpoles
B) Compensating winding
C) Both A and B
D) None
✅ Answer: C
Explanation: Interpoles and compensating windings reduce sparking.
65. The no-load characteristic is also known as—
A) Magnetization characteristic
B) Load characteristic
C) Efficiency curve
D) Voltage regulation curve
✅ Answer: A
Explanation: OCC = Magnetization curve = No-load characteristic.
66. The shape of OCC curve is—
A) Initially linear then flat
B) Hyperbolic
C) Straight line
D) Constant
✅ Answer: A
Explanation: Linear at first, then flattens due to saturation.
67. In a shunt generator, the field winding has—
A) Many turns of thin wire
B) Few turns of thick wire
C) Thick conductors
D) None
✅ Answer: A
Explanation: Shunt field: many turns, high resistance, small current.
68. In a series generator, the field winding has—
A) Few turns of thick wire
B) Many turns of thin wire
C) High resistance
D) None
✅ Answer: A
Explanation: Series field: few turns, low resistance, high current.
69. Compensating winding is placed—
A) In pole shoes
B) On yoke
C) On armature
D) On shaft
✅ Answer: A
Explanation: Compensating windings are embedded in pole faces to oppose armature reaction.
70. Voltage build-up in a shunt generator fails if—
A) Residual magnetism is lost
B) Field connection reversed
C) Field resistance too high
D) All of the above
✅ Answer: D
Explanation: All these prevent self-excitation.
71. The load test on a DC generator is used to determine—
A) Efficiency and regulation
B) Commutation
C) Mechanical losses only
D) No-load losses
✅ Answer: A
Explanation: Load test determines efficiency and voltage regulation.
72. The back emf in a generator is—
A) In opposition to rotation
B) In phase with supply
C) Independent of flux
D) Zero
✅ Answer: A
Explanation: The induced emf always opposes the cause producing it.
73. The terminal voltage can be increased by—
A) Increasing field current
B) Increasing load
C) Decreasing speed
D) Decreasing field flux
✅ Answer: A
Explanation: More field current → more flux → higher emf.
74. In a compound generator, both field windings are—
A) Connected in series
B) One in series, one in parallel
C) Both in parallel
D) On the same circuit
✅ Answer: B
Explanation: Compound = shunt (parallel) + series (series) fields.
75. A DC compound generator used for lighting load is—
A) Cumulatively compounded
B) Differentially compounded
C) Series type
D) Shunt type
✅ Answer: A
Explanation: Cumulative compounding keeps voltage nearly constant.
76. The brushes should make contact—
A) At the neutral plane
B) At the pole axis
C) At 90° to rotation
D) Anywhere
✅ Answer: A
Explanation: Neutral plane minimizes sparking.
77. Eddy current losses can be reduced by—
A) Laminating the core
B) Increasing flux density
C) Using thick iron sheets
D) Using copper core
✅ Answer: A
Explanation: Laminations restrict eddy current paths.
78. Hysteresis loss per unit volume =
A) η × Bmax^1.6 × f
B) k × Bmax^1.6 × f
C) Bmax × f
D) 2B × f
✅ Answer: B
Explanation: Steinmetz’s law:
79. A DC generator can be run as—
A) DC motor
B) AC motor
C) Induction motor
D) Stepper motor
✅ Answer: A
Explanation: DC machine is reversible; generator ↔ motor.
80. The armature reaction in a generator causes flux—
A) To shift in direction of rotation
B) To shift opposite to rotation
C) To increase
D) To decrease
✅ Answer: A
Explanation: Flux shifts in direction of rotation due to armature reaction
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