51. The function of field winding in a synchronous generator is to
A) Produce the main magnetic field
B) Produce armature reaction
C) Carry load current
D) Produce voltage in stator
Answer: A
Explanation:
The field winding on the rotor produces the main magnetic field necessary for voltage generation in the stator.
52. The stator of a 3-phase alternator is usually connected in
A) Star only
B) Delta only
C) Star or Delta
D) Open-delta
Answer: C
Explanation:
The stator can be connected in either star or delta depending on system requirements — star for high voltage, delta for low voltage.
53. For the same power, a 2-pole alternator will run
A) Faster than a 4-pole alternator
B) Slower than a 4-pole alternator
C) At the same speed
D) At variable speed
Answer: A
Explanation:
Speed . Fewer poles → higher speed for the same frequency.
54. The armature of a synchronous generator carries
A) DC current
B) AC current
C) Both AC and DC currents
D) No current
Answer: B
Explanation:
The armature (stator) carries the load current, which is alternating in nature.
55. The field winding of a synchronous generator carries
A) AC
B) DC
C) Both AC and DC
D) No current
Answer: B
Explanation:
DC excitation is supplied to the rotor field winding to produce constant magnetic poles.
56. If excitation of a synchronous generator is increased, the power factor will become
A) More lagging
B) More leading
C) Unity
D) Zero
Answer: B
Explanation:
Over-excitation makes the alternator supply leading reactive power → leading PF.
57. If excitation decreases, alternator supplies
A) Lagging reactive power
B) Leading reactive power
C) No reactive power
D) Real power only
Answer: A
Explanation:
Under-excitation causes the alternator to absorb lagging reactive power (behaves like an inductive load).
58. Armature reaction at unity power factor is
A) Purely cross magnetizing
B) Purely demagnetizing
C) Purely magnetizing
D) Zero
Answer: A
Explanation:
At unity PF, current is in phase with voltage, producing a cross-magnetizing effect.
59. In alternators, short pitch coils are used to
A) Reduce harmonics
B) Increase flux
C) Increase voltage
D) Reduce efficiency
Answer: A
Explanation:
Short pitching reduces harmonic content and copper requirement.
60. A salient pole rotor is preferred for
A) High-speed machines
B) Low-speed machines
C) Medium-speed machines
D) All speeds
Answer: B
Explanation:
Salient pole rotors are used for low-speed (hydro) alternators because of their large diameters and many poles.
61. Cylindrical rotors are used for
A) High-speed alternators
B) Low-speed alternators
C) Variable speed alternators
D) DC machines
Answer: A
Explanation:
Turbo-alternators (steam turbines) operate at high speed and use cylindrical rotors for mechanical strength.
62. The leakage reactance of an alternator depends on
A) Slot design and winding distribution
B) Power factor
C) Frequency only
D) Speed only
Answer: A
Explanation:
Leakage reactance depends on slot geometry, coil span, and winding distribution.
63. In a three-phase alternator, the phase difference between induced EMFs in any two phases is
A) 60°
B) 90°
C) 120°
D) 180°
Answer: C
Explanation:
The three-phase windings are displaced by 120° electrical to produce 120° phase displacement in EMFs.
64. The pitch factor (Kp) for a full-pitched coil is
A) 0
B) 0.5
C) 1
D) 2
Answer: C
Explanation:
For a full-pitch coil, induced voltages in coil sides are in phase → Kp = 1.
65. When a synchronous generator supplies a purely resistive load, the armature current is
A) In phase with voltage
B) Lagging voltage by 90°
C) Leading voltage by 90°
D) Zero
Answer: A
Explanation:
For a resistive load, voltage and current are in phase.
66. In a synchronous generator, the generated EMF waveform is
A) Sinusoidal
B) Trapezoidal
C) Rectangular
D) Sawtooth
Answer: A
Explanation:
The sinusoidal distribution of air-gap flux produces a sinusoidal induced EMF.
67. The regulation of an alternator supplying leading power factor load is
A) Positive
B) Negative
C) Zero
D) Infinity
Answer: B
Explanation:
Leading load → internal EMF less than terminal voltage → negative regulation.
68. The main disadvantage of using a high synchronous reactance is
A) Poor voltage regulation
B) High efficiency
C) High short circuit ratio
D) Reduced armature reaction
Answer: A
Explanation:
High reactance causes large voltage drops, degrading voltage regulation.
69. The short circuit ratio (SCR) of an alternator is
A) Reciprocal of per-unit synchronous reactance
B) Equal to per-unit reactance
C) Equal to power factor
D) Equal to efficiency
Answer: A
Explanation:
SCR = 1 / Xs(pu). A high SCR implies low synchronous reactance.
70. A higher SCR implies
A) Better voltage regulation
B) Poor voltage regulation
C) More stability
D) Both A and C
Answer: D
Explanation:
High SCR → lower reactance → better voltage regulation and higher stability.
71. The excitation system of alternators provides
A) Field current
B) Armature current
C) Load current
D) None of these
Answer: A
Explanation:
The excitation system supplies DC current to the rotor field winding.
72. The armature resistance drop in alternators is usually
A) Neglected
B) Dominant
C) Equal to reactance drop
D) Zero
Answer: A
Explanation:
Resistance drop is very small compared to reactance drop; hence, often neglected.
73. The emf equation of an alternator depends on
A) Frequency and flux
B) Frequency, flux, and turns per phase
C) Speed only
D) Poles only
Answer: B
Explanation:
; depends on f, Φ, and T (turns per phase).
74. The generated voltage increases with
A) Speed
B) Excitation
C) Both A and B
D) None
Answer: C
Explanation:
EMF ∝ fΦ → depends on both speed and excitation.
75. The alternator frequency decreases when
A) Load increases
B) Prime mover slows down
C) Excitation increases
D) Power factor decreases
Answer: B
Explanation:
Frequency depends on speed; if speed decreases, frequency drops.
76. The voltage per phase in alternator winding is induced due to
A) Self-induction
B) Mutual induction
C) Transformer action
D) Rotating magnetic field
Answer: D
Explanation:
Voltage is induced due to the relative motion between stator conductors and rotating magnetic field.
77. The efficiency of an alternator is maximum when
A) Copper loss = Iron loss
B) Copper loss < Iron loss
C) Copper loss > Iron loss
D) Both losses are zero
Answer: A
Explanation:
Maximum efficiency occurs when variable losses (Cu) equal constant losses (core).
78. The prime mover used for a high-speed alternator is
A) Steam turbine
B) Diesel engine
C) Water turbine
D) Wind turbine
Answer: A
Explanation:
Steam turbines operate at very high speeds suitable for cylindrical rotor alternators.
79. Which part of the alternator is stationary?
A) Field system
B) Armature winding
C) Rotor
D) Shaft
Answer: B
Explanation:
In modern alternators, the armature is stationary (stator) for easier insulation and cooling.
80. The main advantage of stationary armature is
A) Easier to insulate
B) Easier to cool
C) Both A and B
D) None
Answer: C
Explanation:
A stationary armature allows simpler insulation and efficient cooling for large currents.
81. The frequency of induced emf in an alternator is
A) Directly proportional to rotor speed
B) Inversely proportional to poles
C) Constant for all speeds
D) Independent of rotor
Answer: A
Explanation:
; directly proportional to speed.
82. The slip of a synchronous generator is
A) Zero
B) Unity
C) Negative
D) Variable
Answer: A
Explanation:
In synchronous operation, rotor speed = synchronous speed → slip = 0.
83. The terminal voltage of an alternator increases when
A) Excitation increases
B) Load increases
C) Speed decreases
D) Field current decreases
Answer: A
Explanation:
More excitation → more flux → higher induced EMF → higher terminal voltage.
84. The power factor of an alternator is controlled by
A) Load
B) Excitation
C) Speed
D) Prime mover
Answer: B
Explanation:
Excitation affects reactive power, hence controlling PF.
85. The ratio of air-gap voltage to field current on open circuit gives
A) Synchronous reactance
B) Armature resistance
C) Open-circuit characteristic (OCC)
D) Short circuit ratio
Answer: C
Explanation:
The OCC represents the relationship between generated EMF and field current at no load.
86. The damper winding also helps in
A) Starting synchronous motor
B) Stopping alternator
C) Reducing armature resistance
D) Improving efficiency
Answer: A
Explanation:
In synchronous motors, damper winding acts as a squirrel cage to start the motor.
87. In an alternator, armature reaction at lagging PF is
A) Demagnetizing
B) Magnetizing
C) Cross magnetizing
D) None
Answer: A
Explanation:
Lagging PF → current lags → demagnetizing effect reduces main field.
88. The induced EMF waveform can be improved by
A) Skewing slots
B) Using short pitch coils
C) Proper distribution
D) All of these
Answer: D
Explanation:
All these methods help in reducing harmonics and improving sinusoidal EMF.
89. The voltage regulation of an alternator can be determined by
A) Synchronous impedance method
B) MMF method
C) Zero power factor method
D) All of these
Answer: D
Explanation:
All methods are used for calculating regulation based on test data.
90. The unity power factor load gives
A) Minimum voltage regulation
B) Maximum voltage regulation
C) Negative voltage regulation
D) Constant regulation
Answer: A
Explanation:
At unity PF, armature reaction is purely cross-magnetizing → minimal voltage drop.
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