In alternator parallel operation, if excitation of one machine is increased

 

51. The synchronous impedance method is also known as

A) EMF method
B) MMF method
C) Potier method
D) Zero power factor method
Answer: A
Explanation:
The synchronous impedance method determines regulation using open- and short-circuit tests and is called the EMF method.

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52. The main disadvantage of EMF method is

A) Requires large setup
B) Gives pessimistic results
C) Requires load test
D) Takes more time
Answer: B
Explanation:
It gives a higher (pessimistic) voltage regulation compared to the actual one.

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53. The zero power factor (ZPF) method is used to find

A) Efficiency
B) Voltage regulation
C) Armature resistance
D) Leakage reactance
Answer: B
Explanation:
ZPF method provides accurate voltage regulation considering armature reaction and leakage reactance separately.

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54. The excitation required for a given terminal voltage increases when

A) Load current increases
B) Load power factor increases
C) Speed increases
D) Frequency increases
Answer: A
Explanation:
With load increase, voltage drops increase → more field excitation is required to maintain terminal voltage.

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55. The alternator power factor depends on

A) Load nature
B) Excitation only
C) Speed only
D) Frequency
Answer: A
Explanation:
Power factor of an alternator depends on the power factor of the connected load.

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56. A short-pitched winding in an alternator

A) Increases EMF
B) Reduces harmonics
C) Increases harmonics
D) Increases voltage regulation
Answer: B
Explanation:
Short-pitching reduces harmonic components (like 5th and 7th) and copper requirement.

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57. The purpose of using a distributed winding in alternator is

A) To reduce flux leakage
B) To improve cooling
C) To reduce harmonics
D) To increase power factor
Answer: C
Explanation:
Distributed winding makes generated voltage waveform smoother and more sinusoidal by reducing harmonics.

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58. In a salient pole alternator, the reluctance of magnetic path is

A) Uniform
B) Non-uniform
C) Zero
D) Constant
Answer: B
Explanation:
Due to the projection of poles, the air-gap length varies → magnetic reluctance is non-uniform.

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59. The reluctance power in salient pole alternator is due to

A) Field winding
B) Saliency of poles
C) Armature current
D) Iron losses
Answer: B
Explanation:
In salient-pole machines, the difference between direct and quadrature axis reactances produces reluctance power.

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60. The two-reaction theory is used for

A) Cylindrical rotor machines
B) Salient pole machines
C) Induction machines
D) DC machines
Answer: B
Explanation:
Two-reaction theory (d-axis and q-axis components) is applied to salient pole synchronous machines.

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61. The expression for power in a salient pole alternator is

A) $P = \frac{EV}{X_s} \sin δ$
B) $P = \frac{EV}{X_d} \sin δ + \frac{V^2 (X_d - X_q)}{2 X_d X_q} \sin 2δ$
C) $P = \frac{V^2}{X_q} \sin δ$
D) None
Answer: B
Explanation:
Salient pole machine develops both electromagnetic and reluctance power components given by that equation.

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62. The maximum electromagnetic power of salient pole alternator occurs when

A) δ = 0°
B) δ = 90°
C) δ = 60°
D) δ = 45°
Answer: B
Explanation:
As in cylindrical rotor, max electromagnetic power occurs at δ = 90°.

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63. The direct-axis reactance (Xd) is always

A) Less than quadrature reactance
B) Equal to quadrature reactance
C) Greater than quadrature reactance
D) Zero
Answer: C
Explanation:
In salient pole machines, $X_d > X_q$ because the d-axis has less air-gap → higher flux linkage.

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64. The reactive power (kVAR) sharing between alternators in parallel is controlled by

A) Prime mover governor
B) Field excitation
C) Load torque
D) Speed control
Answer: B
Explanation:
Excitation controls reactive power (kVAR) sharing between alternators.

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65. The active power (kW) sharing is controlled by

A) Field excitation
B) Prime mover governor setting
C) Load angle
D) Speed of rotor
Answer: B
Explanation:
Governor controls mechanical input to the alternator → controls kW sharing.

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66. Hunting in synchronous machines can be reduced by

A) Using damper windings
B) Reducing load
C) Increasing excitation
D) Decreasing inertia
Answer: A
Explanation:
Damper windings provide damping torque to minimize oscillations (hunting).

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67. The load angle δ of a synchronous generator increases when

A) Load decreases
B) Load increases
C) Excitation increases
D) Speed increases
Answer: B
Explanation:
As load increases, rotor lags further behind stator field → δ increases.

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68. In an alternator, armature reaction voltage drop is

A) Proportional to excitation
B) Proportional to load current
C) Proportional to frequency
D) Constant
Answer: B
Explanation:
Armature reaction effect increases with load current.

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69. The power factor of a lagging load causes armature reaction to be

A) Magnetizing
B) Demagnetizing
C) Cross-magnetizing
D) Neutral
Answer: B
Explanation:
At lagging PF, armature reaction opposes main flux → demagnetizing effect.

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70. For a leading PF load, the armature reaction is

A) Demagnetizing
B) Magnetizing
C) Cross-magnetizing
D) Zero
Answer: B
Explanation:
At leading PF, armature field aids main field → magnetizing effect.

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71. In alternator parallel operation, if excitation of one machine is increased

A) Its kVAR output increases
B) Its kW output increases
C) Both kW and kVAR decrease
D) Frequency changes
Answer: A
Explanation:
Increasing excitation increases machine’s reactive power output (kVAR) without changing kW.

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72. If the prime mover input to one alternator increases

A) It supplies more kVAR
B) It supplies more kW
C) Its excitation increases
D) Voltage decreases
Answer: B
Explanation:
Increasing mechanical power input increases real power (kW) output.

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73. The synchronizing torque will be zero when

A) δ = 0°
B) δ = 45°
C) δ = 90°
D) δ = 180°
Answer: C
Explanation:
Synchronizing torque $T_s ∝ \cos δ$ → zero at δ = 90°.

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74. The condition for stability of a synchronous generator is

A) $\frac{dP}{dδ} > 0$
B) $\frac{dP}{dδ} < 0$
C) $P = 0$
D) δ = 0
Answer: B
Explanation:
For stable operation, a small increase in δ must reduce electromagnetic torque → $dP/dδ < 0$.

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75. The capability curve of a synchronous generator is bounded by

A) Field current, armature current, and power factor limits
B) Frequency only
C) Speed and torque
D) Temperature only
Answer: A
Explanation:
Capability curve (V-curves) shows safe operation limits defined by field current, armature current, and PF constraints.

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76. In alternator testing, the short circuit ratio (SCR) is defined as

A) Ratio of short circuit current to open circuit voltage
B) Ratio of field current for rated voltage (OC) to field current for rated current (SC)
C) Ratio of voltage to current
D) None
Answer: B
Explanation:
$SCR = \frac{I_f(\text{rated voltage, OC})}{I_f(\text{rated current, SC})}$.
Higher SCR → better voltage regulation and stability.

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77. A high SCR machine has

A) Better voltage regulation
B) Poor voltage regulation
C) Low stability
D) Small size
Answer: A
Explanation:
High SCR → low synchronous impedance → better voltage regulation and higher stability.

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78. The air-gap length in a turbo-alternator is

A) Large
B) Small
C) Very small
D) Variable
Answer: C
Explanation:
To withstand high speed and reduce noise, turbo-alternators have very small and uniform air-gap.

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79. The reactive power capability of a generator decreases with

A) Increase in temperature
B) Increase in field current
C) Increase in voltage
D) Increase in armature current
Answer: D
Explanation:
Armature current limit restricts generator’s ability to supply reactive power (kVAR).

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80. The voltage generated per phase increases if

A) Flux increases
B) Speed increases
C) Both A and B
D) Frequency decreases
Answer: C
Explanation:
$E = 4.44 f Φ T$; increasing either flux (Φ) or speed (f) increases generated voltage.

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81. When alternator frequency increases, synchronous reactance

A) Increases
B) Decreases
C) Remains constant
D) Becomes zero
Answer: A
Explanation:
Reactance $X = 2πfL$; hence synchronous reactance increases with frequency.

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82. The terminal voltage of an alternator drops due to

A) Armature reaction
B) Armature impedance drop
C) Both A and B
D) Field weakening
Answer: C
Explanation:
Voltage drop occurs due to both armature reaction and internal impedance effects.

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83. In alternator, iron losses mainly occur in

A) Stator core
B) Rotor core
C) Shaft
D) Slip rings
Answer: A
Explanation:
Stator experiences alternating flux → causes hysteresis and eddy current (iron) losses.

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84. The frequency regulation of a synchronous generator is

A) Controlled by excitation
B) Controlled by governor
C) Controlled by load current
D) None
Answer: B
Explanation:
Frequency depends on rotor speed → controlled by prime mover governor.

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85. The field current required for unity PF operation is

A) Greater than lagging PF
B) Less than lagging PF
C) Greater than leading PF
D) Same for all PFs
Answer: B
Explanation:
At unity PF, less field current is required compared to lagging PF load.

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86. The internal generated EMF at leading PF is

A) Less than terminal voltage
B) Equal to terminal voltage
C) Greater than terminal voltage
D) Same as lagging PF
Answer: A
Explanation:
Leading PF → magnetizing effect → internal EMF < terminal voltage.

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87. The phasor diagram of lagging PF shows

A) E leads V
B) E lags V
C) E in phase with V
D) None
Answer: A
Explanation:
At lagging PF, current lags voltage, so internal EMF leads terminal voltage.

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88. The excitation current in alternator is supplied through

A) Slip rings and brushes
B) Armature
C) Load terminals
D) Transformer
Answer: A
Explanation:
Slip rings and carbon brushes feed DC excitation current to the rotor.

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89. If a synchronous generator runs at over-excitation, it behaves like

A) Inductive load
B) Capacitive load
C) Resistive load
D) Short circuit
Answer: B
Explanation:
Over-excitation → leading PF → behaves as capacitive.

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90. Under-excitation makes the alternator behave as

A) Capacitor
B) Inductor
C) Resistor
D) Transformer
Answer: B
Explanation:
Under-excited → lagging PF → behaves like an inductive load.

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91. Voltage regulation of an alternator is zero when

A) Internal EMF = Terminal voltage
B) No-load voltage = Full-load voltage
C) Excitation constant
D) PF unity
Answer: B
Explanation:
If full-load terminal voltage equals no-load voltage → zero voltage regulation.

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92. Which test is NOT performed on an alternator?

A) Open circuit test
B) Short circuit test
C) Load test
D) Slip test
Answer: C
Explanation:
Load test is not practical for large alternators due to high power consumption.

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93. Slip test is used to determine

A) Armature resistance
B) Xd and Xq of salient pole alternator
C) Synchronous reactance
D) Power factor
Answer: B
Explanation:
Slip test helps find direct-axis (Xd) and quadrature-axis (Xq) reactances.

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94. In parallel operation, alternators must have same

A) Frequency, voltage, and phase sequence
B) Power factor
C) Speed
D) Field current
Answer: A
Explanation:
Synchronization conditions: same frequency, voltage, and phase sequence.

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95. Synchronizing lamps are used to

A) Measure frequency
B) Indicate correct phase relation
C) Control excitation
D) Indicate voltage regulation
Answer: B
Explanation:
Synchronizing lamps show correct phase and frequency match before connecting alternators in parallel.

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96. The dark lamp method shows synchronism when

A) Lamps are bright
B) Lamps are dark simultaneously
C) Lamps flicker alternately
D) Lamps glow continuously
Answer: B
Explanation:
When lamps go dark simultaneously → same voltage, phase, and frequency → alternators in synchronism.

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97. Synchronoscope is used for

A) Measuring torque
B) Synchronizing alternators
C) Measuring power factor
D) Indicating field current
Answer: B
Explanation:
Synchronoscope shows the phase relation and direction of rotation for proper synchronization.

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98. If alternators have same voltage but different frequencies,

A) They can be paralleled safely
B) They will pulsate power
C) They will run smoothly
D) No current will flow
Answer: B
Explanation:
Different frequencies cause cyclic variation in load sharing → power pulsations.

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99. The stability of synchronous generator improves with

A) Higher SCR
B) Lower excitation
C) Lower inertia
D) Higher load angle
Answer: A
Explanation:
High short-circuit ratio (SCR) improves voltage stability and synchronizing power.

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100. The per-unit (p.u.) value of any quantity is given by

A) Actual value / Base value
B) Base value / Actual value
C) (Actual × 100) / Base
D) None
Answer: A
Explanation:
Per-unit system simplifies calculations:
$\text{p.u. value} = \frac{\text{Actual value}}{\text{Base value}}$.