A higher short-circuit ratio results in

 

101. The main purpose of parallel operation of alternators is

A) To increase load capacity
B) For continuity of supply
C) For maintenance flexibility
D) All of the above
Answer: D
Explanation:
Running alternators in parallel increases system reliability, capacity, and allows maintenance without power interruption.

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102. Before synchronizing, alternators must have

A) Same voltage, frequency, and phase sequence
B) Same load
C) Same field current
D) Same size
Answer: A
Explanation:
For proper synchronization, all three parameters — voltage, frequency, and phase sequence — must match.

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103. The instrument used for checking synchronizing conditions is called

A) Megger
B) Synchronoscope
C) Tachometer
D) Frequency meter
Answer: B
Explanation:
A synchronoscope indicates the phase difference and speed between incoming and running alternators.

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104. If phase sequence is wrong during synchronization

A) Alternator will get damaged
B) Alternator will run in reverse direction
C) Heavy circulating current will flow
D) Alternator will stop
Answer: C
Explanation:
Wrong phase sequence causes large circulating currents between alternators due to voltage phase opposition.

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105. When alternators are running in parallel, load sharing depends on

A) Prime mover speed and excitation
B) Frequency only
C) Armature resistance
D) Power factor only
Answer: A
Explanation:

• Speed (governor) controls active power sharing.

• Excitation controls reactive power sharing.

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106. If excitation of one alternator running in parallel increases

A) It supplies more reactive power
B) It supplies more real power
C) Its speed increases
D) Its frequency decreases
Answer: A
Explanation:
Increased excitation → more field flux → machine supplies more leading (reactive) power.

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107. If the prime mover input of one alternator in parallel increases

A) It supplies more active power
B) It supplies less reactive power
C) Its voltage rises
D) It loses synchronism
Answer: A
Explanation:
Increased mechanical power → alternator delivers more active (real) power to the load.

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108. When two alternators share load unequally, it is mainly due to

A) Unequal excitation
B) Unequal speed regulation
C) Unequal reactances
D) All of the above
Answer: D
Explanation:
Load sharing is affected by excitation, speed regulation, and internal impedance of the alternators.

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109. Synchronizing torque is

A) Torque that maintains synchronism between machines
B) Load torque
C) Friction torque
D) Starting torque
Answer: A
Explanation:
Synchronizing torque restores synchronism when a small disturbance causes phase deviation between alternators.

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110. If synchronizing torque is weak, the alternator may

A) Lose synchronism
B) Overheat
C) Over-excite
D) Stop
Answer: A
Explanation:
Weak synchronizing torque cannot restore the rotor to its proper position → alternator slips poles and loses synchronism.

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111. Synchronizing power is maximum when

A) δ = 0°
B) δ = 90°
C) δ = 45°
D) δ = 180°
Answer: A
Explanation:
Synchronizing power $P_s = \frac{EV}{X_s} \cos δ$ → maximum when $δ = 0°$.

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112. The power factor of an alternator improves when

A) Excitation increases (over-excitation)
B) Excitation decreases (under-excitation)
C) Speed increases
D) Load decreases
Answer: A
Explanation:
Over-excited alternator supplies leading reactive power → improves system PF.

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113. During parallel operation, if one alternator’s excitation is reduced

A) It will absorb reactive power
B) It will supply more real power
C) It will speed up
D) It will become over-excited
Answer: A
Explanation:
Under-excited alternator absorbs lagging reactive power from the system.

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114. In alternators, hunting can be minimized by

A) Damper windings
B) Increasing excitation
C) Decreasing speed
D) Reducing load
Answer: A
Explanation:
Damper windings provide damping torque that reduces rotor oscillations (hunting).

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115. The prime mover of a high-speed alternator is usually a

A) Steam turbine
B) Water turbine
C) Diesel engine
D) Wind turbine
Answer: A
Explanation:
Steam turbines run at very high speed (3000 rpm for 2-pole alternators).

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116. The prime mover of a low-speed alternator is usually a

A) Steam turbine
B) Water turbine
C) Gas turbine
D) Jet engine
Answer: B
Explanation:
Hydro turbines operate at low speeds, suitable for salient pole alternators.

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117. Alternator voltage waveform distortion is mainly caused by

A) Slot harmonics
B) Poor winding distribution
C) Short-pitching errors
D) All of these
Answer: D
Explanation:
All these factors can produce harmonics and distort the generated voltage waveform.

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118. Harmonics in alternator EMF cause

A) Heating
B) Torque pulsations
C) Poor voltage waveform
D) All of the above
Answer: D
Explanation:
Harmonics lead to increased losses, noise, and distorted voltage.

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119. The dominant harmonic in a 3-phase alternator is

A) 3rd harmonic
B) 5th harmonic
C) 7th harmonic
D) 11th harmonic
Answer: A
Explanation:
Odd harmonics, especially 3rd, are most significant but are eliminated in 3-phase star-connected systems (they circulate in neutral).

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120. The frequency of the 5th harmonic is

A) 5 × fundamental
B) 1/5 of fundamental
C) 2 × fundamental
D) 1/2 fundamental
Answer: A
Explanation:
nth harmonic has frequency n × fundamental frequency.

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121. The induced emf of an alternator can be increased by

A) Increasing field current
B) Increasing speed
C) Both A and B
D) None
Answer: C
Explanation:
$E = 4.44 f Φ N$; increasing field current or speed increases EMF.

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122. 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) Faraday’s second law
Answer: A
Explanation:
Fleming’s right-hand rule gives the direction of induced current/EMF.

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123. The copper losses in alternator vary with

A) Square of load current
B) Load current
C) Voltage
D) Speed
Answer: A
Explanation:
Copper loss = I²R, varies with square of current.

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124. The iron losses in an alternator depend on

A) Frequency and flux density
B) Load current
C) Armature resistance
D) Speed only
Answer: A
Explanation:
Hysteresis and eddy current losses depend on frequency and magnetic flux density.

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125. Efficiency of an alternator is maximum at

A) Unity PF
B) Lagging PF
C) Leading PF
D) Zero PF
Answer: A
Explanation:
At unity PF, reactive power is zero → minimum current → least copper loss → highest efficiency.

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126. The number of poles required for 50 Hz, 1500 rpm alternator is

A) 2
B) 4
C) 6
D) 8
Answer: B
Explanation:
$N = 120f/P \Rightarrow P = 120×50/1500 = 4$ poles.

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127. For a 50 Hz, 1000 rpm alternator, number of poles = ?

A) 4
B) 6
C) 8
D) 10
Answer: C
Explanation:
$P = 120×50/1000 = 6$? Wait, compute: $P = 6$? Actually $120×50/1000 = 6$. ✅ Correct → 6 poles.

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128. The maximum value of load angle δ before losing synchronism is

A) 90°
B) 120°
C) 180°
D) 270°
Answer: A
Explanation:
Stable operation possible only up to δ = 90°; beyond that, torque becomes negative.

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129. The main factor deciding alternator size is

A) KVA rating
B) Power factor
C) Efficiency
D) Excitation voltage
Answer: A
Explanation:
Alternator is rated in kVA since copper losses depend on current and iron losses depend on voltage, independent of PF.

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130. The alternator voltage regulation at leading PF is

A) Negative
B) Positive
C) Zero
D) Infinite
Answer: A
Explanation:
At leading PF, internal EMF < terminal voltage → negative regulation.

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131. Synchronous reactance of alternator is

A) $X_s = \sqrt{Z_s^2 - R_a^2}$
B) $X_s = R_a + Z_s$
C) $X_s = Z_s / R_a$
D) None
Answer: A
Explanation:
From OC and SC tests: $Z_s = \sqrt{R_a^2 + X_s^2} \Rightarrow X_s = \sqrt{Z_s^2 - R_a^2}$.

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132. The per-unit synchronous reactance of a modern alternator is about

A) 0.1–0.2
B) 0.3–0.5
C) 1.0–1.5
D) 2.0–3.0
Answer: B
Explanation:
Typical alternators have per-unit synchronous reactance between 0.3 and 0.5.

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133. A high value of synchronous reactance causes

A) Poor voltage regulation
B) Low short circuit current
C) Both A and B
D) None
Answer: C
Explanation:
High $X_s$ → large voltage drop and small short-circuit current → poor regulation but safer operation.

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134. The short-circuit ratio (SCR) of an alternator is usually

A) <1
B) 1–2
C) >2
D) Infinite
Answer: B
Explanation:
Typical alternators have SCR between 1 and 2 for stability and economy.

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135. The leakage flux in alternator contributes to

A) Leakage reactance
B) Armature reaction
C) Copper loss
D) Iron loss
Answer: A
Explanation:
Leakage flux does not link all turns → causes reactance known as leakage reactance.

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136. The field poles in a salient pole alternator are

A) Projected from rotor surface
B) Cylindrical and smooth
C) Laminated on stator
D) Placed in slots
Answer: A
Explanation:
Salient pole machines have large, projecting poles bolted to the rotor.

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137. Cylindrical rotors are used because they

A) Can withstand high centrifugal forces
B) Provide smooth air gap
C) Are suitable for high-speed turbines
D) All of the above
Answer: D
Explanation:
Cylindrical rotors offer mechanical strength and uniform air-gap for high-speed alternators.

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138. The core of alternator stator is laminated to

A) Reduce eddy current losses
B) Increase flux
C) Strengthen magnetic field
D) Reduce hysteresis losses
Answer: A
Explanation:
Lamination minimizes eddy current losses by reducing the path for circulating currents.

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139. The insulation resistance of alternator winding is measured by

A) Megger
B) Voltmeter
C) Ohmmeter
D) Ammeter
Answer: A
Explanation:
A Megger applies high voltage to measure insulation resistance.

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140. The alternator nameplate rating is given in

A) kW
B) kVA
C) kVAR
D) Amperes only
Answer: B
Explanation:
Alternators are rated in kVA because the rating is independent of power factor.

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141. The armature winding of alternator is placed in

A) Rotor slots
B) Stator slots
C) Field poles
D) Commutator
Answer: B
Explanation:
Armature (stator) winding is placed in stator slots for ease of cooling and insulation.

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142. The field winding of alternator is placed on

A) Rotor
B) Stator
C) Armature
D) Shaft only
Answer: A
Explanation:
Field winding is placed on the rotor to supply DC excitation easily through slip rings.

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143. For a 4-pole alternator operating at 1500 rpm, frequency is

A) 25 Hz
B) 50 Hz
C) 60 Hz
D) 75 Hz
Answer: B
Explanation:
$f = P×N/120 = 4×1500/120 = 50 \text{ Hz}$

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144. For an 8-pole alternator running at 750 rpm, frequency = ?

A) 25 Hz
B) 50 Hz
C) 100 Hz
D) 60 Hz
Answer: B
Explanation:
$f = 8×750/120 = 50 \text{ Hz}$

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145. The primary cause of armature reaction is

A) Armature current
B) Field current
C) Speed variation
D) Frequency change
Answer: A
Explanation:
Armature reaction is the effect of the magnetic field produced by armature current on the main field.

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146. The open-circuit characteristic (OCC) of an alternator is

A) Non-linear
B) Linear
C) Parabolic
D) Exponential
Answer: A
Explanation:
OCC is non-linear due to magnetic saturation of the iron core at higher excitations.

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147. The short-circuit characteristic (SCC) of an alternator is

A) Straight line
B) Curved
C) Exponential
D) Parabolic
Answer: A
Explanation:
SCC is a straight line because under short-circuit conditions, the flux is small and the core remains unsaturated.

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148. A higher short-circuit ratio results in

A) Better voltage regulation and stability
B) Poor regulation and instability
C) Higher reactance
D) Lower synchronizing power
Answer: A
Explanation:
High SCR means low reactance → improved regulation and dynamic stability.

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149. The relation between synchronous impedance and SCR is

A) Inversely proportional
B) Directly proportional
C) Independent
D) Logarithmic
Answer: A
Explanation:
SCR = 1 / (Synchronous Reactance in p.u.), hence inversely proportional.

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150. The field current required to produce rated voltage on open circuit and rated current on short circuit defines

A) Short circuit ratio
B) Synchronous reactance
C) Voltage regulation
D) Excitation emf
Answer: A
Explanation:
SCR = Field current for rated voltage (OC) / Field current for rated current (SC).