When two alternators are in parallel, they share load depending on

 

11. The field winding of a synchronous generator is placed on

A) Stator
B) Rotor
C) Either stator or rotor
D) Frame
Answer: B
Explanation:
In alternators, the DC field winding is placed on the rotor to avoid supplying DC to a moving stator and to use slip rings only for excitation current.

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12. The armature winding of a synchronous generator is placed on

A) Rotor
B) Shaft
C) Stator
D) Commutator
Answer: C
Explanation:
Armature winding is placed on the stator to generate large AC voltage safely and easily connect it to the external circuit.

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13. For the same machine speed, increasing the number of poles will

A) Increase frequency
B) Decrease frequency
C) Not affect frequency
D) Increase voltage
Answer: B
Explanation:
$f = \frac{P \times N}{120}$. For constant speed, increasing poles decreases frequency.

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14. The function of the exciter in a synchronous generator is to

A) Supply mechanical power
B) Supply AC to the rotor
C) Supply DC to the rotor winding
D) Increase output voltage
Answer: C
Explanation:
Exciter is a small DC generator (or rectifier) that provides DC to the rotor field winding.

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15. The salient pole type rotor is generally used for

A) High-speed alternators
B) Low-speed alternators
C) DC generators only
D) Turbo-alternators
Answer: B
Explanation:
Salient pole rotors are used for low-speed applications like hydro-generators due to their mechanical design and multiple poles.

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16. Cylindrical rotors are preferred for

A) Hydro-generators
B) Diesel engines
C) Steam turbines
D) Wind generators
Answer: C
Explanation:
Cylindrical rotors are used in high-speed turbo-alternators driven by steam turbines.

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17. The EMF generated in each phase of an alternator is

A) Direct current
B) Pulsating DC
C) Alternating and sinusoidal
D) Rectified DC
Answer: C
Explanation:
Alternator output is sinusoidal AC due to sinusoidal flux distribution and uniform rotation.

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18. The leakage reactance of an alternator depends on

A) Armature current
B) Slot design and winding
C) Supply voltage
D) Speed of rotor
Answer: B
Explanation:
Leakage reactance is determined by magnetic leakage paths, affected by slot shape and coil pitch.

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19. The voltage regulation of an alternator is

A) Always positive
B) Depends on power factor of load
C) Independent of power factor
D) Always negative
Answer: B
Explanation:
Regulation varies: positive (lagging PF), negative (leading PF), or zero (unity PF).

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20. If excitation of alternator decreases, its terminal voltage

A) Increases
B) Decreases
C) Remains same
D) Becomes zero
Answer: B
Explanation:
Reduced excitation decreases field strength → less induced EMF → lower terminal voltage.

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21. The load angle (δ) is the angle between

A) Stator current and stator voltage
B) Rotor axis and stator flux
C) Rotor voltage and stator current
D) None
Answer: B
Explanation:
Load angle δ represents displacement between rotor field axis and stator resultant field.

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22. When the power factor of load is leading, the voltage regulation is

A) Positive
B) Negative
C) Zero
D) Infinite
Answer: B
Explanation:
At leading PF, voltage rise occurs on load → negative regulation.

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23. The efficiency of an alternator is maximum when

A) Copper losses = Iron losses
B) Copper losses < Iron losses
C) Iron losses = 0
D) Copper losses = 0
Answer: A
Explanation:
Efficiency $\eta = \frac{Output}{Output + Losses}$ is maximum when variable (Cu) losses = constant (Iron) losses.

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24. Armature reaction in alternator at unity PF is

A) Magnetizing
B) Demagnetizing
C) Cross-magnetizing
D) Neutral
Answer: C
Explanation:
At unity PF, current is in phase with voltage → armature flux is perpendicular to main field → cross-magnetizing.

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25. The open circuit characteristic (OCC) of an alternator gives

A) Relation between field current and load current
B) Field current and generated voltage
C) Field current and power
D) Load current and voltage
Answer: B
Explanation:
OCC shows how generated EMF varies with excitation (field current) at no-load.

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

A) Straight line
B) Parabolic curve
C) Hyperbola
D) Exponential curve
Answer: A
Explanation:
SCC is linear since short-circuit current is proportional to field current (no saturation).

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27. In alternators, the generated EMF waveform depends mainly on

A) Field winding shape
B) Distribution of air-gap flux
C) Rotor speed only
D) Load current
Answer: B
Explanation:
The waveform depends on flux distribution along the air-gap — sinusoidal flux gives sinusoidal EMF.

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28. The synchronous reactance is the sum of

A) Armature resistance and leakage reactance
B) Field resistance and armature resistance
C) Armature resistance and reactance
D) Leakage and armature reaction reactances
Answer: D
Explanation:
Synchronous reactance $X_s = X_l + X_a$, where $X_l$ is leakage reactance and $X_a$ is armature reaction reactance.

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29. The EMF method of voltage regulation gives

A) Accurate result
B) Lower than actual result
C) Higher than actual result
D) Exact result
Answer: C
Explanation:
EMF method is pessimistic — it gives a higher regulation than actual because it neglects magnetic saturation and phase effects.

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30. The MMF method of regulation gives

A) Higher than actual
B) Lower than actual
C) Exact
D) No result
Answer: B
Explanation:
MMF method is optimistic — it predicts lower regulation than actual since it underestimates voltage drops.

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31. Potier triangle method is used to determine

A) Voltage regulation
B) Efficiency
C) Power factor
D) Torque angle
Answer: A
Explanation:
Potier triangle separates the effects of armature reaction and leakage reactance to accurately determine voltage regulation.

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32. The armature copper losses are proportional to

A) I
B) I²
C) 1/I
D) Constant
Answer: B
Explanation:
Copper loss $= I^2 R$, hence proportional to the square of armature current.

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33. If load increases suddenly, alternator speed

A) Increases
B) Decreases
C) Remains same
D) Oscillates
Answer: D
Explanation:
Sudden load change causes rotor oscillations called hunting, damped by damper windings.

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34. The synchronizing power depends on

A) δ only
B) Excitation only
C) Both excitation and δ
D) Supply voltage only
Answer: C
Explanation:
Synchronizing power $P_s = \frac{EV}{X_s} \cos δ$; depends on excitation (E) and load angle (δ).

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35. When two alternators are in parallel, they share load depending on

A) Prime mover speed
B) Excitation and governor setting
C) Terminal voltage only
D) Field current only
Answer: B
Explanation:
Governor setting controls kW load sharing; excitation controls kVAR (reactive power) sharing.

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36. The synchronizing torque is proportional to

A) sin δ
B) cos δ
C) tan δ
D) δ
Answer: B
Explanation:
Synchronizing torque $T_s ∝ \cos δ$, maximum at δ = 0 and zero at δ = 90°.

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37. The prime mover of a turbo-alternator is usually

A) Diesel engine
B) Water turbine
C) Steam turbine
D) Gas turbine
Answer: C
Explanation:
Turbo-alternators operate at very high speed (3000 rpm) driven by steam turbines.

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38. In alternator, the pitch factor (Kp) is always

A) Less than 1
B) Greater than 1
C) Equal to 1
D) Zero
Answer: A
Explanation:
Pitch factor $K_p = \cos(\frac{α}{2})$, where α = chording angle. Chorded coils reduce harmonics, making $K_p < 1$.

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39. The distribution factor (Kd) depends on

A) Coil span
B) Slots per pole per phase
C) Frequency
D) Speed
Answer: B
Explanation:
Distribution factor depends on how many slots per pole per phase are used to distribute winding.

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40. The resultant winding factor (Kw) is given by

A) $Kw = Kp + Kd$
B) $Kw = Kp \times Kd$
C) $Kw = Kp / Kd$
D) $Kw = Kp - Kd$
Answer: B
Explanation:
Winding factor combines pitch and distribution effects: $Kw = Kp \times Kd$.

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41. The field poles of a salient pole alternator are made of

A) Cast iron
B) Cast steel
C) Laminated steel
D) Aluminum
Answer: C
Explanation:
To reduce eddy current losses, field poles are laminated steel.

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42. Slip rings in alternator are used to

A) Connect load
B) Supply DC excitation to rotor
C) Measure voltage
D) Ground the stator
Answer: B
Explanation:
Slip rings transfer DC excitation current to the rotating field winding.

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43. The voltage waveform in alternator becomes distorted due to

A) Non-sinusoidal flux distribution
B) Load variations
C) Winding resistance
D) Rotor speed variation
Answer: A
Explanation:
Harmonics appear when flux distribution deviates from sinusoidal shape.

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44. Harmonics in alternator voltage can be reduced by

A) Using chorded coils
B) Increasing field current
C) Using salient poles
D) Using larger rotor
Answer: A
Explanation:
Short-pitching (chording) the coils reduces certain harmonics like 5th and 7th.

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45. The most common cause of voltage unbalance in alternator is

A) Unequal phase load
B) Magnetic losses
C) Speed variation
D) Winding short
Answer: A
Explanation:
Unequal loading of phases leads to unbalanced voltage drops → voltage unbalance.

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46. Armature heating mainly occurs due to

A) Iron loss
B) Copper loss
C) Eddy currents
D) Magnetic flux
Answer: B
Explanation:
Armature copper loss $I^2R$ causes heating in the stator windings.

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47. The damper winding is located in

A) Stator slots
B) Rotor poles
C) Shaft
D) End shields
Answer: B
Explanation:
Damper winding consists of copper bars embedded in pole faces of the rotor.

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48. The synchronous speed of a 4-pole alternator at 50 Hz is

A) 750 rpm
B) 1500 rpm
C) 3000 rpm
D) 6000 rpm
Answer: B
Explanation:
$N = \frac{120 f}{P} = \frac{120 × 50}{4} = 1500\ rpm$.

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49. The generated voltage in an alternator is directly proportional to

A) Speed only
B) Flux only
C) Both flux and speed
D) Power factor
Answer: C
Explanation:
$E ∝ Φ × N$, where Φ is flux per pole and N is rotational speed.

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50. The main losses in an alternator are

A) Copper, Iron, and Mechanical
B) Copper and Magnetic only
C) Iron only
D) Mechanical only
Answer: A
Explanation:
Alternator losses include copper (I²R), iron (hysteresis + eddy current), and mechanical (friction & windage) losses.