Thursday, October 23, 2025

When alternator supplies leading PF load, terminal voltage

 

251. The main function of a voltage regulator in an alternator is to

A) Control load current
B) Maintain constant terminal voltage
C) Control mechanical speed
D) Maintain constant frequency
Answer: B
Explanation:
A voltage regulator adjusts the field excitation to keep the terminal voltage constant under varying load conditions.

252. The excitation current of a synchronous generator is adjusted to control

A) Frequency
B) Load angle
C) Power factor and terminal voltage
D) Speed
Answer: C
Explanation:
By varying field current, the alternator’s reactive power and terminal voltage are controlled, thus adjusting power factor.

253. Hunting in a synchronous machine occurs due to

A) Load fluctuations
B) Speed variations of prime mover
C) Mechanical vibrations
D) Field circuit faults
Answer: A
Explanation:
Hunting is oscillation of the rotor about its equilibrium due to sudden load changes.

254. The damping torque in a synchronous generator is provided by

A) Damper windings
B) Field windings
C) Armature resistance
D) Load reactance
Answer: A
Explanation:
Damper windings (copper bars embedded in the rotor pole faces) provide damping torque to suppress hunting.

255. Which test provides the magnetization characteristic of an alternator?

A) Short-circuit test
B) Open-circuit test
C) Load test
D) Heat run test
Answer: B
Explanation:
The Open-Circuit Test gives the relationship between field current and generated voltage — known as the magnetization characteristic.

256. The pitch factor in alternator windings is always

A) Equal to 1
B) Greater than 1
C) Less than 1
D) Zero
Answer: C
Explanation:
Pitch factor Kp=cos⁡(α/2)K_p = \cos(\alpha/2). For fractional-pitch windings, Kp<1K_p < 1.

257. The distribution factor depends on

A) Number of slots per pole per phase
B) Flux per pole
C) Speed of rotation
D) Frequency
Answer: A
Explanation:
Distribution factor Kd=sin⁡(mα/2)msin⁡(α/2)K_d = \frac{\sin(m\alpha/2)}{m\sin(\alpha/2)}, depends on the slot angle and number of slots per pole per phase.

258. The voltage per phase of a distributed winding alternator is

A) Higher than concentrated winding
B) Lower than concentrated winding
C) Same as concentrated winding
D) Independent of winding type
Answer: B
Explanation:
Distributed winding reduces EMF due to phase spread, but it improves waveform and reduces harmonics.

259. The synchronous reactance of an alternator is the sum of

A) Leakage reactance and armature reaction reactance
B) Armature resistance and reactance
C) Armature current and voltage
D) Inductive and capacitive reactance
Answer: A
Explanation:
Xs=Xl+XarX_s = X_l + X_{ar}, where XlX_l = leakage reactance and XarX_{ar} = armature reaction reactance.

260. The main factor affecting voltage regulation of an alternator is

A) Load current
B) Power factor of load
C) Excitation
D) All of these
Answer: D
Explanation:
Voltage regulation depends on load current, load power factor, and excitation level.

261. In a salient pole alternator, reluctance power is due to

A) Unequal air gap
B) Armature reaction
C) Field resistance
D) Friction losses
Answer: A
Explanation:
Salient pole machines have variable air-gap reluctance causing an additional torque component called reluctance power.

262. The power developed per phase in a salient pole machine is

A) EVXdsin⁡δ\frac{EV}{X_d}\sin\delta
B) EVXqsin⁡δ\frac{EV}{X_q}\sin\delta
C) EVXdsin⁡δ+V22(1Xq−1Xd)sin⁡2δ\frac{EV}{X_d}\sin\delta + \frac{V^2}{2}( \frac{1}{X_q} - \frac{1}{X_d})\sin2\delta
D) None
Answer: C
Explanation:
Power has two components — excitation power and reluctance power.

263. Which method gives the most accurate voltage regulation?

A) Synchronous impedance method
B) Ampere-turn method
C) Zero power factor method
D) EMF method
Answer: C
Explanation:
ZPF method is most accurate because it accounts for the saturation and leakage effects under load.

264. The armature winding of an alternator is always placed on

A) Rotor
B) Stator
C) Either
D) None
Answer: B
Explanation:
Stator houses armature winding to facilitate insulation and cooling, while rotor carries field winding.

265. Frequency of generated EMF is

A) f=P×N120f = \frac{P \times N}{120}
B) f=N60f = \frac{N}{60}
C) f=2PNf = \frac{2P}{N}
D) f=P×N60f = \frac{P \times N}{60}
Answer: A
Explanation:
This is the fundamental relation linking speed and pole count to output frequency.

266. For a 4-pole, 1500 rpm alternator, frequency is

A) 25 Hz
B) 50 Hz
C) 60 Hz
D) 75 Hz
Answer: B
Explanation:
f=(4×1500)/120=50 Hzf = (4×1500)/120 = 50\,Hz.

267. The number of pole pairs in a 6-pole alternator is

A) 2
B) 3
C) 6
D) 12
Answer: B
Explanation:
Number of pole pairs = P/2 = 6/2 = 3.

268. The function of prime mover in alternator is to

A) Supply field current
B) Rotate rotor at synchronous speed
C) Maintain constant voltage
D) None
Answer: B
Explanation:
Prime mover provides mechanical energy to rotate rotor at the desired speed.

269. The terminal voltage of an alternator drops on load due to

A) Armature reaction
B) Armature impedance drop
C) Both A and B
D) None
Answer: C
Explanation:
Voltage drop occurs due to reactance and resistance of armature and armature reaction effect.

270. The excitation system in alternators is used to

A) Produce magnetic flux
B) Supply mechanical energy
C) Control frequency
D) Reduce armature reaction
Answer: A
Explanation:
Excitation provides DC field current producing the main magnetic flux.

271. Synchronous generators are preferred for power generation because

A) They operate at constant speed
B) They maintain constant frequency
C) They can operate at any power factor
D) All of these
Answer: D
Explanation:
Synchronous generators run at constant speed, maintain system frequency, and handle wide PF range.

272. Slip in a synchronous machine is always

A) Zero
B) One
C) Variable
D) Infinite
Answer: A
Explanation:
Rotor speed = synchronous speed ⇒ Slip = 0.

273. The direction of induced EMF in an alternator is given by

A) Fleming’s left-hand rule
B) Fleming’s right-hand rule
C) Lenz’s law
D) None
Answer: B
Explanation:
Right-hand rule gives the direction of induced EMF in conductors.

274. Armature reaction is strongest at

A) Lagging power factor
B) Leading power factor
C) Unity power factor
D) No-load
Answer: A
Explanation:
At lagging PF, current lags voltage, producing demagnetizing and cross-magnetizing effects.

275. Harmonics in the generated voltage are reduced by

A) Using distributed winding
B) Using short-pitch coils
C) Both A and B
D) Increasing flux
Answer: C
Explanation:
Distributed and short-pitch windings reduce harmonics and improve voltage waveform.

276. A turbo alternator is generally

A) High speed, cylindrical rotor
B) Low speed, salient pole rotor
C) DC machine
D) None
Answer: A
Explanation:
Steam turbines run at high speed, so alternators use cylindrical rotors for mechanical balance.

277. In alternators, salient pole type is preferred for

A) Low-speed hydro units
B) High-speed steam units
C) Both
D) None
Answer: A
Explanation:
Hydro turbines are slow-speed, so salient poles are used to produce required frequency.

278. A 2-pole alternator runs at 3000 rpm. Frequency generated is

A) 25 Hz
B) 50 Hz
C) 60 Hz
D) 100 Hz
Answer: B
Explanation:
f=(2×3000)/120=50 Hzf = (2×3000)/120 = 50\,Hz.

279. If an alternator runs at 1000 rpm and produces 50 Hz, the number of poles is

A) 4
B) 6
C) 10
D) 12
Answer: C
Explanation:
P=120f/N=120×50/1000=6P = 120f/N = 120×50/1000 = 6 poles ⇒ 6 poles.

280. The field winding of an alternator is usually

A) Lap wound
B) Wave wound
C) Coil wound on rotor poles
D) None
Answer: C
Explanation:
Rotor carries field winding supplied by DC excitation system.

281. The main cause of voltage dip at load is

A) Armature reactance
B) Field weakening
C) Resistance of winding
D) Poor cooling
Answer: A
Explanation:
Armature reactance causes large voltage drop at lagging power factors.

282. The frequency of voltage generated is directly proportional to

A) Speed only
B) Poles only
C) Both speed and poles
D) None
Answer: C
Explanation:
f=PN/120f = PN/120. Both speed and poles affect frequency.

283. The maximum efficiency of alternator occurs when

A) Copper losses = Iron losses
B) Load current is maximum
C) Power factor is unity
D) Voltage is maximum
Answer: A
Explanation:
Efficiency is max when variable (Cu) losses = constant (iron) losses.

284. The function of automatic voltage regulator (AVR) is

A) Adjust excitation
B) Maintain power factor
C) Control mechanical torque
D) Synchronize alternators
Answer: A
Explanation:
AVR senses terminal voltage and adjusts excitation to maintain constant voltage.

285. The main advantage of rotating field type alternator is

A) Less weight of rotating part
B) High current capacity
C) Easy cooling
D) All of these
Answer: D
Explanation:
Rotor carries only DC, making it lighter, easier to insulate and cool.

286. Alternator output is connected to

A) DC load
B) AC grid
C) Battery bank
D) Rectifier
Answer: B
Explanation:
Alternators generate AC power, directly supplied to the grid.

287. Synchronization of alternators requires matching

A) Voltage, frequency, phase sequence
B) Power factor only
C) Speed only
D) Excitation only
Answer: A
Explanation:
Before connecting alternators in parallel, their voltage, frequency, and phase sequence must match.

288. The reactive power in alternator is controlled by

A) Excitation current
B) Speed
C) Mechanical torque
D) Load current
Answer: A
Explanation:
Excitation controls reactive power flow in alternator operation.

289. If excitation increases at constant load, alternator

A) Supplies lagging reactive power
B) Supplies leading reactive power
C) Remains unchanged
D) Becomes unstable
Answer: A
Explanation:
Over-excited alternator supplies lagging vars (inductive reactive power).

290. The unit of synchronous reactance is

A) Ohm
B) Henry
C) Volt
D) None
Answer: A
Explanation:
Reactance is measured in ohms.

291. The waveform of generated EMF in alternator is

A) Purely sinusoidal
B) Approximately sinusoidal
C) Square
D) Triangular
Answer: B
Explanation:
Due to slot harmonics, waveform is nearly but not perfectly sinusoidal.

292. The excitation voltage of alternator is supplied by

A) Exciter
B) Transformer
C) Rectifier
D) Inductor
Answer: A
Explanation:
Exciter is a small DC generator or rectifier supplying DC field current.

293. The generated EMF per phase is directly proportional to

A) Flux per pole and speed
B) Current
C) Resistance
D) Frequency only
Answer: A
Explanation:
E=4.44fΦNE = 4.44 f Φ N; since f∝Nf \propto N, E∝Φ×NE \propto Φ \times N.

294. If frequency increases while maintaining flux constant, EMF

A) Increases
B) Decreases
C) Remains same
D) None
Answer: A
Explanation:
EMF E=4.44fΦNE = 4.44 f Φ N; directly proportional to frequency.

295. If field current increases, terminal voltage

A) Increases
B) Decreases
C) Remains same
D) Becomes zero
Answer: A
Explanation:
Higher field current → more flux → higher induced EMF → higher voltage.

296. For parallel operation, alternators must have same

A) Frequency
B) Phase sequence
C) Voltage
D) All of these
Answer: D
Explanation:
Synchronizing requires matching voltage, frequency, and phase sequence.

297. If alternator load changes, frequency

A) May vary slightly
B) Remains constant
C) Decreases greatly
D) Increases linearly
Answer: A
Explanation:
Governor adjusts prime mover torque, but slight transient frequency variations may occur.

298. Synchronous impedance method generally gives

A) Higher regulation
B) Lower regulation
C) Accurate result
D) Zero regulation
Answer: A
Explanation:
EMF method overestimates voltage regulation due to ignoring saturation effects.

299. The alternator efficiency under unity power factor is

A) Maximum
B) Minimum
C) Zero
D) Constant
Answer: A
Explanation:
At unity PF, reactive current component is zero, reducing losses → max efficiency.

300. When alternator supplies leading PF load, terminal voltage

A) Increases
B) Decreases
C) Remains same
D) None
Answer: A
Explanation:
Leading PF load causes a voltage rise due to reactive power behavior in circuit

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