Thursday, October 23, 2025

The alternator efficiency under resistive load is

 

201. If the load on an alternator is increased, its power angle (δ)

A) Decreases
B) Increases
C) Remains constant
D) Becomes zero
Answer: B
Explanation:
As load increases, mechanical torque increases → rotor shifts forward → load angle δ increases.

202. The waveform of generated voltage depends upon

A) Armature resistance
B) Field flux distribution
C) Speed of rotation
D) Load current
Answer: B
Explanation:
Sinusoidal flux distribution in the air gap produces a sinusoidal EMF.

203. The value of generated EMF in an alternator is proportional to

A) Speed only
B) Flux only
C) Both speed and flux
D) Armature current
Answer: C
Explanation:
E=4.44fΦNE = 4.44 f Φ N, and f∝Nf ∝ N, hence E depends on both speed and flux.

204. The direction of rotation of an alternator can be reversed by

A) Reversing field current direction
B) Reversing prime mover direction
C) Reversing supply
D) None
Answer: B
Explanation:
Reversing the direction of the prime mover reverses the rotor’s mechanical rotation.

205. The copper losses in the armature of an alternator are proportional to

A) Field current
B) Armature current squared
C) Load current
D) Power factor
Answer: B
Explanation:
Copper loss = Ia2RaI_a^2 R_a, directly proportional to the square of armature current.

206. Which of the following causes voltage waveform distortion in an alternator?

A) Harmonics
B) DC excitation
C) Constant load
D) Balanced load
Answer: A
Explanation:
Harmonics cause voltage waveform distortion from the ideal sinusoidal form.

207. The field poles in a salient pole alternator are generally

A) Narrow and long
B) Large and projecting
C) Cylindrical
D) Smooth surfaced
Answer: B
Explanation:
Salient pole machines have large, projecting poles suitable for low-speed applications.

208. The term “pitch factor” is associated with

A) Short-pitching of coils
B) Speed variation
C) Flux distribution
D) Armature resistance
Answer: A
Explanation:
Pitch factor Kp=cos⁡(α/2)K_p = \cos(\alpha/2), accounts for coil short-pitching effects on generated EMF.

209. If the alternator is under-excited, it supplies

A) Lagging reactive power
B) Leading reactive power
C) Only real power
D) No power
Answer: B
Explanation:
Under-excited alternator behaves like an inductive load → supplies leading reactive power.

210. The advantage of short-pitch coils is

A) Reduced harmonics
B) Increased voltage
C) More heating
D) Poor waveform
Answer: A
Explanation:
Short-pitching reduces higher-order harmonics → improves voltage waveform.

211. In alternators, the function of a damper winding is to

A) Prevent hunting
B) Provide excitation
C) Increase output
D) Reduce iron losses
Answer: A
Explanation:
Damper windings suppress oscillations (hunting) during load fluctuations.

212. In a synchronous generator, the terminal voltage leads the internal emf at

A) Unity PF
B) Lagging PF
C) Leading PF
D) Zero PF
Answer: C
Explanation:
At leading PF, the internal emf (E) lags terminal voltage (V).

213. The voltage regulation of a synchronous generator is determined by

A) Open circuit and short circuit tests
B) Direct loading
C) Efficiency test
D) None
Answer: A
Explanation:
Voltage regulation is obtained using O.C.C. and S.C.C. tests and phasor diagrams.

214. The short-circuit ratio (SCR) of an alternator is the reciprocal of

A) Synchronous reactance
B) Synchronous impedance
C) Leakage reactance
D) Armature resistance
Answer: A
Explanation:
SCR = 1 / (Synchronous Reactance). High SCR → better voltage regulation.

215. A low short-circuit ratio means

A) High stability
B) Poor voltage regulation
C) High voltage regulation
D) High stability and good PF
Answer: C
Explanation:
Low SCR → high synchronous reactance → poor voltage regulation and low stability.

216. In alternator parallel operation, frequency control is done by

A) Governor
B) Exciter
C) AVR
D) Synchronizing lamps
Answer: A
Explanation:
Governor adjusts prime mover speed, controlling frequency.

217. The exciter used in modern alternators is usually

A) AC exciter with rectifier
B) DC generator
C) Separate alternator
D) Battery
Answer: A
Explanation:
Modern alternators use brushless exciters (AC exciter + rectifier).

218. The power developed by a salient pole alternator includes

A) Fundamental component only
B) Direct and quadrature axis components
C) Cross magnetizing component
D) None
Answer: B
Explanation:
Total power P=EVXdsin⁡δ+V22(1Xq−1Xd)sin⁡2δP = \frac{EV}{X_d}\sinδ + \frac{V^2}{2}\left(\frac{1}{X_q} - \frac{1}{X_d}\right)\sin2δ.

219. The frequency of an alternator is kept constant by

A) Changing excitation
B) Keeping speed constant
C) Changing field resistance
D) Changing load
Answer: B
Explanation:
Since f=PN120f = \frac{PN}{120}, constant speed ensures constant frequency.

220. The generated voltage in an alternator is proportional to

A) Field current
B) Speed
C) Both A and B
D) Load
Answer: C
Explanation:
Generated voltage E∝ΦfE ∝ Φf, and Φ∝IfΦ ∝ I_f, f∝Nf ∝ N.

221. A two-pole alternator runs at 3000 rpm. The frequency is

A) 25 Hz
B) 50 Hz
C) 100 Hz
D) 150 Hz
Answer: B
Explanation:
f=PN120=2×3000120=50 Hzf = \frac{PN}{120} = \frac{2×3000}{120} = 50 \text{ Hz}

222. The field system of an alternator is placed on the rotor because

A) Less weight
B) Easier cooling
C) Easier excitation with slip rings
D) All of these
Answer: D
Explanation:
Rotor field winding is DC-excited → lighter and easier to insulate, cool, and control.

223. The stator of an alternator is laminated to

A) Reduce eddy current losses
B) Increase flux density
C) Reduce hysteresis loss
D) Reduce copper loss
Answer: A
Explanation:
Lamination minimizes eddy current losses in the stator core.

224. The damper winding is made of

A) Copper bars
B) Aluminium bars
C) Both A and B
D) Steel laminations
Answer: C
Explanation:
Damper bars (copper or aluminium) are embedded in pole faces and short-circuited.

225. When the field current of an alternator increases, its terminal voltage

A) Increases
B) Decreases
C) Remains constant
D) Drops to zero
Answer: A
Explanation:
More field current → more flux → higher induced EMF → terminal voltage rises.

226. The leakage reactance of an alternator depends on

A) Slot shape
B) Tooth width
C) Air gap
D) All of these
Answer: D
Explanation:
Leakage flux path depends on slot geometry, tooth design, and air-gap size.

227. In alternator testing, the open-circuit characteristic (OCC) is drawn between

A) Field current and armature voltage
B) Armature current and voltage
C) Speed and voltage
D) Frequency and current
Answer: A
Explanation:
OCC shows variation of generated EMF with field current at constant speed.

228. The load angle δ in a synchronous generator lies between

A) 0° and 90°
B) 90° and 180°
C) 0° and 180°
D) 0° and 270°
Answer: A
Explanation:
Stable operation occurs only when δ<90°δ < 90°.

229. The term "synchronous impedance method" is also called

A) EMF method
B) MMF method
C) Zero power factor method
D) Direct loading method
Answer: A
Explanation:
Voltage regulation by EMF method is called the synchronous impedance method.

230. The main limitation of the EMF method is

A) Overestimates voltage regulation
B) Underestimates voltage regulation
C) Complex calculations
D) Low accuracy
Answer: A
Explanation:
EMF method assumes linear magnetic circuit → overestimates voltage regulation.

231. The MMF method gives

A) Low value of regulation
B) High value
C) Exact value
D) None
Answer: A
Explanation:
MMF method underestimates voltage regulation.

232. The zero power factor (ZPF) method gives

A) Most accurate regulation
B) Least accurate
C) Overestimated result
D) No result
Answer: A
Explanation:
ZPF method accounts for saturation → gives most accurate regulation.

233. Alternator ratings are higher for

A) Salient pole
B) Cylindrical rotor
C) DC generators
D) Induction motors
Answer: B
Explanation:
Cylindrical rotors can run at high speeds → higher ratings.

234. The speed of alternator connected to an infinite bus is

A) Constant
B) Variable with load
C) Depends on excitation
D) Depends on voltage
Answer: A
Explanation:
Speed is locked to system frequency → constant.

235. The armature reaction in an alternator mainly distorts

A) Stator flux
B) Rotor flux
C) Field winding
D) None
Answer: B
Explanation:
Armature reaction distorts rotor field flux, affecting generated EMF.

236. The voltage regulation of alternator operating at unity PF is

A) Minimum
B) Maximum
C) Negative
D) Zero
Answer: A
Explanation:
Voltage drop minimal at unity PF → lowest regulation.

237. The excitation control in alternator affects

A) Power factor
B) Frequency
C) Speed
D) Prime mover torque
Answer: A
Explanation:
Excitation adjustment changes reactive power → affects PF.

238. The frequency of generated voltage in alternator changes with

A) Load
B) Speed
C) Excitation
D) PF
Answer: B
Explanation:
Frequency f=PN120f = \frac{PN}{120} → depends only on speed.

239. The number of slip rings in a three-phase alternator with stationary armature is

A) None
B) Two
C) Three
D) Six
Answer: B
Explanation:
Two slip rings carry DC to the rotor field winding.

240. The purpose of a pilot exciter is to

A) Provide initial excitation
B) Drive alternator
C) Reduce losses
D) Improve PF
Answer: A
Explanation:
Pilot exciter provides initial field current to the main exciter.

241. For constant load, if excitation increases, the power factor

A) Improves
B) Becomes leading
C) Becomes lagging
D) Remains same
Answer: C
Explanation:
More excitation → alternator supplies lagging reactive power → PF becomes lagging.

242. The direction of rotation of an alternator is verified by

A) Synchronoscope
B) Frequency meter
C) Phase sequence indicator
D) Tachometer
Answer: C
Explanation:
Phase sequence indicator checks phase order and rotation direction.

243. The alternator’s voltage regulation is better when

A) Synchronous reactance is small
B) Armature resistance is high
C) PF is low
D) Field current is small
Answer: A
Explanation:
Low XsX_s → less voltage drop → better voltage regulation.

244. The alternator efficiency under resistive load is

A) Maximum
B) Minimum
C) Same as at lagging PF
D) Same as leading PF
Answer: A
Explanation:
At unity PF (resistive load), copper losses are minimal → maximum efficiency.

245. The critical speed of alternator refers to

A) Speed at which vibration occurs
B) Maximum speed
C) Minimum speed
D) Rated speed
Answer: A
Explanation:
Critical speed causes mechanical resonance → excessive vibration.

246. The main advantage of using higher number of poles is

A) Lower speed for same frequency
B) Higher frequency
C) Higher voltage
D) Reduced flux
Answer: A
Explanation:
More poles → lower speed required to generate same frequency.

247. For hydroelectric plants, alternators are usually

A) Salient pole type
B) Cylindrical rotor type
C) Induction type
D) None
Answer: A
Explanation:
Low-speed hydraulic turbines require salient pole alternators.

248. In large alternators, cooling is achieved by

A) Air
B) Hydrogen
C) Water
D) Both B and C
Answer: D
Explanation:
Large alternators use hydrogen or water cooling for efficient heat dissipation.

249. The field current required to generate rated voltage on open circuit is called

A) Normal excitation
B) Rated excitation
C) Critical excitation
D) No-load excitation
Answer: D
Explanation:
Field current needed to produce rated voltage on open circuit = no-load excitation.

250. A 4-pole, 50 Hz alternator runs at

A) 1500 rpm
B) 3000 rpm
C) 750 rpm
D) 1000 rpm
Answer: A
Explanation:
N=120fP=120×504=1500 rpmN = \frac{120f}{P} = \frac{120×50}{4} = 1500 \text{ rpm}

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