Thursday, October 23, 2025

The advantage of skewed slots in alternator armature is

 

151. The emf induced in each phase of a 3-phase alternator is displaced by

A) 60°
B) 90°
C) 120°
D) 180°
Answer: C
Explanation:
In a three-phase alternator, the three windings are placed 120° electrical apart, hence EMFs are 120° phase displaced.

152. The field winding of a synchronous generator is excited by

A) Alternating current
B) Direct current
C) Pulsating current
D) Both AC and DC
Answer: B
Explanation:
The field winding on the rotor is supplied with DC to produce a constant magnetic field.

153. The number of poles in a 1500 rpm alternator generating 50 Hz is

A) 2
B) 4
C) 6
D) 8
Answer: B
Explanation:
f=PN120⇒P=120fN=120×501500=4 polesf = \frac{PN}{120} \Rightarrow P = \frac{120f}{N} = \frac{120×50}{1500} = 4 \text{ poles}

154. When an alternator is loaded with a leading power factor load, the terminal voltage

A) Increases
B) Decreases
C) Remains same
D) Becomes zero
Answer: A
Explanation:
At leading PF, the armature reaction is magnetizing, aiding the main field → terminal voltage increases.

155. Alternators in parallel share load according to

A) Their speed
B) Their excitation
C) Their prime mover torque
D) Both excitation and prime mover torque
Answer: D
Explanation:

  • Excitation controls reactive power sharing

  • Prime mover torque controls real power sharing.

156. The reactive power output of an alternator is controlled by

A) Excitation current
B) Prime mover speed
C) Load current
D) Frequency
Answer: A
Explanation:
Excitation controls the field strength → controls reactive (kVAR) output.

157. The active power output of an alternator is controlled by

A) Load power factor
B) Excitation current
C) Prime mover input
D) Armature resistance
Answer: C
Explanation:
Increasing mechanical input increases load angle δ → increases active power output.

158. The main cause of hunting in a synchronous generator is

A) Fluctuating load
B) Poor excitation
C) Frequency variation
D) Voltage unbalance
Answer: A
Explanation:
Sudden load changes cause oscillations in rotor position (hunting).

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

A) Leakage reactance + armature reaction reactance
B) Armature resistance + leakage reactance
C) Leakage reactance only
D) None
Answer: A
Explanation:
Synchronous reactance Xs=Xl+XarX_s = X_l + X_{ar}.

160. Which test gives the armature resistance of an alternator?

A) Open circuit test
B) Short circuit test
C) Resistance test
D) Load test
Answer: C
Explanation:
Armature resistance is measured using a DC resistance test (voltmeter–ammeter method).

161. The voltage per phase of a 3-phase star-connected alternator is 230 V. The line voltage is

A) 230 V
B) 400 V
C) 460 V
D) 230/√3 V
Answer: B
Explanation:
For star connection, VL=3VP=1.732×230≈400VV_L = \sqrt{3} V_P = 1.732 × 230 ≈ 400 V.

162. When two alternators are operating in parallel, one having more excitation will

A) Supply more kW
B) Supply more kVAR
C) Supply less kVAR
D) None
Answer: B
Explanation:
More excitation → higher internal EMF → supplies more reactive (kVAR) power.

163. Synchronizing lamps connected across two alternators will be dark when

A) Frequencies are same and in phase
B) Frequencies are different
C) Phase is opposite
D) Voltage unequal
Answer: A
Explanation:
When voltage, frequency, and phase are identical, potential difference = 0 → lamps dark.

164. The power factor of an alternator at no load is usually

A) Unity
B) Leading
C) Lagging
D) Zero
Answer: C
Explanation:
Due to magnetizing current, alternator on no-load has lagging PF.

165. In a salient pole alternator, the air gap is

A) Uniform
B) Non-uniform
C) Large
D) Very small
Answer: B
Explanation:
Salient pole machines have non-uniform air gap (small along pole, large between poles).

166. The prime mover used for high-speed alternators is usually

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

167. The voltage waveform in an alternator can be improved by

A) Using short-pitched coils
B) Increasing field current
C) Using concentric windings
D) Increasing slots
Answer: A
Explanation:
Short-pitching reduces harmonics, improving the voltage waveform.

168. The field poles in an alternator are laminated to reduce

A) Eddy current losses
B) Hysteresis losses
C) Copper losses
D) Stray losses
Answer: A
Explanation:
Lamination of poles reduces eddy current losses.

169. The advantage of rotating field type alternator is

A) Less slip rings
B) More output
C) Lower speed
D) Simpler stator
Answer: A
Explanation:
Rotating field type has only two slip rings for DC excitation, simplifying construction.

170. The waveform of voltage generated in an alternator is

A) Triangular
B) Sinusoidal
C) Square
D) Sawtooth
Answer: B
Explanation:
With sinusoidal flux distribution, the generated voltage is sinusoidal.

171. Armature reaction at unity power factor is

A) Magnetizing
B) Demagnetizing
C) Cross-magnetizing
D) Both magnetizing and cross
Answer: C
Explanation:
At unity PF, the armature flux is 90° to field flux → purely cross-magnetizing.

172. Slip rings in an alternator are used to

A) Carry excitation current to the field winding
B) Carry current to armature winding
C) Supply load
D) None
Answer: A
Explanation:
Slip rings transfer DC excitation current from the stationary system to the rotating field.

173. For parallel operation, alternators must have

A) Same frequency
B) Same voltage
C) Same phase sequence
D) All of these
Answer: D
Explanation:
Conditions for synchronization: same voltage, frequency, and phase sequence.

174. The voltage regulation of a leading PF load is

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

175. The constant excitation curve of an alternator is also called

A) V curve
B) OCC
C) Load characteristic
D) Synchronous reactance curve
Answer: C
Explanation:
At constant excitation, varying load gives the load characteristic of the alternator.

176. A 4-pole alternator running at 1500 rpm generates

A) 25 Hz
B) 50 Hz
C) 100 Hz
D) 200 Hz
Answer: B
Explanation:
f=PN120=4×1500120=50 Hzf = \frac{PN}{120} = \frac{4×1500}{120} = 50 \text{ Hz}

177. The main component responsible for voltage drop under load is

A) Synchronous reactance
B) Armature resistance
C) Leakage reactance
D) Load power factor
Answer: A
Explanation:
Voltage drop due to IXsI X_s (synchronous reactance) is dominant under load.

178. The no-load induced emf in an alternator depends on

A) Speed and excitation
B) Load current
C) Armature resistance
D) Frequency only
Answer: A
Explanation:
E=4.44fΦNE = 4.44 f Φ N, both speed (f) and flux (Φ) depend on excitation.

179. Alternators are rated in

A) kW
B) kVAR
C) kVA
D) HP
Answer: C
Explanation:
Alternators are rated in kVA since their capacity depends on current and voltage, independent of power factor.

180. The terminal voltage of an alternator is affected by

A) Armature resistance
B) Armature reactance
C) Load power factor
D) All of these
Answer: D
Explanation:
All contribute to voltage drop → affects terminal voltage.

181. For high-speed alternators, which rotor is preferred?

A) Salient pole
B) Cylindrical rotor
C) Reluctance type
D) None
Answer: B
Explanation:
Cylindrical rotors are used for high-speed (turbine-driven) alternators.

182. The efficiency of an alternator is maximum when

A) Iron losses = Copper losses
B) Iron losses > Copper losses
C) Copper losses = Zero
D) Load = Full
Answer: A
Explanation:
Maximum efficiency occurs when iron losses = copper losses.

183. The load angle (δ) is

A) Angle between E and V
B) Angle between current and voltage
C) Angle between excitation and flux
D) None
Answer: A
Explanation:
Load angle δ is the phase angle between internal emf (E) and terminal voltage (V).

184. When δ increases beyond 90°, the alternator

A) Delivers more power
B) Becomes unstable
C) Remains same
D) Improves efficiency
Answer: B
Explanation:
At δ > 90°, synchronizing torque reverses → machine loses synchronism.

185. The advantage of skewed slots in alternator armature is

A) Reduces noise and cogging
B) Increases harmonics
C) Reduces EMF
D) Increases core losses
Answer: A
Explanation:
Skewed slots smooth magnetic pull → reduces noise and cogging.

186. The alternator’s speed decreases with

A) Increase in load
B) Decrease in excitation
C) Increase in frequency
D) None
Answer: D
Explanation:
Speed is constant (synchronous) — independent of load or excitation.

187. The leakage flux in an alternator

A) Produces useful EMF
B) Causes voltage drop
C) Reduces losses
D) Increases efficiency
Answer: B
Explanation:
Leakage flux induces reactance → voltage drop under load.

188. The term “synchronizing power” refers to

A) Power that maintains synchronism
B) Mechanical losses
C) Power lost due to hunting
D) None
Answer: A
Explanation:
Synchronizing power helps maintain constant load angle δ between generators in parallel.

189. The internal power factor of alternator is

A) Power factor between E and I
B) Between V and I
C) Between E and V
D) None
Answer: A
Explanation:
Internal PF = angle between E (generated emf) and I (armature current).

190. The voltage regulation of an alternator can be improved by

A) Reducing armature resistance
B) Reducing synchronous reactance
C) Improving power factor
D) All of these
Answer: D
Explanation:
All factors reduce voltage drop → better regulation.

191. The effect of armature reaction in alternator depends on

A) Load current
B) Power factor of load
C) Both A and B
D) Frequency
Answer: C
Explanation:
Magnitude depends on current; nature (magnetizing/demagnetizing) depends on PF.

192. The EMF equation neglects

A) Armature reaction
B) Leakage reactance
C) Both A and B
D) Copper loss
Answer: C
Explanation:
EMF equation assumes ideal conditions (no reaction or leakage).

193. The terminal voltage at no load is 440 V, and at full load 400 V. Voltage regulation = ?

A) 10%
B) 5%
C) 8%
D) 12%
Answer: A
Explanation:

VR=440−400400×100=10%VR = \frac{440 - 400}{400} × 100 = 10\%

194. The main function of an exciter in an alternator is

A) Supply field current
B) Regulate speed
C) Regulate frequency
D) Provide load current
Answer: A
Explanation:
Exciter provides DC excitation to the alternator’s field winding.

195. The term “infinite bus” means

A) Bus with infinite voltage
B) Bus with constant voltage and frequency
C) Bus with variable frequency
D) None
Answer: B
Explanation:
Infinite bus = system with constant voltage, frequency, and phase angle unaffected by connected machines.

196. During synchronization, if phase sequence is wrong

A) Alternator will run smoothly
B) Short circuit will occur
C) Voltage will rise
D) Frequency will change
Answer: B
Explanation:
Wrong phase sequence causes opposite phase voltages → severe short-circuit.

197. If two alternators are not synchronized properly

A) Heavy current flows
B) No current flows
C) Voltage drops to zero
D) Frequency doubles
Answer: A
Explanation:
Phase mismatch causes large circulating current → mechanical stress.

198. The alternator’s rotor copper losses depend on

A) Field current
B) Armature current
C) Load current
D) Power factor
Answer: A
Explanation:
Rotor copper losses = If2RfI_f^2 R_f → depend only on field current.

199. The load sharing between parallel alternators can be adjusted by

A) Changing field excitation and governor settings
B) Changing only excitation
C) Changing only governor
D) None
Answer: A
Explanation:
Governor controls real power, excitation controls reactive power sharing.

200. The voltage regulation of a good alternator is about

A) 25–30%
B) 15–20%
C) 5–10%
D) 1–2%
Answer: C
Explanation:
Modern alternators have 5–10% voltage regulation, depending on design and power factor.

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