301 MCQ Questions on Synchronous Machine

151. Find the stator winding voltage of a three phase star connected 5500 V synchronous motor having synchronous reactance per phase of 20 Ω.

(a) 3175.4 V
(b) 5500 V
(c) 9526 V
(d) 3180 V [MPJE – 2016]

152. What is the value of load angle at a point where power output of a salient pole synchronous generator is maximum? [UPRVUNL – 2014]

(a) 0°
(b) 45°
(c) 120°
(d) Less than 90° but not fixed

153. On which of the following parameters does the power factor of an alternator depend? [UPRVUNL 2014]

(a) Load
(b) Speed of rotor
(c) Core losses
(d) Armature losses

154. The rotor of an alternator has

(a) Four slip rings
(b) Three slip rings
(c) Two slip rings
(d) No slip rings [DMRC – 2015]

155. If the speed of rotation of a dynamo is doubled, then the induced emf will ………

(a) Become half.
(b) Become double.
(c) Become four times.
(d) Remain unchanged. [LMRC – 2015]

156. An exciter for a turbo generator is a ……

(a) Separately excited generator
(b) Shunt generator
(c) Series generator
(d) Compound generator [LMRC – 2015]

157. The seventh space harmonic in the M.M.F. developed by balance fundamental-frequency armature currents rotates at X-times the synchronous speed with respect to poles, while

(a) X = 5/7
(b) X = 1/7
(c) X=1
(d) X = 8/7

158. The capacity of large turbo-generators varies from

(a) 20 to 100 MW
(b) 50 to 30 MW
(c) 70 to 400 MW
(d) 100 to 650 MW

159. The integration of a three phase Alternator with the infinite grid requires which quantities to be same?

(a) Voltage
(b) Frequency
(c) Phase sequence
(d) All of the above [UPSSSC – 2015]

160. The phase angle introduced between the induced emf phasor, E and terminal voltage phasor, V during the load condition of an alternator is called ……..

(a) Electric angle
(b) Mechanical angle
(c) Load angle
(d) None of the above [MP JE – 2016]

161. The most common type of prime mover used for low speed alternators is …….

(a) Steam turbine
(b) Petrol engine
(c) Hydraulic turbine
(d) Diesel engine

162. The power factor of an alternator is determined by its

(a) Prime mover
(c) Excitation
(b) Speed
(d) Load

163. If the excitation of an alternator operating in parallel with other alternators decreased, its:

(a) Power factor becomes more leading.
(b) Output kW will change.
(c) Power factor becomes more lagging.
(d) Power factor becomes unity. [DMRC 2016]

164. Drop in alternator frequency is corrected by:

(a) Damper winding.
(b) Increased prime mover output.
(c) Automatic voltage regulator.
(d) None of these. [DMRC 2016]

165. Two identical alternators are running in parallel and carry equal loads. If the excitation of one alternator is increases without changing the steam supply then

(a) KVAR supplied by it would decrease
(b) power factor will increase
(c) it will keep supplying the same load
(d) kVA supplied by it will decrease [LMRC – 2015]

166. Armature reaction in alternator primarily affects:

(a) Terminal voltage per phase.
(b) Frequency of armature current.
(c) Generated voltage per phase.
(d) Rotor speed. [LMRC – 2015]

167. For successful parallel operation of two alternators it is desirable that both have:

(a) Low reactance in comparison to resistance.
(b) Same resistance.
(c) Same reactance.
(d) High reactance in comparison to resistance.

168. Because of which one of the following reasons the phasor addition of stator and rotor mmf is possible in a cylindrical rotor synchronous machine?

(a) Two mmfs are rotating in opposite direction .
(b) Two mmfs are rotating in same direction at different speed.
(c) Two mmfs are stationary with respect to each other.
(d) One mmf is stationary and the other mmf is rotating.

169. An alternator A is connected to bus bar and another alternator B supplies its own constant impedance load. If field excitation is changed, then

 (a) pf of both A and B changes.
(b) pf of A changes but pf of B remains unchanged.
(c) Armature current of both A and B changes.
(d) terminal voltage of both A and B changes.

170. If both the generator active and reactive power are doubled, kVA loading on the generator …….

(a) Quadruples
(b) Doubles
(c) Increases by a factor of √2
(d) Remains the same

171. Average value of flux density wave over one pole pitch is…. times the maximum flux density.

(a) 2
(b) 2/π
(c) π /2
(d) 2π

172. Which one of the following is NOT a necessary condition to be satisfied for synchronizing an incoming alternator to an already operating alternator?

(a) Same voltage.
(b) Same frequency.
(c) Same prime mover speed.
(d) Same phase sequence.

173. If a P-pole alternator rotates N completes rotations, it produces …………. cycles of generated voltage. (As N in the number of rotations made per second) [UPPCL (JE) 2016]

(a) NP/2
(b) P/2N
(c) 2N/P
(d) 2NP

174. Magnetic field poles are the ……… part of an alternator. [UPPCL (JE) – 2016]

(a) Static
(b) Rotary
(c) Reciprocating
(d) Circuital

175. At lagging loads, the effect of armature reaction in an alternator is .………

(a) Neutralizing
(b) Cross magnetizing
 (c) Magnetizing
(d) Demagnetizing [UPPCL (JE) 2016]

176. By matching which of the following individual machines, the load frequency control is achieved? [UPPCL (JE) 2016]

(a) Reactive power
(b) Turbine inputs
(c) Generated voltage
(d) Turbine and generator ratings

177. Speed of synchronous motor on increasing load

(a) Becomes infinity.
(b) Remains constant.
(c) Increases.
(d) Decreases. [UPPCL (JE) – 2016]

178. A 3-phase synchronous motor connected to AC mains, is running at full load and unity power factor. If its shaft load is reduced to half, with field current held constant, its new power factor will be ……….

(a) Lagging
(b) Leading
(c) Unity
(d) Dependent on machine parameters [Coal India 2017]

179. Which of the following motor is not self starting? [UJVNL-2016]

(a) Squirrel cage Induction motor
(b) Wound rotor Induction motor
(c) Synchronous motor
(d) D.C. series motor

180. A synchronous compensator is a synchronous motor running:

(a) Without a mechanical load and variable excitation.
(b) Without a mechanical load and fixed excitation.
(c) With a mechanical load and fixed excitation.
(d) With a mechanical load and variable excitation. [UPRVUNL 2016]

181. In a synchronous motor the load angle and internal angle are denoted by and θ respectively. The maximum torque or pull out torque of a synchronous motor occurs when:

(a) α + θ = 0°
(b) α – θ = 0°
(c) α + θ = 90° [UPRVUNL JE 2016]

182. The magnitude of armature current in synchronous motor operation:

(a) Has small value and leads for low excitation .
(b) has large value and lags for low excitation.
(c) Has large value and leads for low excitation.
(d) Has small value and lags for low excitation. [UPRVUNL JE – 2016]

183. The unexcited single phase synchronous motors are of:

(a) Neither reluctance nor hysteresis type.
(b) Reluctance type.
(c) Both reluctance and hysteresis type.
(d) Hysteresis type.

184. Hunting of a synchronous motor may be caused due to:

(a) Pulsating supply frequency & Varying load conditions.
(b) Neither pulsating supply frequency nor varying load conditions.
(c) Pulsating supply frequency.
(d) Varying load conditions.

185. The synchronous motor is:

(a) Not a self starting machine and does not run at synchronous speed.
(b) A self starting machine and does not run at synchronous speed.
(c) Not a self starting machine and runs at synchronous speed.
(d) A self starting machine and runs at synchronous speed.

186. A synchronous machine has P number of poles, find the relation between electrical angular momentum (ω_e) and mechanical angular momentum (ω_m).

(a) ω_e = ω_m
(b) ω_e = P x ω_m
(c) ω_e = P/2 X ω_m
(d) ω_e = 2/P x ω_m

187. Which motor is generally used in rolling mills, paper and cement industries?

(a) D.C. shunt motor.
(b) Double squirrel cage motor.
(c) Slip-ring induction motor.
(d) Three-phase synchronous motor.

188. Damping winding in a synchronous motor:

(a) Improves power factor of the motor.
(b) Increase hunting of the motor.
(c) Reduces windage losses.
(d) Increases starting torque. [NMRC – 2017]

189. When over excited, synchronous motor has:

(a) Leading power factor.
(b) Zero power factor.
(c) Unity power factor.
(d) Lagging power factor. [DMRC JE- 2017]

190. The machine used for power factor correction is:

(a) Induction generator
(b) Universal motor
(c) Stepper motor
(d) Synchronous motor [DMRC JE- 2017]

191. Overexcited synchronous motors are used for

(a) Improving power factor of power systems with more inductive load.
(b) Reducing the copper losses of inductive load.
(c) Reducing power factor of power systems with more inductive load.
(d) Maintaining the power factor to zero. [LMRC JE – 2016]

192. In case the field of a synchronous motor is under excited, the power factor will be:

(a) Leading
(b) Lagging
(c) Zero
(d) Unity [UJVNL-2016]

193. An unexcited single phase synchronous motor is:

(a) Reluctance motor.
(b) Repulsion motor.
(c) Universal motor.
(d) A.C. series motor. [UJVNL-2016]

194. A synchronous motor is said to be ‘floating’ when it operates:

(a) On no load and without loss.
(b) On constantly varying load.
(c) On pulsating load.
(d) On high load & variable supply voltage. [UJVNL-2016]

195. Maximum power developed in a synchronous motor occurs at a coupling angle of? [J&K JE – 2016]

(a) 120°
(b) 60°
(c) 90°
(d) 0°

196. What type of excitation is given to the rotor winding of a synchronous motor? [J&K JE 2016]

(a) DC supply at 100 – 250V
(b) AC supply at 400V
(c) Revolving field
(d) Induction from stator current

197. A three phase synchronous generator is connected to an infinite bus. If the excitation of the generator fails then it acts as a

(a) Synchronous motor.
(b) Induction motor.
(c) Induction generator.
(d) None of these.

198. A cylindrical rotor synchronous motor is switched on to the supply with its field winding shorted on itself. It will:

(a) No start.
(b) Start but not run at synchronous speed.
(c) Start as an induction motor and then run as a synchronous motor.
(d) Start and run as a synchronous motor.

199. For 100% excitation in synchronous motor, the relationship between back emf E and applied voltage V is

(a) E > V
(b) E < V
(c) E = V
(d) E≥ V [Uttrakhand AE -2013]

200. The power factor of the input power to a synchronous motor is adjusted by adjusting

(a) Magnitude of excitation.
(b) Magnitude of armature reaction.
(c) Number of poles.
(d) None of these.