 ## electrical

### Synchronous Motor MCQ Questions and Answers Page - 2

1.
 The phase of emf induced in the armature of a synchronous motor (Eb) depends upon A Rotor speed B Load C Both load and speed D None of the above Answer : Select Correct Ans Explanation :
2.
 In a synchronous motor, the synchronizing power come into action when A Rotor speed either exceeds or falls below the synchronous speed B Rotor speed is equal to synchronous speeds C Rotor speed falls  below the synchronous speed D Rotor speed exceeds the synchronous speed Answer : Select Correct Ans Explanation :
3.
 Synchronizing power  of a synchronous motor varies A Directly  as synchronous reactance Xs B Directly as the square of the synchronous reactance Xs2 C Inversely as the synchronous reactance 1/Xs D None of the above Answer : Select Correct Ans Explanation :
4.
 The coupling  angle or load angle of a synchronous motor is defiend as the space angle between the A Rotor and stator poles of the same polarity B Rotor and stator poles of opposite polarity C Rotor and stator teeth D None of the above Answer : Select Correct Ans Explanation :
5.
 The torque angle is defined as the space angle between A Stator field axis and resultant field axis B Rotor field axis and resultant field axis C Stator field axis and rotor field axis D Stator field axis and mutual field axis Answer : Select Correct Ans Explanation :
6.
 The value of load angle for a synchronous motor depends mainly upon its A Excitation B Load C Speed D Supply voltage Answer : Select Correct Ans Explanation :
7.
 Electromagnetic torque  is present in rotating machines when A Stator winding carried current B Rotor winding carried current C both stator and rotor windings carry current D None of the above Answer : Select Correct Ans Explanation :
8.
 In a synchronous motor electromagnetic power varies A Directly as applied voltage B Directly as square of the applied voltage C Inversely as applied voltage D None of the above Answer : Select Correct Ans Explanation :
9.
 The interaction  torque in a synchronous motor depends upon A Stator field strength alone B Rotor field strength alone C Torque angle alone D All of the above i.e on stator field and rotor field strengths and torque angle Answer : Select Correct Ans Explanation :
10.
 The breakdown torque of a synchronous motor varies as A Applied voltage V B V2 C 1/V D √V Answer : Select Correct Ans Explanation :
11.
 A 3-phse synchronous motor connected to an infinite bus bar with constant excitation is driving a certain load and operating is leading power factor. If the shaft load is reduced A The load angle will increase B The load angle will decrease C Power factor will increase D Owner factor will decrease Answer : Select Correct Ans Explanation :
12.
 A synchronous motor is supplying a load at unity pf . If the load on the motor is increased keeping its excitation and terminal voltage constant , the power factor A Will remain the same B Will become leading C Will become lagging D None of the above Answer : Select Correct Ans Explanation :
13.
 When does a synchronous  motor operate with leading power factor current A While it is underexcited B While it is critically excited C While it is overexcited D While it is heavily loaded Answer : Select Correct Ans Explanation :
14.
 A synchronous generator connected to an infinite bus is over excited. Considering only he reactive power, from the point of view of the system  the machine acts as A Capacitor B Inductor C Resistor D None of the above Answer : Select Correct Ans Explanation :
15.
 A synchronous machine connected to a power system , grid bus-bar is operating as a generator . To make the machine operate as a motor the A Direction of rotation is to be reversed B Phase sequence is to be changed C Field excitation is to be decreased D Mechanical input is to be less than the losses at the shaft Answer : Select Correct Ans Explanation :
16.
 A salient  pole synchronous motor  is running on no load. If its excitation is made off, it will A Continue running at synchronous speed B Continue running but at a speed slightly less than synchronous one C Stop D None of these Answer : Select Correct Ans Explanation :
17.
 What is the effect of the field failure of salient pole synchronous motor connected with infinite bus A Reduce motor torque and speed B Not charge motor torque and speed C Stop the motor D Reduce motor torque but motor will continue to run at synchronous speed Answer : Select Correct Ans Explanation :
18.
 A salient pole synchronous motor continues running even if its excitation current is reduced to zero. This is due to A Rotating magnetic field of stator B Rotating magnetic field of rotor C Magnetization of rotor salient poles by stator magnetic field D Inter looking action between stator and rotor rotating magnetic fields Answer : Select Correct Ans Explanation :
19.
 When the excitation of normally operaitn unloaded salient pole synchronous motor suddenly gets disconnected, it continues to run as a A Charge motor B Spherical motor C Switched reluctance motor D Variable reluctance motor Answer : Select Correct Ans Explanation :
20.
 A smooth cylindrical synchronous motor will always stop when A Excitation winding gets disconnected B Load varies C Supply voltage fluctuates D Supply frequency varies Answer : Select Correct Ans Explanation :
21.
 The speed of a synchronous motor can be varied by varying its A Excitation B Supply voltage C Supply frequency D Load Answer : C Explanation :
22.
 For a 3-phase, 4-pole, 50 Hz synchronous motor the frequency, pole number and the load torque are all halved. The motor speed will be A 375 rpm B 75 rpm C 1,500 rpm D 3,000 rpm Answer : C Explanation :
23.
 A synchronous motor with comparatively large air gap gives A Higher stability limit B Higher synchronizing power making the machine less sensitive to load variations C Both (a) and (b) D None of these Answer : C Explanation :
24.
 In a 3-phase synchronous motor, the magnitude of back emf setup in the stator depends on A Rotor excitation B Supply voltage C Rotor speed D Load on motor Answer : A Explanation :
25.
 The magnitude of counter or back emf in a s synchronous motor A Is always less than supply voltage B Is always more than supply voltage C Is always equal to supply voltage D May be either equal to or less than or more than supply voltage Answer : D Explanation :
26.
 In a synchronous motor the excitation voltage on no load is approximately equal to the applied voltage. This will happen A When developed torque is maximum B With zero excitation C With 100% excitation D None of the above Answer : C Explanation :
27.
 Which motor can conveniently operate at lagging as well as leading power factor? A Squirrel cage induction motor B Wound rotor induction motor C Synchronous motor D DC shunt motor Answer : C Explanation :
28.
 At constant load, the magnitude of armature current drawn by a synchronous motor is large for A Low excitation B High excitation C Both low and high values of field excitation D 100% excitation Answer : C Explanation :
29.
 Variation in dc excitation of a synchronous motor causes variation in A Speed motor B Power factor C Armature current D Both armature current and power factor. Answer : D Explanation :
30.
 If the field of a synchronous motor is under excited, the power factor will be A Lagging B Leading C Unity D More than unity Answer : A Explanation :
31.
 A synchronous motor is operating on no load at unity power factor. If the filed current is increased, the power factor will become A Leading and the current will decrease B Lagging and the current will increase C Lagging and the current will decrease D Leading and the current will increase Answer : D Explanation :
32.
 The operating speed of a synchronous motor can be changed to new fixed value by A Changing the load B Changing the supply voltage C Changing frequency D Using brakes Answer : C Explanation :
33.
 A synchronous motor will always stop when A Supply voltage fluctuates B Load in motor varies C Excitation winding gets disconnected D Supply voltage frequency changes Answer : C Explanation :
34.
 Hunting in a synchronous motor takes place A When supply voltage fluctuates B When load varies C When power factor is unity D Motor is under loaded Answer : B Explanation :
35.
 When load on an over-excited or under excited synchronous motor is increased, rate of change of its armature current as compared with that of power factor is A More B Less C Equal D Twice Answer : B Explanation :
36.
 The rotor copper losses, in a synchronous motor are met by A D.C source B Armature input C Motor input D Supply lines Answer : A Explanation :
37.
 The maximum power developed in a synchronous motor occurs at a coupling angle of A 30° B 60° C 90° D 180° Answer : C Explanation :
38.
 When the stator windings are connected in such a fashion that the number of poles are made half, the speed of a rotor of a synchronous motor A Remain same as the original value B Decreases to half the original value C  Tends to become zero D Increases to two times the original value Answer : D Explanation :
39.
 In which of the following motors the stator and rotor magnetic field rotate at the same speed A Universal motor B Synchronous motor C Induction motor D Reluctance motor Answer : B Explanation :
40.
 Synchronizing power of a synchronous machine is A Directly proportional to the synchronous reactance B Inversely proportional to the synchronous reactance C Equal to synchronous reactance D None of the above Answer : B Explanation :
41.
 Synchronous motors are A Not-self starting B Self-starting C Essentially self-starting D None of the above Answer : A Explanation :
42.
 The standard full-load power factor rating for synchronous motors are A Zero or 0.8 leading B Unity or 0.8 lagging C Unity or 0.8 leading D Unity or zero Answer : C Explanation :
43.
 A synchronous motor running with normal excitation adjust to load increases essentially by increase in A Back e.m.f B Armature current C Power factor D Torque angle Answer : B Explanation :
44.
 A synchronous motor has better power factor as compared to that of an equivalent induction motor. This is mainly because A Synchronous motor has no slip B Stator supply is not required to produce magnetic field C Mechanical rotor on the rotor remains constant D Synchronous motor has large air gap Answer : B Explanation :
45.
 A synchronous motor working at leading power factor can be used as A Voltage booster B Phase advancer C Noise generator D Mechanical synchronizer Answer : B Explanation :
46.
 Slip rings are usually made of A Carbon or graphite B Brass or steel C Silver or gold D Copper or aluminium Answer : B Explanation :
47.
 An over-excited synchronous motor is used for A Fluctuating loads B Variable speed loads C Low torque loads D Power factor corrections Answer : D Explanation :
48.
 When the voltage is applied to a synchronous motor is increased, which of the following will reduce? A Stator flux B Pull in torque C Both (a) and (b) D None of the above Answer : C Explanation :
49.
 The speed of synchronous motor A Increases with increase in load B Decreases as load increase C Varies with power factor D Always remain constant Answer : D Explanation :
50.
 Dust and dirt accumulation on motor winding will result in A Rise in winding temperature B Increase core losses C Shorting the coil D Eddy current flow Answer : A Explanation :
51.
 The efficiency of a properly designed synchronous motor will usually fall in range A 60 to 70 % B 75 to 80% C 85 to 95% D 99 to 99.5% Answer : D Explanation :
52.
 To limit the operating temperature an electrical machine should have proper A Voltage rating B Current rating C Power factor D Speed Answer : B Explanation :
53.
 Slip-ring in a synchronous motor carry A Direct current B Alternating current C No current D All of the above Answer : A Explanation :
54.
 A synchronous machine with large air gap has A A higher value of stability limit B A small value of inherent regulation C A higher synchronizing power which makes the machine less sensitive to load variations D All of the above Answer : D Explanation :
55.
 The armature current of the synchronous motor has higher value for A High excitation only B Low excitation only C Both (a) and (b) D None of this Answer : C Explanation :
56.
 In a synchronous motor running with fixed excitation, when the load is increased three times, its torque angle becomes approximately A One-third B Twice C Thrice D Six times Answer : C Explanation :
57.
 The angle between the rotating stator flux and rotor poles and rotor is called …………. Angle A Torque B Obtuse C Synchronizing D Power factor Answer : A Explanation :
58.
 Which of the following methods is used to start a synchronous motor? A Damper winding B Star-delta stator C Damper winding on conjunction with star-delta stator D Resistance stator in the armature circuit Answer : C Explanation :
59.
 When the rotor speed, in a synchronous machine become more than the synchronous speed during hunting, the damper bar develops A Inductor motor torque B Induction generator torque C Synchronous motor torque D D.C motor torque Answer : B Explanation :
60.
 An important advantage of synchronous motor over wound induction motor is that A Its power factor may be varied at will B Its speed is independent of supply frequency C Its speed may be controlled more easily D None of the above Answer : A Explanation :