## electrical

### Synchronous Motor MCQ Questions and Answers Page - 3

1.
 A three-phase, salient pole synchronous motor is connected to an infinite bus. It is operated at no load at normal excitation. The field excitation of the motor is first reduced to zero and then increased in the reverse direction gradually. Then the armature current A Increases continuously B First increases and then decreases steeply C First decrease and then increases steeply D Remains constant Answer : B Explanation :
2.
 A 3-phase synchronous motor is operating at a given load. If an increase in excitation reduces the armature current, it can be concluded that the motor is A Operating at lagging power factor and absorbing reactive power from the mains B Operating at leading power factor and delivering reactive power to the mains C Either (a) and (b) D None of the above Answer : A Explanation :
3.
 A synchronous motor installed at end of a transmission line is operating at lagging power factor. With the fall in supply voltage, the power factor of the synchronous motor will A Go down B Improve C Remain unchanged D None of the these Answer : B Explanation :
4.
 A synchronous motor operates at 0.8 pf lagging. If the filed current of the motor is continuously increased A The power factor decreases up to a certain value of the filed current and there after it increases. B The power factor increases up to a certain value of field current and there after it decreases. C The armature current decreases upto a certain value of field current and their after it increases D Both (b) and (c) Answer : D Explanation :
5.
 Stability of a synchronous motor_____ with the increase in excitation A Increases B Decreases C Remains unaffected D None of these Answer : A Explanation :
6.
 Armature reaction in a synchronous motor at rated voltage and zero power factor (lead) is A Magnetizing B Cross magnetizing C Both magnetizing and cross magnetizing D Demagnetizing Answer : D Explanation :
7.
 The torque angle of a synchronous machine operating from a constant voltage bus, is usually defined as the space angle between A Rotor mmf wave and stator mmf wave B Rotor mmf wave and resultant flux density wave C Stator mmf wave and resultant flux density wave D Stator mmf wave and resultant mmf wave Answer : A Explanation :
8.
 A synchronous motor will deliver maximum power when A Load angle is equal to internal angle B Input power factor is unity C Load angle is 450 D Load angle is 00 Answer : A Explanation :
9.
 A synchronous motor with negligible armature resistance runs at a load angle of 200 at the rated frequency. If supply frequency is increased by 10%, keeping other parameters constant, the new load angle will be A 160 B 180 C 200 D 220 Answer : C Explanation :
10.
 The power developed by a synchronous motor for constant supply voltage and constant excitation will be maximum when load angle    is A 900 B 00 C Slightly less than 900 D Slightly more than 900 Answer : C Explanation :
11.
 For a given developed power, a synchronous motor operating from a constant voltage and constant frequency supply, will draw the minimum and maximum armature currents. Imin and Imax respectively, corresponding to A Imin at unity pf, but Imax at zero pf B Imax at unity pf, but Imin at zero pf C Both Imin and Imax at unity pf D Both Imin and I max at zero pf Answer : A Explanation :
12.
 The V-Curve of synchronous motor shows the relation between which of the parameters? A Armature current and field current B Power factor and field current C Armature current and torque D Torque and field current Answer : A Explanation :
13.
 An inverted V-curve of synchronous motor shows the variation of A Power factor and dc excitation at constant load B Supply voltage and field current at constant excitation C Power factor and supply voltage during hunting D Supply voltage and excitation current at constant load Answer : A Explanation :
14.
 Keeping the excitation voltage same, the load on the motor is increased such that the motor current increases by 20%. The operating power factor will become A 0.995 lagging B 0.995 leading C 0.791 lagging D 0.848 leading Answer : D Explanation :
15.
 Synchronous capacitor is A An ordinary static capacitor bank B An overexcited synchronous motor driving mechanical load C An overexcited synchronous motor without mechanical load D None of the above Answer : C Explanation :
16.
 An induction motor and synchronous motor are connected to a common feeder line. To operate the feeder line at a unity pf, the synchronous motor should be A Under excited B Overexcited C Normally excited D Disconnected from the common terminals Answer : B Explanation :
17.
 Synchronous condensers, when operated at power factor ranging from lagging through unity to leading for voltage control, are called the A Voltage boosters B Synchronous reactors C Mechanical synchronous D None of the above Answer : B Explanation :
18.
 The power factor of a synchronous motor A Improves with increase in excitation and may even become leading at higher excitation B Decreases with increase in excitation C Is independent of excitation D Increases with loading for a given excitation Answer : A Explanation :
19.
 A 3-phsae induction motor draws 1,000 kVA at a pf of 0.8 lag. A synchronous condenser is connected in parallel to draw an additional 750kVA at a power factor of 0.6 led. The pf of the total load supplied by the mains is A Unity B 0.707 lead C 0.6 lag D Zero Answer : A Explanation :
20.
 synchronous condenser means A a synchronous motor with capacitor connected across terminals to improve pf B a synchronous motor operating at full load with leading pf C An overexcited synchronous motor partially supplying mechanical load, and also improving pf of the system to which it is connected D An overexcited synchronous motor operating at no load with leading pf used in large power stations for improvement of pf. Answer : D Explanation :
21.
 Which of the following devices can be used as phase advancer? A 3-phase induction motor squirrel cage type B 3-phase induction motor slip ring type C Synchronous motor working at leading power factor D Synchronous motor working at lagging power factor Answer : C Explanation :
22.
 The phenomenon of oscillation in of the rotor of a synchronous motor about its equilibrium position corresponding to new load on sudden throwing off or increasing of load is called the A Swining B Crawling C Hunting D None of these Answer : C Explanation :
23.
 A 3-phase synchronous motor hunts due to A Fluctuating load B Fluctuating supply voltage C Excessive filed current D Either fluctuating load or fluctuating supply voltage Answer : D Explanation :
24.
 In a synchronous motor hunting can be reduced to minimum possible by A Providing damper winding in the rotor pole faces B Using a flywheel C Designing the motor for adequate synchronizing power D Any of the above methods Answer : D Explanation :
25.
 The damper windings also called the squirrel cage windings or damper girds A Are provided in a synchronous motor to make itself starting and to prevent hunting B Consists of short-circuited copper bars embedded in the field pole faces of a synchronous motor C Are provided on the stator of a synchronous motor for improving the power factor D Both (a) and (b) Answer : D Explanation :
26.
 In a synchronous machine, damper windings are used to A Help in starting as a motor B Run it as an induction motor C Help in starting as a motor and to reduce hunting D Increase efficiency Answer : C Explanation :
27.
 During hunting of synchronous motor A Negative phase sequence currents are generated B Harmonics are developed in the armature circuit C Damper bar develops torque D Field excitation increases Answer : C Explanation :
28.
 When a synchronous motor is running at synchronous speed the damper winding produces A Damping torque B Eddy current torque C Torque aiding the developed torque D No torque Answer : D Explanation :
29.
 In a synchronous motor when the rotor speed exceeds the synchronous speed during hunting A The damper bars develop synchronous motor torque B The damper bars develop induction motor torque C The damper bars develop induction generator torque D Harmonics are produced in the armature circuit Answer : C Explanation :
30.
 The mechanical displacement of the rotor with respect to stator, in poly phase multipolar synchronous motor running at full load is of the order of A Zero degree B Two degree C Five degree D Ten degree Answer : C Explanation :
31.
 Power factor of a synchronous motor is unity when A The armature current is maximum B The armature current is minimum C The armature current is zero D None of the above Answer : B Explanation :
32.
 Change of D.C excitation of a synchronous motor changes A Applied voltage of the motor B Motor speed C Power factor to power drawn by the motor D All of the above Answer : C Explanation :
33.
 While starting a synchronous motor by induction motor action, field winding is usually A Connected to D.C supply B Short-circuited by low resistance C Kept open circuited D None of the above Answer : B Explanation :
34.
 Which of the following motors will be used in electric clocks? A D.C shunt motor B D.C series motor C A.C induction motor D A.C synchronous motor Answer : D Explanation :
35.
 If in a synchronous motor, driving mechanical load and drawing current voltage supply, its field excitation is increased, then its power factor A Become more B Become less C Remain constant D None of the above Answer : B Explanation :
36.
 A synchronous motor installed at the receiving and substation operates with such an excitation that takes power at lagging power factor. Now if the applied voltage of the synchronous motor goes down, the power factor of the synchronous motor will A Remain same’ B Go down C Improve D None of the above Answer : C Explanation :
37.
 While starting a salient pole synchronous motor by induction motor action and connecting field discharge resistance across field, starting and accelerating torque is produced by A Induction motor torque in field winding B Induction motor torque in damper winding C Eddy current and hysteresis torque in pole faces D All of the above methods Answer : D Explanation :
38.
 Armature of a synchronous machine is kept fixed because A Of reducing number of slip rings on the rotor B Armature is associated with large power as compared to the field circuits C Of difficulty of providing high voltage insulation on rotor D All of the above reasons Answer : D Explanation :
39.
 If excitation of a synchronous motor running with a constant load is running with a constant, load is decreased from its normal value, ignoring effects of armature reaction, it leads to A Increase in both armature current and power factor angle B Increase in back e.m.f but decrease in armature current C Increase in both armature current and power factor which is lagging D Increase in torque angle but decrease in back e.m.f Answer : A Explanation :
40.
 When a 3-phase synchronous generator is supplying a zero-power factor lagging loads, the armature field affects the main field in the following ways A Augments it directly B Directly opposes it C Cross-magnetize it D None of the above Answer : B Explanation :
41.
 Stability of a synchronous machine A Decrease with increase in its excitation B Increases with increase in its excitations C Remains unaffected with increase in excitation D Any of the above Answer : B Explanation :
42.
 The power factor of a synchronous motor is better than that of induction motor because A Stator supply is relieved of responsibility of producing magnetic field B Mechanical load on the motor can be adjusted C Synchronous motor runs at synchronous speed D Synchronous motor has large air gap Answer : A Explanation :
43.
 If in a synchronous motor is running with normal excitation. When the load is increased, the armature current drawn by its increase because A Speed of the motor is reduced B Power factor is decreased C Will remain unchanged D None of the above Answer : B Explanation :
44.
 A synchronous motor, driving a given mechanical load and drawing current at a leading power factor from constant voltage supply its field excitation is increased, its power factor A Will become more B Will become less C Will remain unchanged D None of the above Answer : D Explanation :
45.
 If one –phase of a 3-phase synchronous motor is short circuited, motor A Will refuse to start B Will overheat in spots C It continues to run at same speed D It runs at a very high speed Answer : A Explanation :
46.
 If the field current of an unloaded salient-pole synchronous motor gets suddenly open-circuited, then A It runs at a slower speed B The motor stops C It continues to run at the same speed D It runs at a very high speed Answer : B Explanation :
47.
 In which of the following motors the stator and rotor field rotates simultaneously? A D.C motor B Universal motor C Synchronous motor D Induction motor Answer : C Explanation :
48.
 The speed of a synchronous motor A Increase as the load increase B Decrease as the load decreases C Always remain constant D None of the above Answer : C Explanation :
49.
 A rotary converter can also be run as a A D.C shunt motor B D.C compound motor C Induction motor D Synchronous motor Answer : D Explanation :
50.
 The maximum speed variation in a 3-phase synchronous motor is A 10% B 4% C 2% D Zero Answer : D Explanation :
51.
 Which of the following resistance can be measured by conducting insulation resistance test on a synchronous motor? A Phase to phase winding resistance B Stator winding to earthed frame C Rotor winding to earthed shaft D All of the above Answer : D Explanation :
52.
 Due to which of the following reasons a synchronous motor fails to pull into synchronism after applying D.C field current? A High field current B Low short circuit ratio C High core losses D Low field current Answer : D Explanation :
53.
 In a synchronous motor, the maximum power developed depends on all of the following except A Rotor excitation B Maximum value of coupling angle C Direction of rotation D Supply voltage Answer : C Explanation :
54.
 In a 3-phase synchronous motor, the negative phase sequence exists when the motor is A Supplied with unbalanced voltage B Under-loaded C Over-loaded D None of the above Answer : A Explanation :
55.
 In asynchronous motor, damper windings are provided on A Stator frame B Rotor shaft C Pole faces D None of the above Answer : C Explanation :
56.
 The induced e.m.f in a synchronous motor working on leading power factor will be A More than the supply voltage B Less than the supply voltage C Equal to the supply voltage D None of the above Answer : A Explanation :
57.
 The effect of increasing the load on a synchronous motor running with normal excitation is to A Decrease both armature current and power factor B Decrease armature current but increase power factor C Increase armature current but decrease power factor D Increase both its armature current and power factor Answer : C Explanation :
58.
 The net armature voltage of a synchronous motor is equal to the A Vector sum of Eb and V B Arithmetic sum of Eb and V C Arithmetic difference of Eb and V D Vector difference of Eb and V Answer : D Explanation :
59.
 The ratio of starting torque to running torque in a synchronous motor is A Zero B One C Two D Infinity Answer : A Explanation :