McqMate
These multiple-choice questions (MCQs) are designed to enhance your knowledge and understanding in the following areas: Electrical Engineering .
| 51. |
Slip-rings in a synchronous motor carry |
| A. | direct current |
| B. | alternating current |
| C. | no current |
| D. | all of the above |
| Answer» A. direct current | |
| 52. |
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. all of the above | |
| 53. |
The armature current of the synchronous motor has higher values for |
| A. | high excitation only |
| B. | low excitation only |
| C. | both (a) and (b) |
| D. | none of the above |
| Answer» C. both (a) and (b) | |
| 54. |
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. thrice | |
| 55. |
The angle between the rotating stator flux and rotor poles is called _____ angle. |
| A. | torque |
| B. | obtuse |
| C. | synchronizing |
| D. | power factor |
| Answer» A. torque | |
| 56. |
Which of the following methods is used to start a synchronous motor ? |
| A. | Damper winding |
| B. | Star-delta starter |
| C. | Damper winding in conjunction with star-delta starter |
| D. | Resistance starter in the armature circuit |
| Answer» C. Damper winding in conjunction with star-delta starter | |
| 57. |
When the rotor speed, in a synchronous machine, becomes more than the synchronous speed during hunting, the damper bars develop |
| A. | inductor motor torque |
| B. | induction generator torque |
| C. | synchronous motor torque |
| D. | d.c. motor toque |
| Answer» B. induction generator torque | |
| 58. |
An important advantage of a synchronous motor over wound round 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. its power factor may be varied at will | |
| 59. |
The mechanical displacement of the rotor with respect to the stator, in polyphase multipolar synchronous motors running at full load, is of the order of |
| A. | zero degree |
| B. | two degrees |
| C. | five degrees |
| D. | ten degrees |
| Answer» C. five degrees | |
| 60. |
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. the armature current is minimum | |
| 61. |
Change of D.C. excitation of a synchronous motor changes |
| A. | applied voltage of the motor |
| B. | motor speed |
| C. | power factor of power drawn by the motor |
| D. | any of the above |
| Answer» C. power factor of power drawn by the motor | |
| 62. |
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. short-circuited by low resistance | |
| 63. |
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. A.C. synchronous motor | |
| 64. |
If in a synchronous motor, driving mechanical load and drawing current at lagging power factor from constant 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. become less | |
| 65. |
A synchronous motor installed at the receiving end substation operates with such an excitation that it 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. improve | |
| 66. |
While starting a salient pole synchronous motor by induction motor action and connecting field discharge resistance across field, starting and accelerting 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. all of the above methods | |
| 67. |
Armature of a synchronous machine is |
| 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. all of the above reasons | |
| 68. |
If excitation of a synchronous motor 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. increase in both armature current and power factor angle | |
| 69. |
When a 3-phase synchronous generator is supplying a zero power factor lagging load, the armature field affects the main field in the following way |
| A. | augments it directly |
| B. | directly opposes it |
| C. | cross-magnetises it |
| D. | none of the above |
| Answer» B. directly opposes it | |
| 70. |
Stability of a synchronous machine |
| A. | decreases with increase in its excitation |
| B. | increases with increase in its excitation |
| C. | remains unaffected with increase in excitation |
| D. | any of the above |
| Answer» B. increases with increase in its excitation | |
| 71. |
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. stator supply is relieved of responsibility of producing magnetic field | |
| 72. |
If in 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» B. will become less | |
| 73. |
A synchronous motor is running with normal excitation. When the load is increased, the armature current drawn by it increases because |
| A. | speed of the motor is reduced |
| B. | power factor is decreased |
| C. | Eb (back e.m.f.) becomes less than V (applied voltage) |
| D. | Er (net resultant voltage) in armature is increased |
| Answer» D. Er (net resultant voltage) in armature is increased | |
| 74. |
If one-phase of a 3-phase synchronous motor is short-circuited, motor |
| A. | will refuse to start |
| B. | will overheat in spots |
| C. | will not come upto speed |
| D. | will fail to pull into step |
| Answer» A. will refuse to start | |
| 75. |
If the field circuit of an unloaded salientpole 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. the motor stops | |
| 76. |
In which of the following motors the stator and rotor fields rotate simultaneously ? |
| A. | D.C. motor |
| B. | Reluctance motor |
| C. | Universal motor |
| D. | Synchronous motor |
| Answer» D. Synchronous motor | |
| 77. |
The speed of a synchronous motor |
| A. | increases as the load increases |
| B. | decreases as the load decreases |
| C. | always remains constant |
| D. | none of the above |
| Answer» C. always remains constant | |
| 78. |
A rotory converter can also be run as a |
| A. | d.c. shunt motor |
| B. | d.c. series motor |
| C. | d.c. compound motor |
| D. | synchronous motor |
| Answer» D. synchronous motor | |
| 79. |
The maximum speed variation in a 3-phase synchronous motor is |
| A. | 10 per cent |
| B. | 6 per cent |
| C. | 4 per cent |
| D. | zero |
| Answer» D. zero | |
| 80. |
Which of the following resistances 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. All of the above | |
| 81. |
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. Low field current | |
| 82. |
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. direction of rotation | |
| 83. |
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. supplied with unbalanced voltage | |
| 84. |
In a synchronous motor, damper windings are provided on |
| A. | stator frame |
| B. | rotor shaft |
| C. | pole faces |
| D. | none of the above |
| Answer» C. pole faces | |
| 85. |
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 |
| Answer» A. more than the supply voltage | |
| 86. |
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. increase armature current but decrease power factor | |
| 87. |
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 Eh and V |
| Answer» D. vector difference of Eh and V | |
| 88. |
The ratio of starting torque to running torque in a synchronous motor is |
| A. | zero |
| B. | one |
| C. | two |
| D. | infinity |
| Answer» A. zero | |
| 89. |
In a synchronous motor, the magnitude of stator back e.m.f. Eb depends on |
| A. | load on the motor |
| B. | d.c. excitation only |
| C. | both the speed and rotor flux |
| D. | none of the above |
| Answer» B. d.c. excitation only | |
| 90. |
A 3-phase synchronous motor is running clockwise. If the direction of its field current is reversed |
| A. | the motor will stop |
| B. | the motor continue to run in the same direction |
| C. | the winding of the motor will burn |
| D. | the motor will run in the reverse direction |
| Answer» B. the motor continue to run in the same direction | |
| 91. |
The magnitude of field flux in a 3-phase synchronous motor |
| A. | remains constant at all loads |
| B. | varies with speed |
| C. | varies with the load |
| D. | varies with power factor |
| Answer» A. remains constant at all loads | |
| 92. |
The torque angle, in a synchronous motor, is the angle between |
| A. | the supply voltage and the back e.m.f. |
| B. | magnetising current and back e.m.f. |
| C. | the rotating stator flux and rotor poles |
| D. | none of the above |
| Answer» C. the rotating stator flux and rotor poles | |
| 93. |
Hunting in a synchronous motor cannot be due to |
| A. | windage friction |
| B. | variable load |
| C. | variable frequency |
| D. | variable supply voltage |
| Answer» A. windage friction | |
| 94. |
By which of the following methods the constant speed of a synchronous motor can be changed to new fixed value ? |
| A. | By changing the supply frequency |
| B. | By interchanging any two phases |
| C. | By changing the applied voltage |
| D. | By changing the load. |
| Answer» A. By changing the supply frequency | |
| 95. |
In a synchronous motor, V-curves represent relation between |
| A. | armature current and field current |
| B. | power factor and speed |
| C. | field current and speed |
| D. | field current and power factor |
| Answer» A. armature current and field current | |
| 96. |
In a 3-phase, 4-pole, 50 Hz synchronous motor, the frequency, pole number and load torque all are halved. The motor speed will be |
| A. | 3000 r.p.m. |
| B. | 1500 r.p.m. |
| C. | 750 r.p.m. |
| D. | none of the above |
| Answer» B. 1500 r.p.m. | |
| 97. |
A synchronous motor connected to infinite bus-bars has at constant full load, 100% excitation and unity power factor. On changing the excitation only, the armature current will have |
| A. | no change of power factor |
| B. | lagging power factor with over-excitation |
| C. | leading power factor with under-excitation |
| D. | leading power factor with over-excitation |
| Answer» D. leading power factor with over-excitation | |
| 98. |
Which of the following motors is non-self starting ? |
| A. | D.C. series motor |
| B. | synchronous motor |
| C. | Squirrel cage induction motor |
| D. | Wound round induction motor |
| Answer» B. synchronous motor | |
| 99. |
In a synchronous motor it the back e.m.f. generated in the armature at noload is approximately equal to the applied voltage, then |
| A. | the motor is said to be fully loaded |
| B. | the torque generated is maximum |
| C. | the excitation is said to be zero per cent |
| D. | the excitation is said to be hundred per cent |
| Answer» D. the excitation is said to be hundred per cent | |
| 100. |
In a synchronous motor, the damping winding is generally used to |
| A. | prevent hunting and provide the starting torque |
| B. | reduce the eddy currents |
| C. | provide starting torque only |
| D. | reduce noise level |
| Answer» A. prevent hunting and provide the starting torque | |
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