MCQOPTIONS
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MCQOPTIONS
This section includes 517 Mcqs, each offering curated multiple-choice questions to sharpen your Electrical Engineering knowledge and support exam preparation. Choose a topic below to get started.
| 51. |
When the excitation of an unloaded salient pole synchronous motor suddenly getsdisconnected |
| A. | the motor stops |
| B. | it runs as a reluctance motor at the same speed |
| C. | it runs as a reluctance motor at a lower speed |
| D. | none of the above |
| Answer» B. it runs as a reluctance motor at the same speed | |
| 52. |
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» C. cross-magnetises it | |
| 53. |
A synchronous motor which works on a leading power factor and does not drive amechanical load is called as |
| A. | static condenser |
| B. | condenser |
| C. | synchronous condenser |
| D. | none of the above |
| Answer» D. none of the above | |
| 54. |
In a synchronous motor running with fixed excitation, when the load is increasedthree times, its torque angle becomes approximately |
| A. | one-third |
| B. | twice |
| C. | thrice |
| D. | six times |
| Answer» D. six times | |
| 55. |
The duration of sudden snort-circuit test on a synchronous motor is usually about |
| A. | one hour |
| B. | one minute |
| C. | one second |
| D. | none of the above |
| Answer» D. none of the above | |
| 56. |
While starting a synchronous motor by induction motor action, field winding isusually |
| A. | connected to D.C. supply |
| B. | short-circuited by low resistance |
| C. | kept open-circuited |
| D. | none of the above |
| Answer» C. kept open-circuited | |
| 57. |
Which of the following losses is not dissipated by the stator core surface in asynchronous motor ? |
| A. | Eddy current losses in the conductors |
| B. | Iron losses in the stator |
| C. | Copper losses in the slot portion of the conductors |
| D. | Windage losses |
| Answer» E. | |
| 58. |
If load (or torque) angle of a 4-pole synchronous motor is 6° electrical, its value inmechanical degrees is |
| A. | 2 |
| B. | 3 |
| C. | 4 |
| D. | 6 |
| Answer» C. 4 | |
| 59. |
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» C. it continues to run at the same speed | |
| 60. |
When load on an over-excited or under excited synchronous*motor is increased, rateof change of its armature current as compared with that of power factor is |
| A. | more |
| B. | less |
| C. | equal |
| D. | twice |
| Answer» C. equal | |
| 61. |
As the load is applied to a synchronous motor, the motor takes more armature currentbecause |
| A. | the increased load has to take more current |
| B. | the rotor by shifting its phase backward causes motor to take more current |
| C. | the back e.m.f. decreases causing an increase in motor current |
| D. | the rotor strengthens the rotating field casuing more motor current |
| Answer» C. the back e.m.f. decreases causing an increase in motor current | |
| 62. |
At which power factor is DG set rated in kW? |
| A. | 0.7 |
| B. | 0.8 |
| C. | 0.9 |
| D. | 1 |
| Answer» C. 0.9 | |
| 63. |
In a synchronous generator supplying power at rated voltage with zero power factor lagging, the effect of armature reaction ampere turns is: |
| A. | Demagnetising |
| B. | Cross magnetising |
| C. | Magnetising |
| D. | Magnetizing and cross magnetising |
| Answer» B. Cross magnetising | |
| 64. |
In the V curves of a synchronous motor, what are the quantities on Y-axis and X-axis respectively? |
| A. | Armature current and torque |
| B. | Torque and field current |
| C. | Power factor and field current |
| D. | Armature current and field current |
| Answer» E. | |
| 65. |
Ignoring the effect of armature reaction, if excitation of a synchronous motor running with constant load is increased, its torque angle must: |
| A. | decrease |
| B. | increase |
| C. | become twice the no-load value |
| D. | remain constant |
| Answer» B. increase | |
| 66. |
In a synchronous motor, which of the following losses is NOT met by motor AC input? |
| A. | Iron loss |
| B. | Stator Cu loss |
| C. | Rotor Cu loss |
| D. | Friction loss |
| Answer» D. Friction loss | |
| 67. |
How can we increase the frequency of the voltage generated by a generator? |
| A. | Adjusting the governor |
| B. | Using reactors |
| C. | Increasing the load |
| D. | Reducing the terminal voltage |
| Answer» B. Using reactors | |
| 68. |
A stationary alternator should not be connected to a live busbar because of it |
| A. | will get short-circuited |
| B. | will disturb generated e.m.fs of other alternators connected in parallel |
| C. | is likely to run as a synchronous motor |
| D. | will decrease busbar voltage though momentarily |
| Answer» B. will disturb generated e.m.fs of other alternators connected in parallel | |
| 69. |
In an alternator, if the winding is short-pitched by 60 electrical degrees, its pitch factor will be: |
| A. | 1 |
| B. | 0.28 |
| C. | 0.75 |
| D. | 0.866 |
| Answer» E. | |
| 70. |
If the load angle of a 4-pole synchronous motor is 8° (electrical), its value in mechanical degrees is: |
| A. | 0.25 |
| B. | 4 |
| C. | 2 |
| D. | 0.5 |
| Answer» C. 2 | |
| 71. |
Consider a system consisting of a synchronous generator working at a lagging power factor, a synchronous motor working at an overexcited condition and a directly grid-connected induction generator. Consider capacitive VAr to be a source and inductive VAr to be a sink of reactive power. Which one of the following statements is TRUE? |
| A. | Synchronous motor and synchronous generator are sources and induction generator is a sink of reactive power. |
| B. | Synchronous motor and induction generator are sources and synchronous generator is a sink of reactive power. |
| C. | Synchronous motor is a source and induction generator and synchronous generator are sinks of reactive power. |
| D. | All are sources of reactive power. |
| Answer» B. Synchronous motor and induction generator are sources and synchronous generator is a sink of reactive power. | |
| 72. |
A three phase synchronous generator is to be connected to the infinite bus. The lamps are connected as shown in the figure for the synchronization. The phase sequence of bus voltage is R – Y – B and that of incoming generator voltage is R’ – Y’ – B’.It was found that the lamps are becoming dark in the sequence La – Lb – Lc. It means that the phase sequence of incoming generator is |
| A. | Opposite to infinite bus and its frequency is more than infinite bus |
| B. | Opposite to infinite bus but its frequency is less than infinite bus |
| C. | Same as infinite bus and its frequency is more than infinite bus |
| D. | Same as infinite bus and its frequency is less than infinite bus |
| Answer» B. Opposite to infinite bus but its frequency is less than infinite bus | |
| 73. |
In a synchronous generator, the effect of armature reaction is completely magnetising if the power factor of the load is: |
| A. | Unity |
| B. | Zero lagging |
| C. | Zero leading |
| D. | 0.5 |
| Answer» D. 0.5 | |
| 74. |
A 720 kVA, 240√3 V, star connected synchronous machine generates rated open circuit voltage at a field current of 20 A. A field current of 10 A produces rated armature current under short circuit condition. Per unit saturated synchronous reactance is: |
| A. | 2 |
| B. | 0.5 |
| C. | 0.4 |
| D. | 1.5 |
| Answer» C. 0.4 | |
| 75. |
In order to make concentric winding in a alternator: |
| A. | The number of slots is equal to three times the number of coils |
| B. | The number of slots is equal to twice the number of coils |
| C. | The number of slots is less than the number of coils |
| D. | The number of slots is equal to the number of coils |
| Answer» C. The number of slots is less than the number of coils | |
| 76. |
If the prime mover input of an alternator connected directly to an infinite bus is increased, then its |
| A. | Active power output increases |
| B. | Terminal voltage increases |
| C. | Frequency increases |
| D. | Reactive power output increases |
| Answer» B. Terminal voltage increases | |
| 77. |
A synchronous generator is feeding a zero power factor (lagging) load at rated current, the armature reaction is |
| A. | magnetizing |
| B. | demagnetizing |
| C. | cross-magnetizing |
| D. | ineffective |
| Answer» C. cross-magnetizing | |
| 78. |
An incoming alternator is to be synchronised with an already operating alternator. Which of the following conditions is NOT necessary to be satisfied? |
| A. | Same frequency |
| B. | Same voltage magnitude |
| C. | Same phase sequence |
| D. | Same prime mover speed |
| Answer» E. | |
| 79. |
Fifth harmonic component of the induced voltage in three phase AC generator can be eliminated by using a winding pitch to: |
| A. | 2/3 |
| B. | 4/5 |
| C. | 5/6 |
| D. | 6/7 |
| Answer» C. 5/6 | |
| 80. |
A synchronous motor on load draw a current at a leading power factor angle ϕ. If the internal power factor angle which is the phase angle between the excitation emf and the current in the time phasor diagram is ψ. Then the air gap excitation mmf lags the armature mmf by |
| A. | Ψ |
| B. | π/2 + ψ |
| C. | π/2 – ψ |
| D. | Ψ+ ϕ |
| Answer» D. Ψ+ ϕ | |
| 81. |
An alternator is being synchronised to the power station bus bars whose frequency is 50 Hz. If the bulb of the synchroscope glows with maximum brightness, the frequency of the alternator is: |
| A. | 50 Hz |
| B. | 40 Hz |
| C. | 55 Hz |
| D. | 45 Hz |
| Answer» B. 40 Hz | |
| 82. |
In a three-phase alternator the rotating part is the: |
| A. | Commutator |
| B. | Field system |
| C. | Slip rings |
| D. | Armature system |
| Answer» C. Slip rings | |
| 83. |
A slip test is conducted on a synchronous generator to determine: |
| A. | Positive and negative sequence reactance |
| B. | Sub-transient reactance |
| C. | Direct and quadrature axis reactance |
| D. | Slip |
| Answer» D. Slip | |
| 84. |
In a salient pole synchronous machine, the MMF acting along the d-axis is: |
| A. | Only armature MMF |
| B. | The field MMF and armature MMF |
| C. | No MMF |
| D. | Only field MMF |
| Answer» C. No MMF | |
| 85. |
In an alternator at leading power factor, the armature flux _____ the rotor flux. |
| A. | does not affect |
| B. | opposes |
| C. | distorts |
| D. | aids |
| Answer» E. | |
| 86. |
In a synchronous machine, all of the following losses are independent of the load EXCEPT: |
| A. | iron loss |
| B. | bearing friction |
| C. | windage loss |
| D. | Copper loss |
| Answer» E. | |
| 87. |
Inverted V curve of a synchronous motor represents the relation between: |
| A. | field current and power factor |
| B. | power factor and speed |
| C. | armature current and field current |
| D. | field current and speed |
| Answer» B. power factor and speed | |
| 88. |
Once a synchronous machine is synchronized, it will tend to remain in synchronism with the other alternators. Any tendency to depart from the condition of synchronism is ________ produced due to circulating current flowing through the alternators. |
| A. | Opposed by a motor torque |
| B. | Opposed by a load torque |
| C. | opposed by a synchronizing torque |
| D. | Aided by a synchronizing torque |
| Answer» D. Aided by a synchronizing torque | |
| 89. |
A synchronous motor can operate at______. |
| A. | Leading power factor only |
| B. | Lagging power factor only |
| C. | Unity power factor only |
| D. | Lagging, leading and unity power factors |
| Answer» E. | |
| 90. |
Let α be the chording angle in electrical degrees for the fundamental flux wave of an alternator. Its pitch-factor for the nth harmonic will be equal to: |
| A. | cos (nα/2) |
| B. | cos α |
| C. | cos (nα) |
| D. | cos (3α) |
| Answer» B. cos α | |
| 91. |
A 6 pole star connected three phase alternator, driven at 1000 rpm, has 108 slots with 10 conductors per slot The slot angle and the number of turns per phase, respectively are: |
| A. | 7.5° and 360 |
| B. | 7.5° and 180 |
| C. | 10° and 360 |
| D. | 10° and 180 |
| Answer» E. | |
| 92. |
In case of two alternators running in parallel and perfectly synchronized, the synchronizing power is |
| A. | zero |
| B. | positive |
| C. | negative |
| D. | ideally infinite |
| Answer» B. positive | |
| 93. |
A 4-pole, 1500 RPM alternator will generate EMF at: |
| A. | 20 Hz |
| B. | 60 Hz |
| C. | 40 Hz |
| D. | 50 Hz |
| Answer» E. | |
| 94. |
In a synchronous motor running with fixed excitation, if the load is increased three times, then its torque angle becomes nearly: |
| A. | One-third |
| B. | Six times |
| C. | Nine times |
| D. | Thrice |
| Answer» E. | |
| 95. |
A ‘P’ pole, 50 Hz alternator is driven by a 60 Hz synchronous motor. Calculate the number of poles for the synchronous motor. |
| A. | 1.2P |
| B. | 2.4/P |
| C. | 1.2/P |
| D. | 2.4P |
| Answer» B. 2.4/P | |
| 96. |
In a factory, a three-phase, 4-kV, 400-kVA synchronous machine is installed along with other induction motors. The following are the loads on the machine:(i) Induction motors: 500 kVA at 0.8 power factor lagging(ii) Synchronous motor: 300 kVA at unity power factorDetermine the overall power factor of the factory loads. |
| A. | \(\frac{7}{\sqrt{58}}\) lagging |
| B. | \(\frac{7}{\sqrt{58}}\) leading |
| C. | \(\frac{9}{\sqrt{58}}\) leading |
| D. | \(\frac{9}{\sqrt{58}}\) lagging |
| Answer» B. \(\frac{7}{\sqrt{58}}\) leading | |
| 97. |
A synchronous motor will deliver maximum power when: |
| A. | Load angle is equal to internal angle |
| B. | Load angle is 45° |
| C. | Load angle is 0° |
| Answer» B. Load angle is 45° | |
| 98. |
Consider the following statements regarding synchronous machines:1. When a synchronous motor is over-excited, its back emf is greater than the supply voltage2. When a synchronous motor is over-excited, its power factor is leading3. Synchronous motor is used as capacitor where load is so large that construction of a static capacitor is impracticalWhich of these statements are correct? |
| A. | 1 and 2 |
| B. | 1 and 3 |
| C. | 2 and 3 |
| D. | 1, 2 and 3 |
| Answer» E. | |
| 99. |
In synchronous motors, the phenomenon of successive overshoots and undershoots in the motor speed due to sudden changes in the load is called ______. |
| A. | Crawling |
| B. | Humming |
| C. | Hunting |
| D. | Damping |
| Answer» D. Damping | |
| 100. |
A doubly excited salient pole motor will have |
| A. | reluctance torque |
| B. | mutual torque |
| C. | reluctance torque and mutual torque at the same instant |
| D. | reluctance torque followed by mutual torque |
| Answer» D. reluctance torque followed by mutual torque | |