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MCQOPTIONS
This section includes 309 Mcqs, each offering curated multiple-choice questions to sharpen your Electrical Machines knowledge and support exam preparation. Choose a topic below to get started.
| 51. |
If a 400 V, 50 Hz, star connected, 3 phase squirrel cage induction motor is operated from a 400 V, 75 Hz supply, the torque that the motor can now provide while drawing rated current from the supply |
| A. | reduces |
| B. | increases |
| C. | remains the same |
| D. | increases or reduces depending upon the rotor resistance |
| Answer» B. increases | |
| 52. |
Armature reaction at of a synchronous generator at rated voltage zero power factor lagging is |
| A. | magnetizing |
| B. | demagnetizing |
| C. | cross-magnetizing |
| D. | both magnetizing and cross-magnetizing |
| Answer» C. cross-magnetizing | |
| 53. |
Voltage regulation of a synchronous generator calculated by synchronous impedance method is |
| A. | higher than actual because of saturation of magnetic circuit |
| B. | lower than actual because of saturation of magnetic circuit |
| C. | nearly accurate because it takes account of magnetic circuit. |
| D. | nearly accurate because the generator is normally operated with an unsaturated magnetic circuit. |
| Answer» B. lower than actual because of saturation of magnetic circuit | |
| 54. |
A synchronous motor is floating on infinite mains at no load. If its excitation is now increased, it will draw |
| A. | unity power factor current |
| B. | zero power factor lagging current |
| C. | zero power factor loading current |
| D. | no current |
| Answer» D. no current | |
| 55. |
Short-circuit ratio of a synchronous machine is defined as the ratio of |
| A. | field current required to produce rated voltage on full load and field current required to produce rated current on SC. |
| B. | field current required to produce rated voltage on OC and field current required to produce rated armature current on SC. |
| C. | field current required to produce rated voltage on SC and field current required to produce rated current on SC. |
| D. | field current required to produce rated voltage on full-load and field current required to produce rated voltage on SC. |
| Answer» C. field current required to produce rated voltage on SC and field current required to produce rated current on SC. | |
| 56. |
The flux per pole in a synchronous motor with the field circuit ON and the startor disconnected from the supply is found to be 25 mWb. When startor is connected to the rated supply with the field excitation unchanged, the flux per pole in the machine is found to be 20 mWb while motor is running on no load. Assuming no load losses to be zero, the no load current drawn by the motor from the supply |
| A. | lags the supply voltage |
| B. | leads the supply voltage |
| C. | is in phase with the supply voltage |
| D. | is zero |
| Answer» C. is in phase with the supply voltage | |
| 57. |
If armature current in a generating synchronous machine is in phase with the excitation emf, then armature mmf vector |
| A. | lags behind the field mmf vector by 90 . |
| B. | is in the same direction as the field mmf vector. |
| C. | is in a direction opposite to the field mmf vector. |
| D. | leads the field mmf vector by 90 as observed from the startor |
| Answer» B. is in the same direction as the field mmf vector. | |
| 58. |
The pole shoes of a synchronous machine are so shaped that the air-gap is least in the middle and increases towards the pole edges. This type of construction |
| A. | increases flux/pole for a given excitation |
| B. | eliminates a particular harmonic |
| C. | gives a nearly sinusoidal waveform of output voltage |
| D. | facilitates machining of pole shoes. |
| Answer» D. facilitates machining of pole shoes. | |
| 59. |
For a given number of turns/pole/phase, the effect of winding distribution in a synchronous generator is to |
| A. | increase induced emf but to reduce the harmonic content of the wave form |
| B. | increase induced emf and also the harmonic content of the wave form |
| C. | reduce induced emf but to increase the harmonic content of the wave form |
| D. | reduce induced emf and also the harmonic content of the wave form |
| Answer» B. increase induced emf and also the harmonic content of the wave form | |
| 60. |
A single-phase transformer is feeding a lagging load. What will be the effect on t he voltage regulation of the transformer if |
| A. | decrease in both the cases. |
| B. | increase in both the cases. |
| C. | increase in case (i) but decrease in case (ii). |
| D. | decrease in case (i) but increase in case (ii). |
| Answer» D. decrease in case (i) but increase in case (ii). | |
| 61. |
Two 3-limb, 3-phase delta-star connected transformers are supplied from the same source. One of the transformers is of Dy 1 and the other is of Dy 11 connection. The phase difference between the corresponding phase voltages of the two secondaries would be |
| A. | 0 |
| B. | 30 |
| C. | 60 |
| D. | 120 |
| Answer» D. 120 | |
| 62. |
A single -winding single-phase motor has |
| A. | low starting torque |
| B. | zero starting torque |
| C. | high starting torque |
| D. | starting torque equal to full-load torque. |
| Answer» C. high starting torque | |
| 63. |
If a single-winding single-phase motor is running in a particular direction |
| A. | both the rotating fields (rotating in opposite directions) have the same strength. |
| B. | forward rotating field is slightly stronger than the backward field. |
| C. | forward rotating field is much stronger than the backward field. |
| D. | backward field is stronger than the forward field. |
| Answer» D. backward field is stronger than the forward field. | |
| 64. |
A synchronous motor is operating on no-load at unity power factor. If field 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» E. | |
| 65. |
In a dc shunt motor for given field and armature currents |
| A. | speed will be higher if the magnetic circuit is saturated than if the magnetic circuit is unsaturated |
| B. | speed will be higher if the magnetic circuit is unsaturated than if the magnetic circuit is saturated |
| C. | state of saturation of the magnetic circuit will have no effect on the speed of the motor. |
| D. | with unsaturated magnetic circuit the motor can acquire a dangerously high speed. |
| Answer» B. speed will be higher if the magnetic circuit is unsaturated than if the magnetic circuit is saturated | |
| 66. |
A dc generator running at fixed speed and with fixed shunt field resistance has short-circuit current |
| A. | less than the maximum load current that it can feed |
| B. | more than the maximum load current that it can feed. |
| C. | current equal to the maximum load current that it can feed. |
| D. | equal to the full load current. |
| Answer» B. more than the maximum load current that it can feed. | |
| 67. |
Armature reaction AT in a dc machine |
| A. | are in the same direction as the main poles |
| B. | are in direct opposition to the main poles |
| C. | make an angle of 90 with the main pole axis. |
| D. | make an angle with the main pole axis which is load dependent. |
| Answer» D. make an angle with the main pole axis which is load dependent. | |
| 68. |
In a level compound generator, the series field ampere-turns are |
| A. | in direct opposition to the shunt field ampereturns. |
| B. | in the same direction as the shunt field ampere-turns |
| C. | at 90 (elect) to the shunt field ampere-turns |
| D. | placed on the interpoles |
| Answer» C. at 90 (elect) to the shunt field ampere-turns | |
| 69. |
In a level compound gener ator the terminal voltage at half full load is |
| A. | same as the full load voltage |
| B. | same as the no-load voltage |
| C. | more than the no-load voltage |
| D. | less than the no-load voltage |
| Answer» D. less than the no-load voltage | |
| 70. |
In a dc series motor. Assume negligible armature circuit resistance; the magnetic circuit is linear |
| A. | the speed varies inversely as the square root of the load torque; at a given torque load speed is directly proportional to the number of field turns. |
| B. | the speed varies inversely as the square of the load torque; at a given torque load speed is directly proportional to the number of field turns. |
| C. | the speed varies inversely as the load torque; at a given torque load speed is inver sely proportional to the number of field turns. |
| D. | the speed varies inversely as the square root of the load torque; at a given torque load speed is inversely proportional to the number of field turns. |
| Answer» E. | |
| 71. |
In a dc shunt motor |
| A. | no-load speed is directly proportional to flux/ pole and inversely proportional armature voltage; the speed drops off linearly with load torque. |
| B. | no-load speed is directly proportional to flux/ pole and directly proportional to armature voltage; the speed drops of linearly with load torque. |
| C. | no-load speed is inversely proportional to flux/pole and directly proportional to armature voltage; the speed rises linearly with load torque. |
| D. | no-load speed is inversely proportional to flux/pole and directly proportional to armature voltage; the speed rises linearly with load torque |
| Answer» D. no-load speed is inversely proportional to flux/pole and directly proportional to armature voltage; the speed rises linearly with load torque | |
| 72. |
When st ar t i ng a di ffer ent i al l y connect ed compound motor, it is desirable to short-circuit the series field winding to |
| A. | avoid very high starting time |
| B. | avoid excessive starting speed |
| C. | prevent the motor from starting in the reverse direction |
| D. | avoid heavy in-rush of current |
| Answer» C. prevent the motor from starting in the reverse direction | |
| 73. |
A dc shunt gener at or when driven without connecting field winding shows as open circuit terminal voltage of 12 V. When field winding is connected and excited the terminal voltage drops to zero because |
| A. | field resistance is higher than critical resistance |
| B. | there is no residual magnetism in the field circuit |
| C. | field winding has got wrongly connected |
| D. | there is a fault in armature circuit |
| Answer» D. there is a fault in armature circuit | |
| 74. |
An electric motor with constant output power will have a torque-speed characteristic in the form of a |
| A. | straight line through the origin |
| B. | straight line parallel to the speed axis |
| C. | circle about the origin |
| D. | rectangular hyperbola |
| Answer» C. circle about the origin | |
| 75. |
A 200 V, 2000 rpm, 10A, separately excited dc motor has an armature resistance of 20Ω. Rated dc voltage is applied to both the armature and field winding of the motor. If the armature draws 5 A from the source, the torque developed by the motor is |
| A. | 4.30 Nm |
| B. | 4.77 Nm |
| C. | 0.45 Nm |
| D. | 0.50 Nm |
| Answer» C. 0.45 Nm | |
| 76. |
Neglecting all losses, the developed torque (T) of a d.c. separately excited motor, operating under constant terminal voltage, is related to its output power (P) as |
| A. | T |
| B. | <span style=" text-decoration: overline;">P</span> |
| C. | T P |
| D. | T P |
| E. | T is independent of P |
| Answer» D. T P | |
| 77. |
In a dc motor the torque (Nm) developed is |
| A. | <table><tr><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center> <sub>m</sub></center></td><td rowspan="2">opposite to the direction of <sub>m</sub> (speed in mech rad/s)</td></tr><tr><td style="text-align: center;">E<sub> </sub>I<sub> </sub></td></tr></table> |
| B. | <table><tr><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center> <sub>m</sub></center></td><td rowspan="2">in the direction of <sub>m</sub></td></tr><tr><td style="text-align: center;">E<sub> </sub>I<sub> </sub></td></tr></table> |
| C. | <table><tr><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center>E<sub> </sub>I<sub> </sub></center></td><td rowspan="2">opposite to the direction of <sub>m</sub></td></tr><tr><td style="text-align: center;"> <sub>m</sub></td></tr></table> |
| D. | <table><tr><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center>E<sub> </sub>I<sub> </sub></center></td><td rowspan="2">in the direction of <sub>m</sub></td></tr><tr><td style="text-align: center;"> <sub>m</sub></td></tr></table> |
| Answer» E. | |
| 78. |
Starting torque can be obtained in case of a single phase induction motor with identical main and auxiliary windings by connecting |
| A. | a capacitor across the mains |
| B. | a capacitor in series with the machine |
| C. | a capacitor in series with the auxiliary winding |
| D. | main and auxiliary windings in series |
| Answer» D. main and auxiliary windings in series | |
| 79. |
The capacitor-star single-phase induction motor develop much larger starting torque in comparison with the split-phase motor, because use of capacitors in the auxilliary winding enables |
| A. | provision of larger number of turns in the auxilliary winding |
| B. | larger starting current to be drawn from the supply |
| C. | torque-slip characteristic in general to get a shape to give a large starting torque |
| D. | current division between main and auxilliary winding |
| Answer» D. current division between main and auxilliary winding | |
| 80. |
In a shaded-pole induction motor, the rotor runs from |
| A. | shaded portion to the unshaded portion of the pole while flux in the former leads that of the latter |
| B. | shaded portion to the unshaded portion of the pole while flux in the former lags that in the latter |
| C. | unshaded portion to the shaded portion while flux in the former leads that in the latter |
| D. | unshaded portion to the shaded portion while flux in the former lags that in the latter |
| Answer» D. unshaded portion to the shaded portion while flux in the former lags that in the latter | |
| 81. |
A single phase induction motor is running on the main winding only. The rotor impedance seen by two rotating fields are related as impedance seen by |
| A. | forward field in finite but the backward field is seen as an open circuit. |
| B. | two fields are nearly equal. |
| C. | forward field is much more than that seen by the backward field. |
| D. | forward field is much less than that seen by the backward field. |
| Answer» D. forward field is much less than that seen by the backward field. | |
| 82. |
Which one of the following statements regarding capacitor start, capacitor run single-phase induction motor is correct? |
| A. | One of the capacitors is in the circuit during starting and both are in the circuit during running |
| B. | Both capacitors are in the circuit during starting and one of them is in the circuit during running |
| C. | Both capacitors are in the circuit during starting and running |
| D. | One of the capacitors is in the circuit during starting and other is in circuit during running |
| Answer» C. Both capacitors are in the circuit during starting and running | |
| 83. |
Speed of 3-phase induction motor is controlled from 1 pu to 2 pu using a variable frequency inverter. The equivalent circuit parameter most likely to vary is |
| A. | startor leakage inductance |
| B. | rotor leakage inductance |
| C. | magnetising inductance |
| D. | core loss resistance |
| Answer» C. magnetising inductance | |
| 84. |
A 3 phase squirrel case induction motor has a full load efficiency of 0.8 and a maximum efficiency of 0.9. If it is operated at a slip of 0.6 by applying a reduced voltage, then efficiency of the motor at this operating point |
| A. | less than 0.4 |
| B. | greater than 0.6 |
| C. | in the range of 0.8 to 0.1 |
| D. | none of these |
| Answer» B. greater than 0.6 | |
| 85. |
Synchronous generators used in thermal power stations have |
| A. | cylindrical rotors |
| B. | slip-rings and brushes |
| C. | startor slots in multiples of 6 |
| D. | all of these |
| Answer» C. startor slots in multiples of 6 | |
| 86. |
A 6-pole, 50 Hz wound rotor induction motor when supplied at the rated voltage and frequency with slip-ring open-circuited, developed a voltage of 100 V between any two rings. If rotor is driven by an external means at 1000 rpm opposite to then direction of startor field, the frequency of voltage across an slip-rings will be |
| A. | zero |
| B. | 50 Hz |
| C. | 100 Hz |
| D. | 200 Hz |
| Answer» B. 50 Hz | |
| 87. |
A 3-phase induction machine draws active power P and reactive power Q form the grid. If it is operated as a generator, P and Q will respectively be |
| A. | positive and negative |
| B. | negative and negative |
| C. | positive and positive |
| D. | negative and positive |
| Answer» E. | |
| 88. |
Phase splitting can be accomplished in a singlephase induction motor by |
| A. | adding a capacitor in series with auxiliary winding. |
| B. | causing auxiliary winding to have high resistance |
| C. | both (a) and (b) |
| D. | none of these |
| Answer» D. none of these | |
| 89. |
In an induction motor the rotor field runs with respect to the startor at |
| A. | synchronous speed in the same direction as the startor field |
| B. | synchronous speed in opposite direction as the startor field |
| C. | slip speed in the same direction as the startor field |
| D. | slip speed in opposite direction as the startor field. |
| Answer» B. synchronous speed in opposite direction as the startor field | |
| 90. |
Crawling in an induction motor is due to |
| A. | time harmonics in supply |
| B. | slip ring rotor |
| C. | space harmonics produce by winding currents |
| D. | insufficient starting torque |
| Answer» D. insufficient starting torque | |
| 91. |
If load current and flux of d.c motor are held constant and voltage applied across it s armature is increase by 5%, then speed of motor will |
| A. | increase by about 5% |
| B. | reduce by about 5% |
| C. | remain unaltered |
| D. | depends on other factors |
| Answer» B. reduce by about 5% | |
| 92. |
A 230-V d.c. series motor has an armature circuit resistance of 0.2Ω and field resistance of 0.1Ω. At rated votage, the motor draws a line current of 40 amps and runs at speed of 1000 r.p.m. What is the speed of the motor for a line current of 20 A at 230 V? Assume that the flux at 20 A line current is 60% of the flux at 40 A line current. |
| A. | 1213 rpm |
| B. | 1513 rpm |
| C. | 1713 rpm |
| D. | 1913 rpm |
| Answer» D. 1913 rpm | |
| 93. |
For a level compounded dc generator to run at constant speed, the series field mmf must effectively compensate |
| A. | armature reaction mmf |
| B. | armature resistance voltage drop |
| C. | brush contact voltage drop |
| D. | none of these |
| Answer» B. armature resistance voltage drop | |
| 94. |
Armature mmf of a dc machine has |
| A. | triangular space distribution and rotates at the speed of armature |
| B. | trapezoidal space distribution and is stationary in space |
| C. | stepped distribution and rotates at the speed of the armature |
| D. | triangular space distribution and is stationary in space |
| Answer» E. | |
| 95. |
A 6-pole, 3-phase, 60 Hz induction motor runs at 1000 rpm developing maximum torque. Rotor resistance per phase is 1.2 ohms. Neglecting star tor impedance, for developing maximum torque, the external resistance to be connected in series with each rotor phase will be |
| A. | 7.2 ohms |
| B. | 6 ohms |
| C. | 1.44 ohms |
| D. | 1.2 ohms |
| Answer» C. 1.44 ohms | |
| 96. |
During no-load test, an induction motor draws power |
| A. | for core loss and windage and friction loss |
| B. | for core loss and copper loss |
| C. | for copper loss and windage and friction loss |
| D. | only for the very small copper loss. |
| Answer» C. for copper loss and windage and friction loss | |
| 97. |
An ideal synchronous motor has no starting torque because the |
| A. | rotor is made up of salient poles |
| B. | relative velocity between the startor and the rotor mmf's is zero |
| C. | relative velocity between stator and rotor mmf's is not zero |
| D. | rotor winding is highly reactive. |
| Answer» D. rotor winding is highly reactive. | |
| 98. |
In an induction motor, if air gap is increased, then |
| A. | speed will reduce |
| B. | efficiency will improve |
| C. | power factor will be lowered |
| D. | breakdown torque will reduce |
| Answer» D. breakdown torque will reduce | |
| 99. |
If a three phase slip ring induction motor is fed from the rotor side with startor winding short circuited, then frequency of the current flowing in the short circuited startor is |
| A. | slip frequency |
| B. | supply frequency |
| C. | frequency corresponding to rotor speed |
| D. | zero |
| Answer» B. supply frequency | |
| 100. |
A three phase, 8 pole induction motor runs on 50Hz supply. The rotor bar current makes 150 complete cycles per minute. Then the rotor moves at a speed of |
| A. | 710 rpm |
| B. | 713.5 rpm |
| C. | 712.5 rpm |
| D. | none of these |
| Answer» E. | |