MCQOPTIONS
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MCQOPTIONS
This section includes 122 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. |
A 4-pole 50-Hz induction motor has a slip of 0.5. What will be the value of rotor induced voltage frequency? |
| A. | 125 Hz |
| B. | 100 Hz |
| C. | 50 Hz |
| D. | 25 Hz |
| Answer» E. | |
| 52. |
If a machine has P poles and the armature rotates at N revolutions per minute, then the frequency of induced emf in the armature winding of the machine is: |
| A. | \(f = \frac{{50}}{{PN}}Hz\) |
| B. | \(f = \frac{{120}}{{PN}}Hz\) |
| C. | \(f = \frac{{PN}}{{120}}Hz\) |
| D. | \(f = \frac{{PN}}{{50}}Hz\) |
| Answer» D. \(f = \frac{{PN}}{{50}}Hz\) | |
| 53. |
In an induction motor, stator is the |
| A. | rotary part |
| B. | part that is stationary |
| C. | part that controls the speed |
| D. | rotary and part that controls the speed |
| Answer» C. part that controls the speed | |
| 54. |
If a delta-connected 6-pole 50 Hz induction motor has a rotor resistance of 0.15 Ω per phase and exerts the maximum torque at 880 rpm, then the percentage maximum torque that will be exerted at standstill is: |
| A. | 33.43% |
| B. | 11.83% |
| C. | 47.32% |
| D. | 23.66% |
| Answer» E. | |
| 55. |
A 3-ϕ induction motor drives a constant torque load. It operates at a slip of 4% at rated voltage. If the voltage is reduced by 10% the slip will (approximately) |
| A. | Increase by 25% |
| B. | Decrease by 20% |
| C. | Decrease by 67% |
| D. | Increase by 33% |
| Answer» B. Decrease by 20% | |
| 56. |
Due to low supply voltage for a 40 HP, 3-phase induction motor, which of the following equipment can be used to boost the voltage? |
| A. | Synchronous condenser |
| B. | Additional tap changing transformer |
| C. | Series capacitors |
| D. | Shunt capacitors |
| Answer» E. | |
| 57. |
A three-phase slip-ring induction motor, provided with a commutator winding, is shown in the figure. The motor rotates in clockwise direction when the rotor windings are closed.If the rotor winding is open circuited and the system is made to run at rotational speed fr with the help of prime-mover in anti-clockwise direction, then the frequency of voltage across slip ring is f1 and frequency of voltage across commutator brushes is f2. The values of f1 and f2 respectively are |
| A. | f + fr and f |
| B. | f - fr and f |
| C. | f - fr and f + fr |
| D. | f + fr and f - fr |
| Answer» B. f - fr and f | |
| 58. |
Induction motor is also known as: |
| A. | Asynchronous motor |
| B. | AC series motor |
| C. | Commutator motor |
| D. | Repulsion motor |
| Answer» B. AC series motor | |
| 59. |
A balanced three-phase, 50 Hz voltage is applied to a 3 phase, 4 pole, induction motor. When the motor is delivering rated output, the slip is found to be 0.05. The speed of the rotor m.m.f. relative to the rotor, structure is |
| A. | 1500 r.p.m |
| B. | 1425 r.p.m |
| C. | 25 r.p.m |
| D. | 75 r.p.m |
| Answer» E. | |
| 60. |
A 4-pole, 3-phase induction motor operates from a supply whose frequency is 50 Hz. Calculate the speed of the rotor and frequency of the rotor currents when the slip is 0.04. |
| A. | N = 1450 rpm and frequency is 4 Hz |
| B. | N = 1440 rpm and frequency is 48 Hz |
| C. | N = 1460 rpm and frequency is 4 Hz |
| D. | N = 1440 rpm and frequency is 2 Hz |
| Answer» E. | |
| 61. |
After using Auto transformer starter, starting current gets limited to |
| A. | 7-8 times full load current |
| B. | 2-3 times full load current |
| C. | 2.5-4 times full load current |
| D. | 10 times full load current |
| Answer» D. 10 times full load current | |
| 62. |
Changing the electrical connections to reverse the direction of rotation of a motor running at full speed is called ________. |
| A. | slugging |
| B. | plugging |
| C. | dynamic braking |
| D. | brush shifting |
| Answer» C. dynamic braking | |
| 63. |
In an Induction motor, at speed near to synchronous speed, the torque-speed and torque-slip curves are approximately: |
| A. | Hyperbola |
| B. | Parabola |
| C. | Ellipses |
| D. | Straight lines |
| Answer» E. | |
| 64. |
No load test was conducted on a three phase induction motor at different input voltages and the input power obtained was plotted against the input voltage. The intersection of the extrapolated curve on the Y-Axis, would given: |
| A. | Rated core loss |
| B. | windage and friction loss |
| C. | Rated copper loss |
| D. | Rated core loss and Windage and friction loss |
| Answer» C. Rated copper loss | |
| 65. |
By adding resistance in the rotor circuit of a slip ring induction motor, the starting current |
| A. | as well as torque reduce |
| B. | as well as torque increase |
| C. | reduces but the starting torque increases |
| D. | increases but the starting torque decreases |
| Answer» D. increases but the starting torque decreases | |
| 66. |
Plugging method of braking is applied to induction motor by: |
| A. | Reducing the applied voltage |
| B. | Reversing the rotor current externally |
| C. | Interchanging two phases of stator |
| D. | Opening the rotor and connecting across the resistance |
| Answer» D. Opening the rotor and connecting across the resistance | |
| 67. |
An induction motor when started on load does not accelerate up to full speed but runs at 1/7th of the rated speed. The motor is said to be. |
| A. | Locking |
| B. | Plugging |
| C. | Crawling |
| D. | Logging |
| Answer» D. Logging | |
| 68. |
A 4 pole 400 V, 50 Hz, 3 phase induction motor is running at 1425 RPM. What will be the frequency of rotor current at this speed? |
| A. | 50 Hz |
| B. | 100 Hz |
| C. | 2.5 Hz |
| D. | 5 Hz |
| Answer» D. 5 Hz | |
| 69. |
Directions: It consists of two statements, one labeled as the ‘Statement (I)’ and the other as ‘Statement (II)’. Examine these two statements carefully and select the answer using the codes given below:Statement (I): The rotor of an induction motor rotates in the direction of the rotating magnetic field.Statement (II): The rotor will start rotating in such a direction that it will oppose the cause, i.e., the relative speed between the rotating field and stationary rotor conductors should decrease. |
| A. | Statement (I) and Statement (II) are individually true and statement (II) is the correct explanation of statement (II) |
| B. | Statement (I) and Statement (II) are individually true but statement (II) is not the correct explanation of statement (I) |
| C. | Statement (I) is true but statement (II) is false |
| D. | Statement (I) is false but statement (II) is true |
| Answer» B. Statement (I) and Statement (II) are individually true but statement (II) is not the correct explanation of statement (I) | |
| 70. |
"Cogging" in induction motor occurs when __________ |
| A. | number of stator teeth – number of rotor teeth = odd number |
| B. | number of stator teeth – number of rotor teeth = zero |
| C. | number of stator teeth – number of rotor teeth = even number |
| D. | number of stator teeth – number of rotor teeth = negative number |
| Answer» C. number of stator teeth – number of rotor teeth = even number | |
| 71. |
In a 4 pole induction motor, the rotor speed is 1400 rpm. What will be the air gap field speed? |
| A. | 1400 rpm |
| B. | 1700 rpm |
| C. | 1600 rpm |
| D. | 1500 rpm |
| Answer» E. | |
| 72. |
Find the input power when a 7.46 kW, three-phase induction motor having 85% efficiency is connected to a 400 V, 50 Hz AC supply. |
| A. | 74.6 kW |
| B. | 7.46 kW |
| C. | 6.87 kW |
| D. | 8.78 kW |
| Answer» E. | |
| 73. |
A 3-phase induction motor connected in star draws 5 amps. When connected in delta, it will draw a current of |
| A. | 5 A |
| B. | 15 A |
| C. | \(\frac{5}{{\sqrt 3 }}A\) |
| D. | \(\frac{5}{{\sqrt 2 }}A\) |
| Answer» C. \(\frac{5}{{\sqrt 3 }}A\) | |
| 74. |
Imbalance in shaft of an induction motor occurs due to: |
| A. | Non uniform air gap |
| B. | Overheating of the winding |
| C. | Slip rings design |
| D. | Rugged construction |
| Answer» B. Overheating of the winding | |
| 75. |
In a variable speed induction motor derive V/f kept constant over a wide range of its frequency variation. The motor operates in the following mode: |
| A. | Constant power mode |
| B. | Constant torque mode |
| C. | Constant slip mode |
| D. | Variable power and variable torque mode |
| Answer» C. Constant slip mode | |
| 76. |
In a 3-phase induction motor for large value of slip the torque is: |
| A. | Proportional to square of slip |
| B. | Proportional to slip |
| C. | Inversely proportional to square of slip |
| D. | inversely proportional to slip |
| Answer» E. | |
| 77. |
On light loads, induction type A.C motors works on power factor in the range of _______. |
| A. | 0.8 - 0.9 |
| B. | 0.2 - 0.3 |
| C. | 0.5 - 0.6 |
| D. | 0.6 - 0.8 |
| Answer» C. 0.5 - 0.6 | |
| 78. |
At full load, the speed of an induction motor is |
| A. | more than its synchronous speed |
| B. | same as its synchronous speed |
| C. | half of its synchronous speed |
| D. | slightly less than synchronous speed |
| Answer» E. | |
| 79. |
A 4-pole, 3-phase IM operates from a supply with frequency 50 Hz. Calculate the speed at which the magnetic field of stator rotates. |
| A. | 90 rpm |
| B. | 1500 rpm |
| C. | 1440 rpm |
| D. | 600 rpm |
| Answer» C. 1440 rpm | |
| 80. |
A reciprocating pump, which is required to start under load, will need a: |
| A. | synchronous motor |
| B. | squirrel-cage induction motor |
| C. | repulsion motor |
| D. | double squirrel-cage induction motor |
| Answer» E. | |
| 81. |
Consider the following statements:As a three-phase induction motor is loaded from no-load to rated load.1. There is an improvement in the power factor.2. The torque increases almost in proportion to slip.3. The air-gap flux falls sharplyWhich of the above statements are correct? |
| A. | 1, 2 and 3 |
| B. | 1 and 2 only |
| C. | 1 and 3 only |
| D. | 2 and 3 only |
| Answer» C. 1 and 3 only | |
| 82. |
A 50 Hz, 3-phase induction motor has a full load speed of 1440 rpm. The number of poles of the motor are |
| A. | 4 |
| B. | 6 |
| C. | 12 |
| D. | 8 |
| Answer» B. 6 | |
| 83. |
In an induction motor, if the air gap is increased |
| 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 | |
| 84. |
Induction motor rotor always rotate at a speed ________ than synchronous speed |
| A. | same |
| B. | greater |
| C. | less |
| D. | infinite |
| Answer» D. infinite | |
| 85. |
If a 3-phase slip ring induction motor is fed from the rotor side with stator winding short circuited, then frequency of currents flowing in the short circuited stator is |
| A. | Slip × frequency |
| B. | Supply frequency |
| C. | Frequency corresponding to rotor speed |
| D. | Zero |
| Answer» B. Supply frequency | |
| 86. |
In the method of speed control of induction motor by inducing emf in the rotor circuit, if the injected voltage is in phase opposition to the induced rotor emf, then: |
| A. | the rotor resistance decreases |
| B. | the rotor resistance increases |
| C. | the rotor reactance decreases |
| D. | the rotor reactance increases |
| Answer» C. the rotor reactance decreases | |
| 87. |
For a three-phase induction motor, if T is the starting torque when a DOL starter is used, then what is the starting torque when a star-delta starter used is? |
| A. | T/3 |
| B. | T/√3 |
| C. | T/4 |
| D. | T/2 |
| Answer» B. T/√3 | |
| 88. |
Consider the following data regarding the name plate of 1-phase, 4-pole induction motor: Output = 373 W; 230 V, frequency = 50 Hz, input current = 2.9 A, power factor = 0.71, speed = 1410 rpm. The efficiency of motor will be nearly |
| A. | 72.8% |
| B. | 78.8% |
| C. | 84.4% |
| D. | 88.4% |
| Answer» C. 84.4% | |
| 89. |
In a 3-phase induction motor a balanced three phase supply is with electrical angle separation |
| A. | 30° |
| B. | 60° |
| C. | 90° |
| D. | 120° |
| Answer» E. | |
| 90. |
Induction motor can be regarded as a generalized transformer due to certain similarities except rated |
| A. | Frequency |
| B. | Flux |
| C. | Speed |
| D. | Induced emf |
| Answer» D. Induced emf | |
| 91. |
During the starting of a slip ring induction motor using rotor resistance starter, the insertion of resistance in the rotor circuit causes: |
| A. | stator current to increase and torque to decrease |
| B. | stator current to decrease and torque to increase |
| C. | stator current to increase and power factor to decrease |
| D. | power factor to decrease and torque to increase |
| Answer» C. stator current to increase and power factor to decrease | |
| 92. |
In order to run an induction motor at synchronous speed __________ |
| A. | load must be reduced |
| B. | friction at bearing must be reduced |
| C. | e.m.f must be injected in the rotor in a phase with rotor e.m.f |
| D. | e.m.f. must be injected in the rotor in a phase opposite to the rotor e.m.f. |
| Answer» D. e.m.f. must be injected in the rotor in a phase opposite to the rotor e.m.f. | |
| 93. |
In an Induction motor, what will be the value of slip to get maximum torque under running condition? |
| A. | s = R2 / X2 |
| B. | s = 1 |
| C. | s = X2 / (R2 + X2) |
| D. | s = X2 / R2 |
| Answer» B. s = 1 | |
| 94. |
A 4-pole, 50 Hz, 3-ϕ induction motor has a slip of 6%. Calculate the speed of the motor. |
| A. | 1250 rpm |
| B. | 1350 rpm |
| C. | 1500 rpm |
| D. | 1410 rpm |
| Answer» E. | |
| 95. |
If rotor resistance in an induction motor is small, the rotor current will be: |
| A. | Zero |
| B. | Low |
| C. | Large |
| D. | Infinite |
| Answer» D. Infinite | |
| 96. |
A three phase four pole induction motor is operating on an input frequency of 75 Hz and slip of 4%.If the rotor resistance of the machine is 1 Ω. The torque developed by the motor is given by: (Assume the operating voltage is 415 V. assuming stator voltage drop = 0) |
| A. | 6889 syn.watts |
| B. | 7898 syn.watts |
| C. | 8998 syn.watts |
| D. | 9998 syn.watts |
| Answer» B. 7898 syn.watts | |
| 97. |
An induction machine works at generating mode when slip is: |
| A. | Zero |
| B. | 0 - 1 |
| C. | More than one |
| D. | Negative |
| Answer» E. | |
| 98. |
A small 3-phase induction motor has a short-circuit current 5 times of full-load current and full-load slip 5%. If starting resistance starter is used to reduce the impressed voltage to 60% of normal voltage, the starting torque obtained in terms of full load torque would be |
| A. | 30% |
| B. | 45% |
| C. | 55% |
| D. | 80% |
| Answer» C. 55% | |
| 99. |
Directions: It consists of two statements one labeled as statement I and the other as statement II. Examine these two statements carefully and select the correct answer using the code given below.Statement I: An induction motor always runs at a speed less than its synchronous speed.Statement II: Synchronous speed of induction motor depends on the frequency of supply and number of poles. |
| A. | Both statements I and statement II are individually true and statement II is the correct explanation of statement I |
| B. | Both statement I and statement II are individually true but statement II is not the correct explanation of statement I |
| C. | Statements I is true but statement II is false |
| D. | Statement I is false but statement II is true |
| Answer» C. Statements I is true but statement II is false | |
| 100. |
Pull out torque of a squirrel cage induction motor occurs at that value of slip where rotor power factor equals |
| A. | 0.5 |
| B. | 0.866 |
| C. | 0.707 |
| D. | 1 |
| Answer» D. 1 | |