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.
| 1. |
Cogging in an induction motor is caused |
| A. | If the number of stator slots are unequal to number of rotor slots |
| B. | If the number of stator slots are an integral multiple of rotor slots |
| C. | If the motor is running at fraction of its rated speed |
| D. | Due to 5th harmonic |
| Answer» C. If the motor is running at fraction of its rated speed | |
| 2. |
In an induction motor, if the number of poles is doubled, the speed will be ______. |
| A. | same |
| B. | doubled |
| C. | Inverse |
| D. | Half |
| Answer» E. | |
| 3. |
From the design perspective, how can the torque of a squirrel-cage induction motor be increased? |
| A. | By increasing the supply voltage |
| B. | By increasing the rotor resistance |
| C. | By increasing the rotor reactance |
| D. | By increasing rotor EMF |
| Answer» C. By increasing the rotor reactance | |
| 4. |
If during a no-load test on an induction motor it takes 10 A current and 300 watts of power at a line voltage of 200 V, the stator core loss will be (assume stator resistance /phase as 0.3 Ω) |
| A. | 165 W |
| B. | 300 W |
| C. | 210 W |
| D. | 192 W |
| Answer» D. 192 W | |
| 5. |
A three phase induction motor is driving full-load torque which is independent of speed. If the line voltage drops to 90% of the rated value. % increase in motor copper losses |
| A. | 23% |
| B. | -19% |
| C. | 123% |
| D. | None of these |
| Answer» B. -19% | |
| 6. |
Consider the following statements with regards to an induction motor:1. Maximum torque is independent of rotor resistance.2. Starting torque is maximum when rotor resistance equals rotor reactance.3. Torque is very sensitive to any changes in supply voltage.Which of the above statements are correct? |
| A. | 1 and 2 only |
| B. | 1 and 3 only |
| C. | 2 and 3 only |
| D. | 1, 2 and 3 |
| Answer» E. | |
| 7. |
A 3-phase, 400/200 V, Y— Y connected wound-rotor induction motor has 0.06 Ω rotor resistance and 0.3 Ω standstill reactance per phase. To make the starting torque equal to the maximum torque, the additional resistance required in the rotor circuit will be |
| A. | 0.24 Ω /phase |
| B. | 0.34 Ω /phase |
| C. | 0.42 Ω /phase |
| D. | 0.52 Ω /phase |
| Answer» B. 0.34 Ω /phase | |
| 8. |
For controlling the speed of 3ϕ induction motor, which of the following scheme is used: |
| A. | Fixed voltage and fixed frequency scheme |
| B. | Fixed voltage and variable frequency scheme |
| C. | Variable voltage and variable frequency scheme |
| D. | Variable voltage and fixed frequency scheme |
| Answer» D. Variable voltage and fixed frequency scheme | |
| 9. |
_______ is used to support the rotor of an induction motor. |
| A. | Ball bearing |
| B. | Needle bearing |
| C. | Plummer block |
| D. | Bush bearing |
| Answer» B. Needle bearing | |
| 10. |
A balanced 3 – phase induction motor runs at slip s. If ωs is its synchronous speed, what is the relative speed between the stator mmf and rotor mmf ? |
| A. | s ωs |
| B. | (1 – s) ωs |
| C. | ωs |
| D. | Zero |
| Answer» E. | |
| 11. |
In a 3-phase induction motor the torque under running conditions will be maximum when: |
| A. | the rotor resistance per phase is equal to slip times the rotor reactance per phase |
| B. | the rotor resistance becomes zero |
| C. | the rotor resistance is not equal to the rotor reactance |
| D. | the rotor resistance is twice the rotor reactance |
| Answer» B. the rotor resistance becomes zero | |
| 12. |
At slip equal to infinity, the torque of a three-phase induction motor is: |
| A. | Zero |
| B. | Very small |
| C. | High |
| D. | Very high |
| Answer» B. Very small | |
| 13. |
A three-phase induction motor rated for 500 kW, 400 V, 50 Hz and 6 poles runs at a speed of 960 rpm when fully loaded. Slip of the machine will be: |
| A. | 0.04 |
| B. | 0.01 |
| C. | 0.02 |
| D. | 0.03 |
| Answer» B. 0.01 | |
| 14. |
In torque-slip characteristics of an induction motor, at normal speeds close to synchronism the torque is: |
| A. | directly proportional to the slip |
| B. | not dependent on the slip |
| C. | maximum |
| D. | inversely proportional to the slip. |
| Answer» B. not dependent on the slip | |
| 15. |
__________ are employed for the operation of Jaw Crushers. |
| A. | DC shunt-wound motor |
| B. | Squirrel cage induction motor |
| C. | Belted slip ring induction motor |
| D. | Any DC motor |
| Answer» D. Any DC motor | |
| 16. |
In an electric drive using a three-phase induction motor, it is intended to apply regenerative braking. This is possible only if: |
| A. | The rotor speed is greater than the speed of the magnetic field produced by stator currents, both rotating in the same direction. |
| B. | The rotor speed is equal to the speed of the magnetic field produced by stator currents, both rotating in the same direction. |
| C. | The rotor speed is greater than the speed of the magnetic field produced by stator currents, both rotating in opposite directions. |
| D. | The rotor speed is less than the speed of the magnetic field produced by stator currents, both rotating in the same direction. |
| Answer» B. The rotor speed is equal to the speed of the magnetic field produced by stator currents, both rotating in the same direction. | |
| 17. |
Power supply to a 10 pole induction motor is supplied by 4 pole alternator which is driven at 1500 RPM. If the motor runs with slip of 4%, what is the speed? |
| A. | 750 RPM |
| B. | 576 RPM |
| C. | 600 RPM |
| D. | 240 RPM |
| Answer» C. 600 RPM | |
| 18. |
In induction motor, the short circuit test is a ___________ test. |
| A. | Blocked rotor |
| B. | Stator resistance |
| C. | Open circuit |
| D. | No-load |
| Answer» B. Stator resistance | |
| 19. |
For controlling the speed of an induction motor the supply frequency is increased by 10%. For maximum toque to remain constant, the supply voltage must |
| A. | Be increased by 10% |
| B. | Remain constant |
| C. | Be decreased by 10% |
| D. | Be reduced or increased by 20% |
| Answer» B. Remain constant | |
| 20. |
A three phase induction machine, when it is operating like a motor, the range of slip ‘s’ is given by: |
| A. | 0 < s < 1 |
| B. | 1 < s < 2 |
| C. | -1 > s > -2 |
| D. | 0 > s > -1 |
| Answer» B. 1 < s < 2 | |
| 21. |
A squirrel cage induction motor has a starting current of 6 times the full load current and a full load slip of 0.05. If it is started using a star-delta starter, the values of the starting line current and the starting torque, respectively are: |
| A. | 2 pu and 0.6 pu |
| B. | 4 pu and 1.2 pu |
| C. | 1.15 pu and 0.35 pu |
| D. | 3.46 pu and 0.17 pu |
| Answer» B. 4 pu and 1.2 pu | |
| 22. |
In a three-phase induction motor, if the air gap is increased then its: |
| A. | power factor will reduce |
| B. | efficiency will improve |
| C. | speed will reduce |
| D. | breakdown torque will reduce |
| Answer» B. efficiency will improve | |
| 23. |
During single phasing, the unbalanced stator current have a negative sequence component, which cause |
| A. | Magnetic flux rotating in opposite direction to main flux |
| B. | Magnetic flux rotating in same direction to main field |
| C. | Increase the speed of the motor above synchronous speed |
| D. | Motor starts rotating in opposite direction |
| Answer» B. Magnetic flux rotating in same direction to main field | |
| 24. |
A three phase induction motor connected to a three phase supply with a phase sequence of R-Y-B rotates in clockwise direction. Which of the following effects are observed if the phase sequence is changed to R-B-Y? |
| A. | Rotate in anticlockwise direction |
| B. | Continue to rotate in clockwise direction |
| C. | Not rotate |
| D. | Oscillate |
| Answer» B. Continue to rotate in clockwise direction | |
| 25. |
An Induction Motor is expected to run at 950 rpm at full load. With a power supply of 50 Hz. What could be the number of poles in the IM? |
| A. | four |
| B. | Six |
| C. | Eight |
| D. | Two |
| Answer» C. Eight | |
| 26. |
For a slip ring induction motor if the rotor resistance is increased the |
| A. | Starting torque and efficiency decrease. |
| B. | Starting torque and efficiency increase. |
| C. | Starting torque increases but efficiency decreases. |
| D. | Starting torque decreases but efficiency increases. |
| Answer» D. Starting torque decreases but efficiency increases. | |
| 27. |
In an induction motor, number of stator slots should never be equal to number of rotor slots in order to prevent: |
| A. | Crawling |
| B. | Cogging |
| C. | Over heating |
| D. | Humming |
| Answer» C. Over heating | |
| 28. |
Choose the WRONG statement from the below with regard to the 3-ϕ induction motor.Blocked rotor test on a 3-ϕ induction motor helps to find |
| A. | Short-circuit current with normal voltage |
| B. | Motor resistance as referred to the stator |
| C. | Short-circuit power factor |
| D. | Fixed losses |
| Answer» E. | |
| 29. |
In the per phase rotor equivalent circuit of a three-phase induction motor as shown below:Expression for R1 will be given by: |
| A. | R |
| B. | 0 |
| C. | \(R\left( {\frac{{1 - s}}{s}} \right)\) |
| D. | \(R\left( {\frac{1}{s}} \right)\) |
| Answer» D. \(R\left( {\frac{1}{s}} \right)\) | |
| 30. |
In a 3 phase induction motor, the speed of the revolving magnetic field is |
| A. | directly proportional to the frequency of the source |
| B. | inversely proportional to the frequency of the source |
| C. | independent of the frequency of the source |
| D. | equal to the frequency of the source |
| Answer» B. inversely proportional to the frequency of the source | |
| 31. |
In case of 3 phase induction motor, synchronous speed is: |
| A. | Speed of stator |
| B. | Speed of rotor |
| C. | Speed of rotor and rotor flux |
| D. | Speed of stator flux |
| Answer» E. | |
| 32. |
A 6-pole, 50-Hz, three-phase induction motor develops mechanical power 16.252 kW at a rotor frequency of 1.5 Hz. The value of the rotor copper loss will be: |
| A. | 235 W |
| B. | 0.5026 kW |
| C. | 1.23 kW |
| D. | 0.6234 kW |
| Answer» C. 1.23 kW | |
| 33. |
In an induction motor for a value slip(s) = 0, the torque (T) is |
| A. | maximum |
| B. | zero |
| C. | minimum |
| D. | Twice the value of full torque |
| Answer» C. minimum | |
| 34. |
In a 3-phase induction motor, the relative speed of stator flux with respect to ______ is zero. |
| A. | Stator winding |
| B. | Rotor |
| C. | Rotor flux |
| D. | Space |
| Answer» D. Space | |
| 35. |
A three phase induction motor running at 1440 rpm on rated supply will run at the following rpm when fuse of one phase gets burnt |
| A. | 1440 |
| B. | 1380 |
| C. | 1500 |
| D. | Zero |
| Answer» C. 1500 | |
| 36. |
A 4 pole, 400 V, 60 Hz, 1750 RPM, 3 phase induction motor is connected to 400 V, 50 Hz supply. The motor will: |
| A. | not run at all |
| B. | burn out |
| C. | run at speed more than 1750 RPM |
| D. | run at speed less than 1750 RPM |
| Answer» E. | |
| 37. |
Blocked rotor test is used to find which of the following losses? |
| A. | Constant losses |
| B. | Variable losses |
| C. | Constant losses and Variable losses both |
| D. | Rotational loss |
| Answer» C. Constant losses and Variable losses both | |
| 38. |
A four-pole, three-phase Induction Motor operates from a supply with 50 Hz. What is the frequency of the rotor currents when the slip is 0.06? |
| A. | 50 Hz |
| B. | 48 Hz |
| C. | 3 Hz |
| D. | 47 Hz |
| Answer» D. 47 Hz | |
| 39. |
Magnetic locking of the rotor with stator in case of an induction motor may be reduced by: |
| A. | Using interpoles |
| B. | Laminating the rotor core |
| C. | Equaliser connections |
| D. | Skewed rotor slots |
| Answer» E. | |
| 40. |
An 8 pole alternator runs at 750 rpm. It supplies power to 6 pole induction motor, which has full load slip of 4%. Full load speed of the motor is |
| A. | 705 rpm |
| B. | 750 rpm |
| C. | 960 rpm |
| D. | 970 rpm |
| Answer» D. 970 rpm | |
| 41. |
An induction machine is operated with two values of frequencies with the ratio of 1:2 without changing the voltage. Then the starting torque of the machine for two cases will be in the ratio of |
| A. | 1 : 2 |
| B. | 1 : 4 |
| C. | 8 : 1 |
| D. | 4 : 1 |
| Answer» D. 4 : 1 | |
| 42. |
For obtaining very quick braking of a 3-phase, wound-rotor induction motor running on load |
| A. | a large external resistance has to be inserted in the rotor-circuit |
| B. | a large external resistance has to be inserted in the stator-circuit |
| C. | interchange any two terminals of the stator supply |
| D. | interchange any two terminals of the rotor to the sliprings |
| Answer» D. interchange any two terminals of the rotor to the sliprings | |
| 43. |
In the circle diagram for a 3-phase induction motor, the diameter of the circle is determined by |
| A. | total stator current |
| B. | exciting current |
| C. | rotor current |
| D. | rotor current referred to stator |
| Answer» E. | |
| 44. |
In three induction motor the initial inrush of current is produce |
| A. | large line voltage drop |
| B. | large phase voltage drop |
| C. | damage the rotor windings |
| D. | high starting torque |
| Answer» B. large phase voltage drop | |
| 45. |
A three-phase, 4-pole, self excited induction generator is feeding power to a load at a frequency f1. If the load is partially removed, the frequency becomes f2. If the speed of the generator is maintained at 1500 rpm in both the cases, then |
| A. | f1, f2 > 50 and f1 > f2 |
| B. | f1 < 50 Hz and f2 > 50 Hz |
| C. | f1, f2 < 50 Hz and f2 > f1 |
| D. | f1 > 50 Hz and f2 < 50 Hz |
| Answer» D. f1 > 50 Hz and f2 < 50 Hz | |
| 46. |
A variable speed three-phase induction motor drive obtained by varying the applied voltage is suitable for |
| A. | Driving constant torque loads |
| B. | Driving fan type loads where torque proportional to square of speed |
| C. | Driving loads where torque is inversely proportional to square of the speed |
| D. | Driving all type of loads |
| Answer» C. Driving loads where torque is inversely proportional to square of the speed | |
| 47. |
Find the frequency of rotor induced EMF of a 3-phase, 440 V, 50 Hz induction motor having a slip of 10%. |
| A. | 50 Hz |
| B. | 2.5 Hz |
| C. | 5 Hz |
| D. | 25 Hz |
| Answer» D. 25 Hz | |
| 48. |
Induction generators deliver power at ______ power factor |
| A. | Lagging |
| B. | Leading |
| C. | Unity |
| D. | Zero |
| Answer» C. Unity | |
| 49. |
In a three-phase induction motor, the slip is generally: |
| A. | 15% to 25% |
| B. | 2% to 5% |
| C. | 0 |
| D. | More than 45% |
| Answer» C. 0 | |
| 50. |
In a 3-phase, 4-pole 50 Hz induction motor running at 1450 rpm, the frequency of rotor currents will be |
| A. | 50 Hz |
| B. | 1.66 Hz |
| C. | 6.1 Hz |
| D. | 1.72 Hz |
| Answer» C. 6.1 Hz | |