McqMate
These multiple-choice questions (MCQs) are designed to enhance your knowledge and understanding in the following areas: Electrical Engineering .
| 1. |
Synchronous motors are generally not self-starting because |
| A. | the direction of rotation is not fixed |
| B. | the direction of instantaneous torque reverses after half cycle |
| C. | startes cannot be used on these machines |
| D. | starting winding is not provided on the machines |
| Answer» B. the direction of instantaneous torque reverses after half cycle | |
| 2. |
In case one phase of a three-phase synchronous motor is short-circuited the motor will |
| A. | not start |
| B. | run at 2/3 of synchronous speed |
| C. | run with excessive vibrations |
| D. | take less than the rated load |
| Answer» A. not start | |
| 3. |
A pony motor is basically a |
| A. | small induction motor |
| B. | D.C. series motor |
| C. | D.C. shunt motor |
| D. | double winding A.C./D.C. motor |
| Answer» A. small induction motor | |
| 4. |
A synchronous motor can develop synchronous torque |
| A. | when under loaded |
| B. | while over-excited |
| C. | only at synchronous speed |
| D. | below or above synchronous speed |
| Answer» C. only at synchronous speed | |
| 5. |
A synchronous motor can be started by |
| A. | pony motor |
| B. | D.C. compound motor |
| C. | providing damper winding |
| D. | any of the above |
| Answer» D. any of the above | |
| 6. |
A three-phase synchronous motor will have |
| A. | no slip-rings |
| B. | one slip-ring |
| C. | two slip-rings |
| D. | three slip-rings |
| Answer» C. two slip-rings | |
| 7. |
Under which of the following conditions hunting of synchronous motor is likely to occur ? |
| A. | Periodic variation of load |
| B. | Over-excitation |
| C. | Over-loading for long periods |
| D. | Small and constant load |
| Answer» A. Periodic variation of load | |
| 8. |
When the excitation of an unloaded salient pole synchronous motor suddenly gets disconnected |
| 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» A. the motor stops | |
| 9. |
When V is the applied voltage, then the breakdown torque of a synchronous motor varies as |
| A. | V |
| B. | V312 |
| C. | V2 |
| D. | 1/V |
| Answer» A. V | |
| 10. |
The power developed by a synchronous motor will be maximum when the load angle is |
| A. | zero |
| B. | 45° |
| C. | 90° |
| D. | 120° |
| Answer» C. 90° | |
| 11. |
A synchronous motor can be used as a synchronous capacitor when it is |
| A. | under-loaded |
| B. | over-loaded |
| C. | under-excited |
| D. | over-excited |
| Answer» D. over-excited | |
| 12. |
A synchronous motor is running on a load with normal excitation. Now if the load on the motor is increased |
| A. | power factor as well as armature current will decrease |
| B. | power factor as well as armature current will increase |
| C. | power factor will increase but armature current will decrease |
| D. | power factor will decrease and armature current will increase |
| Answer» D. power factor will decrease and armature current will increase | |
| 13. |
Mostly, synchronous motors are of |
| A. | alternator type machines |
| B. | induction type machines |
| C. | salient pole type machines |
| D. | smooth cylindrical type machines |
| Answer» C. salient pole type machines | |
| 14. |
The synchronous motor is not inherently self-starting because |
| A. | the force required to accelerate the rotor to the synchronous speed in an instant is absent |
| B. | the starting device to accelerate the rotor to near synchronous speed is absent |
| C. | a rotating magnetic field does not have enough poles |
| D. | the rotating magnetic field is produced by only 50 Hz frequency currents |
| Answer» A. the force required to accelerate the rotor to the synchronous speed in an instant is absent | |
| 15. |
As the load is applied to a synchronous motor, the motor takes more armature current because |
| 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» B. the rotor by shifting its phase backward causes motor to take more current | |
| 16. |
Synchronous motor always runs at |
| A. | the synchronous speed |
| B. | less than synchronous speed |
| C. | more than synchronous speed |
| D. | none of the above |
| Answer» A. the synchronous speed | |
| 17. |
An over-excited synchronous motor takes |
| A. | leading current |
| B. | lagging current |
| C. | both (a) and (b) |
| D. | none of the above |
| Answer» A. leading current | |
| 18. |
The working of a synchronous motor is similar to |
| A. | gear train arrangement |
| B. | transmission of mechancial power by shaft |
| C. | distribution transformer |
| D. | turbine |
| Answer» B. transmission of mechancial power by shaft | |
| 19. |
The minimum armature current of the synchronous motor corresponds to operation at |
| A. | zero power factor leading |
| B. | unity power factor |
| C. | 0.707 power factor lagging |
| D. | 0.707 power factor leading |
| Answer» B. unity power factor | |
| 20. |
In a synchronous motor, the magnitude of stator back e.m.f. £& depends on |
| A. | d.c. excitation only |
| B. | speed of the motor |
| C. | load on the motor |
| D. | both the speed and rotor flux |
| Answer» A. d.c. excitation only | |
| 21. |
If load (or torque) angle of a 4-pole synchronous motor is 6° electrical, its value in mechanical degrees is |
| A. | 2 |
| B. | 3 |
| C. | 4 |
| D. | 6 |
| Answer» B. 3 | |
| 22. |
For V-curves for a synchronous motor the graph is drawn between |
| A. | field current and armature current |
| B. | terminal voltage and load factor |
| C. | power factor and field current |
| D. | armature current and power factor |
| Answer» A. field current and armature current | |
| 23. |
The back e.m.f. of a synchronous motor depends on |
| A. | speed |
| B. | load |
| C. | load angle |
| D. | all of the above |
| Answer» C. load angle | |
| 24. |
A synchronous motor can operate at |
| A. | lagging power factor only |
| B. | leading power factor only |
| C. | unity power factor only |
| D. | lagging, leading and unity power factors |
| Answer» D. lagging, leading and unity power factors | |
| 25. |
In a synchronous motor which loss varies with load ? |
| A. | Windage loss |
| B. | Bearing friction loss |
| C. | Copper loss |
| D. | Core loss |
| Answer» C. Copper loss | |
| 26. |
A synchronous motor can be made self starting by providing |
| A. | damper winding on rotor poles |
| B. | damper winding on stator |
| C. | damper winding on stator as well as rotor poles |
| D. | none of the above |
| Answer» D. none of the above | |
| 27. |
The oscillations in a synchronous motor can be damped out by |
| A. | maintaining constant excitation |
| B. | running the motor on leading power factors |
| C. | providing damper bars in the rotor pole faces |
| D. | oscillations cannot be damped |
| Answer» C. providing damper bars in the rotor pole faces | |
| 28. |
The shaft of synchronous motor is made of |
| A. | mild steel |
| B. | chrome steel |
| C. | alnico |
| D. | stainless steel |
| Answer» A. mild steel | |
| 29. |
When the field of a synchronous motor is under-excited, the power factor will be |
| A. | leading |
| B. | lagging |
| C. | unity |
| D. | zero |
| Answer» B. lagging | |
| 30. |
The speed regulation of a synchronous motor is always |
| A. | 1% |
| B. | 0.5% |
| C. | positive |
| D. | zero |
| Answer» D. zero | |
| 31. |
The percentage slip in case of a synchronous motor is |
| A. | 1% |
| B. | 100% |
| C. | 0.5% |
| D. | zero |
| Answer» D. zero | |
| 32. |
The operating speed of a synchronous motor can be changed to new fixed value by |
| A. | changing the load |
| B. | changing the supply voltage |
| C. | changing frequency |
| D. | using brakes |
| Answer» C. changing frequency | |
| 33. |
A synchronous motor will always stop when |
| A. | supply voltage fluctuates |
| B. | load in motor varies |
| C. | excitation winding gets disconnected |
| D. | supply voltage frequency changes |
| Answer» C. excitation winding gets disconnected | |
| 34. |
riunting in a synchronous motor takes place |
| A. | when supply voltage fluctuates |
| B. | when load varies |
| C. | when power factor is unity |
| D. | motor is under loaded |
| Answer» B. when load varies | |
| 35. |
When load on an over-excited or under excited synchronous*motor is increased, rate of change of its armature current as compared with that of power factor is |
| A. | more |
| B. | less |
| C. | equal |
| D. | twice |
| Answer» B. less | |
| 36. |
The rotor copper losses, in a synchronous motor, are met by |
| A. | d.c. source |
| B. | armature input |
| C. | motor input |
| D. | supply lines |
| Answer» A. d.c. source | |
| 37. |
The maximum power developed in a synchronous motor occurs at a coupling angle of |
| A. | 30° |
| B. | 60° |
| C. | 90° |
| D. | 180° |
| Answer» C. 90° | |
| 38. |
When the stator windings are connected in such a fashion that the number of poles are made half, the speed of the rotor of a synchronous motor |
| A. | remains same as the original value |
| B. | decreases to half the original value |
| C. | tends to becomes zero |
| D. | increases to two times the original value |
| Answer» D. increases to two times the original value | |
| 39. |
In which of the following motors the stator and rotor magnetic field rotate at the same speed ? |
| A. | Universal motor |
| B. | Synchronous motor |
| C. | Induction motor |
| D. | Reluctance motor |
| Answer» B. Synchronous motor | |
| 40. |
Synchronsizingpower of a synchronous machine is |
| A. | direcly proportional to the synchronous reactance |
| B. | inversely proportional to the synchronous reactance |
| C. | equal to the synchronous reactance |
| D. | none of the above |
| Answer» B. inversely proportional to the synchronous reactance | |
| 41. |
Synchronous motors are |
| A. | not-self starting |
| B. | self-starting |
| C. | essentially self-starting |
| D. | none of the above |
| Answer» A. not-self starting | |
| 42. |
The standard full-load power factor ratings for synchronous motors are |
| A. | zero or 0.8 leading |
| B. | unity or 0.8 lagging |
| C. | unity or 0.8 leading |
| D. | unity or zero |
| Answer» C. unity or 0.8 leading | |
| 43. |
A synchronous motor running with normal excitation adjusts to load increases essentially by increase in |
| A. | back e.m.f. |
| B. | armature current |
| C. | power factor |
| D. | torque angle |
| Answer» B. armature current | |
| 44. |
A synchronous motor has better power factor as compared to that of an equivalent induction motor. This is mainly because |
| A. | synchronous motor has no slip |
| B. | stator supply is not required to produce magnetic field |
| C. | mechanical load on the rotor remains constant |
| D. | synchronous motor has large airgap |
| Answer» B. stator supply is not required to produce magnetic field | |
| 45. |
A synchronous motor working at leading power factor can be used as |
| A. | voltage booster |
| B. | phase advancer |
| C. | noise generator |
| D. | mechanical synchronizer |
| Answer» B. phase advancer | |
| 46. |
Slip rings are usually made of |
| A. | carbon or graphite |
| B. | brass or steel |
| C. | silver or gold |
| D. | copper or aluminium |
| Answer» B. brass or steel | |
| 47. |
An over excited synchronous motor is used for |
| A. | fluctuating loads |
| B. | variable speed loads |
| C. | low torque loads |
| D. | power factor corrections |
| Answer» D. power factor corrections | |
| 48. |
When the voltage applied to a synchronous motor is increased, which of the following will reduce ? |
| A. | Stator flux |
| B. | Pull in torque |
| C. | Both (a) and (b) |
| D. | None of the above |
| Answer» D. None of the above | |
| 49. |
The efficiency of a properly designed synchronous motor will usually fall in range |
| A. | 60 to 70% |
| B. | 75 to 80% |
| C. | 85 to 95% |
| D. | 99 to 99.5% |
| Answer» C. 85 to 95% | |
| 50. |
To limit the operating temperature an electrical machine should have proper |
| A. | voltage rating |
| B. | current rating |
| C. | power factor |
| D. | speed |
| Answer» B. current rating | |
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