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Sunday, 17 July 2016

Synchronous Motors - Multiple Choice Questions

Synchronous Motors - Multiple Choice Questions Points : Synchronous Motors, multiple choice questions and answers, electrical objective type question answers, mcqs 1. A synchronous motor rims at speeds ranging from .............
(i) 1800 to 3600 r.p.m.
(ii) 3600 to 6000 r.p.m.
(iii) 150 to 1800 r.p.m.
(iv) none of the above

2. The rotor of a synchronous motor is excited with direct current while the motor …………..
(i) is at standstill (i.e. at starting)
(ii) approaches synchronous speed
(ii) approaches half synchronous speed
(iv) none of the above

3. A synchronous motor runs at only one speed (i. e. synchronous speed) because it ………………
(i) has no losses
(ii) is a doubly fed machine
(iii) has a damper winding
(iv) none of the above

4. A 50 Hz synchronous motors runs at 200 r.p.m. The number of salient poles on the rotor are ………..
(i) 36
(ii) 16
(iii) 30
(iv) none of the above

5. The pull-out torque of a synchronous motor is generally............... times its full—load torque.
(i) 1.25 to3.5
(ii) 5 to 10
(iii) 6 to 9
(iv) 0.5

6. For a given load, the normal field excitation of synchronous motor is that which gives ............... power factor.
(i) 0.8 leading
(ii) 0.8 lagging
(iii) unity
(iv) none of the above

7. Rotor of a synchronous motor is ……….
(i) salient-pole type
(ii) nonsabent-pole type
(iii) identical to that of a d.c. motor
(iv) none of the above

8. The pull-out torque of a synchronous motor will............ as the field excitation is increased.
(i) decrease
(ii) remain unchanged
(iii) increase
(iv) none of the above

9. In order to reverse the direction of a synchronous motor …………….
(i) interchange any to stator lines
(ii) reduce d.c. field excitation to zero
(iii) change supply frequency
(iv) none of the above

10. For a given load, the armature current of a synchronous motor will be minimum for ...............power factor.
(i) unity
(ii) 0.8 leading
(iii) 0.8 lagging
(iv) none of the above

11. When the synchronous motor is on no load, the axes of the rotor poles.............. that of stator poles.
(i) nearly coincide with
(ii) are far ahead
(iii) are far behind
(iv) none of the above

12. The increased load demand in a synchronous motor is met by ………..
(i) reduction in speed
(ii) increase in speed
(iii) decrease in stator current
(iv) relative shift between stator and rotor poles

13. The speed of a synchronous motor can be changed by varying ………..
(i) mechanical load
(ii) field excitation
(iii) supply frequency
(iv) none of the above

14. When the synchronous motor runs at synchronous speed, voltage induce in damper winding is ………….
(i) maximum
(ii) minimum
(iii) zero
(iv) none of the above

15. If a synchronous motor is operating at a supply frequency of 50 cycles/second, then, rotor revolves at ……………
(i) 25 cycles/second
(ii) 50 cycles/second
(iii) 100 cycles/second
(iv) none of the above

16. When pull-out torque occurs In a synchronous motor, the poles of the rotor are …………..
(i) midway between N and S poles of
(ii) coincident with stator poles
(iii) 45° (electrical) behind stator poles
(iv) none of the above

17. Damper winding in a synchronous motor ………….
(i) reduces windage losses
(ii) serves to start the motor
(iii) improves p.f. of the motor
(iv) increases bunting of the motor

18. Small synchronous motors are started with
(i) pony motor
(ii) damper winding
(iii) variable-frequency
(iv) none of the above

19. The full-load slip of a synchronous motor is ………….
(i) 5%
(ii) 1%
(iii) 2%
(iv) zero

20. For a given load, if the field excitation of a synchronous motor is increased, then armature current ............
(i) decreases
(ii) increases
(iii) becomes minimum
(iv) remains unchanged

21. A synchronous motor will be more stable in its operation when it is operated at. .............. power factor.
(i) unity
(ii) leading
(iii) tagging
(iv) none of the above

22. When a synchronous motor is on no load, the torque angle is about ...............
(i) 45° electrical
(ii) 90° electrical
(iii) 30° electrical
(iv) 0° electrical

23. If supply frequency of synchronous motors in 60 cycles second, then the rotor have to revolve at ……….
(i) 25 cycles/second
(ii) 60 cycles/second
(iii) 100 cycles/second
(iv) none of the above

24. For the same horsepower output the efficiency of a synchronous motor is ………….. that of 3 - phase induction motor.
(i) about the same as
(ii) greater
(iii) lesser than
(iv) none of the above

25. The power-factor correction capability of a loaded synchronous motor .............. with the increase in mechanical load.
(i) decreases
(ii) increases
(iii) remains unchanged
(Iv) none of the above

26. A synchronous motor is a ............... motor
(i) variable p.f.
(ii) variable speed
(iii) singly fed
(iv) none of the above

27. The size of a synchronous motor is determined by its ..............
(i) kVA rating
(ii) kW rating
(iii) KVAR rating
(iv) none of the above

28. Most of the synchronous motors are ………….. .power factor motors.
(i) unity
(ii) 0.8 leading
(iii) 0.8 lagging
(iv) 0.5 leading

29. At full-load, the rotor poles of a synchronous motor are displaced by a mechanical angle of 1° from their no load position. If the machine has 40 poles, then torque angle is …………..
(i) 40° electrical
(ii) 20° electrical
(iii) 10° electrical
(iv) none of the above

30. The mechanical power developed by a synchronous motor is independent of …………..
(i) applied stator voltage
(ii) torque angle
(iii) field excitation
(iv) speed

31. For the same active power consumed, a synchronous motor draws .............. that of 3-phase induction motor.
(i) more apparent power thin
(ii) the same apparent power thin
(iii) less apparent power than
(iv) none of the above

32. A synchronous motor Is cheaper than 3-phase induction motor for ............
(i) high speed, high output
(ii) high speed, low output
(iii) low speed, high output
(iv) none of the above

33. When a synchronous motor is pulled out of synchronism., the stator current ............
(i) becomes zero,
(ii) is increased too much
(iii) is decreased too much
(iv) remain unchanged

34. When a unity p.f. synchronous motor is operated at a leading p.f. it results in ………….
(i) more than rated horsepower output
(ii) more than rated stator current
(iii) less than rated horsepower output
(iv) none of the above

35. Synchronous motors are generally used in applications requiring ……………
(i) infrequent starting
(ii) variable speed
(iii) sudden application of heavy loads
(iv) frequent stopping

36. If the mechanical angle a between the stator and rotor poles increases, then, stator current ……………
(i) is decreased
(ii) remains unchanged
(iii) is increased
(iv) none of the above

37. Leading power factor synchronous motors cost ............... the unity of p.f. motors.
(i) less than
(ii) more than
(iii) about the same as
(iv) none of the above

38. The putt-out torque of a practical synchronous motor will occur when the torque angle is about .......
(i) 0°
(ii) 30°
(iii) 45°
(iv) 75°

39. For the same active power consumed, the losses in the stator winding of a synchronous motor are .............. that of a 3-phase induction motor.
(i) more
(ii) less
(iii) much more
(iv) about the same as

40. The change of load on a synchronous motor will not result in the change of ...............
(i) armature current
(ii) speed
(iii) torque angle
(iv) power factor

41. An over-excited synchronous motor behaves as ............
(i) a resistor
(ii) an inductor
(iii) a capacitor
(iv) none of the above

42. An under-excited synchronous motor behaves as ...............
(i) an inductor
(ii) a capacitor
(iii) a resistor
(iv) none to the above

43. While running, a synchronous motor is compelled to run at synchronous speed because of ............
(i) damper winding in its pole faces
(ii) magnetic locking between stator and rotor poles
(iii) induced e.m.f in rotor field winding by stator flux
(iv) compulsion due to Lenz’s Law

44. The mechanical power developed by a synchronous motor is .............. stator voltage.
(i) directly proportional to
(ii) directly proportional to square of
(iii) independent of
(iv) inversely proportional to

45. A synchronous capacitor is an over-excited motor running at ..........
(i) full-load
(ii) half full load
(iii) no-load
(iv) none of the above

46. A synchronous motor delivers reactive power when ........
(i) over-excited
(ii) under-excited
(iii) normally excited
(iv) none of the above

47. For reciprocating compressor, the synchronous motor is designed to have pull-in torque of about .............. times the fill-load torque.
(i) 2
(ii) 5
(iii) 0.5
(iv) 06

48. The capacity of a synchronous capacitor is generally in the range of...............
(i) VAR
(ii) MVAR
(iii) kVAR
(iv) none of the above

49. For the same active power consumed, the stator current of a synchronous motor is ............ that of a 3-phase induction motor.
(i) more than
(ii) less than
(iii) the same as
(iv) none of the above

50. In a synchronous motor, the magnitude of stator blade e.m.f. Eb depends on ……………
(i) speed of the rotor
(ii) bad on the motor
(iii) both the speed and rotor flux
(iv) d.c. excitation only

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