Practice Set on Diploma Electrical Engineering

1. The effect of increasing the length of air-gap in the induction motor will be increase the
(a) Power factor
(b) Speed
(c) Magnetising current
(d) Air gap flux

2. In 3ph induction motor, the rotor field rotates at synchronous speed with respect to
(a) Stator
(b) Rotor
(c) Stator flux
(d) None of these.

3. The main reason why the rotor of an induction motor runs in the same direction as the rotating stator magnetic flux is to meet the requirement of
(a) Lenz’s law
(b) Faraday’s law
(c) Fleming’s Left-hand rule
(d) Fleming’s Right-hand rule

4. A 6-pole, 50Hz, 3ph induction motor has a full-load speed of 950 rpm. At half-load, its speed would be ____ rpm
(a) 475
(b) 500
(c) 975
(d) 1000

5. The principle of operation of a 3ph induction motor is most similar to that of a
(a) Synchronous motor
(b) Repulsion-start induction motor
(c) Transformer with a shorted secondary
(d) Capacitor start, induction motor

6. The efficiency and pf of a squirrel-cage induction motor increase in proportion to its
(a) Speed
(b) Mechanical load
(c) Voltage
(d) Rotor torque

7. An unexcited single phase synchronous motor is
(a) Reluctance motor
(b) Repulsion motor
(c) Universal motor
(d) AC series motor

8. The maximum power developed in the synchronous motor will depend on
(a) rotor excitation only
(b) maximum value of coupling angle
(c) supply voltage only
(d) rotor excitation supply voltage and maximum value of coupling angle

9. In case the field of a synchronous motor is under excited, the power factor will be
(a) Leading
(b) Lagging
(c) Zero
(d) unity

10. A synchronous motor is switched on to supply with its field windings shorted on themselves. It will
(a) not start
(b) start and continue to run as an induction motor
(c) start as an induction motor and then run as synchronous motor
(d) bum immediately

11. If the flux per pole of a shunt-wound DC generator is halved, the generated e.m.f. at constant speed
(a) is doubled
(b) is halved
(c) remains the same
(d) none of these

12. An over excited synchronous motor draws current at
(a) lagging power factor
(b) leading power factor
(c) unity power factor
(d) depends on the nature of load

13. The starting torque of a simple squirrel-cage motor is:
(a) Low
(b) Increase as rotor current rises
(c) Decreases as rotor current rises
(d) High

14. What is the ratio of no load speed to full load speed of a 200 kVA, 12 pole, 2200 V, 3 phase, 60 Hz synchronous motor?
(a) 1
(b) 1.1
(c) 1.21
(d) infinite

15. The hunting in a synchronous motor takes place when
(a) friction in bearings is more
(b) air gap is less
(c) load is variable
(d) load is constant

16. V curves for a synchronous motor represent relation between
(a) field current and speed
(b) field current and power factor
(c) power factor and speed
(d) armature current and field current

17. A 4-pole three-phase induction motor has a synchronous speed of 25 revls. The frequency of the supply to the stator is:
(a) 50 Hz
(b) 100 Hz
(c) 25 Hz
(d) 12.5 Hz

18. A capacitor start single phase induction motor will usually have a power factor of
(a) Unity
(b) 0.8 leading
(c) 0.6 leading
(d) 0.6 lagging

19. The starting torque of a capacitor start motor is
(a) Zero
(b) Low
(c) same as rated torque
(d) more than rated torque.

20. The direction of rotation of universal motor can be reversed by
(a) reversing the supply terminals
(b) switching over from ac to dc
(c) interchanging the brush leads
(d) any of these

21. For ceiling fans generally the single phase motor used is
(a) split phase type
(b) capacitor start type
(c) capacitor start and run type
(d) permanent capacitor type

22. Reluctance motors are
(a) doubly excited
(b) singly excited
(c) either doubly excited or singly excited
(d) none of these

23. The snubber circuit is used in thyristor circuits for
(a) Triggering
(b) dv/dt protection
(c) di/dt protection
(d) phase shifting

24. Traic cannot be used in
(a) Ac voltage regulator
(b) Cycloconverter
(c) Solid state type of switch
(d) Inverter

25. Two thyristors A and B have rated gate current of 100 mA and 2 A respectively.
(a) B is GTO and A is conventional thyristor
(b) A is GTO and B is conventional thyristor
(c) Thyristor A may opearate as transistor
(d) None of these

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26. In a thyristor controlled reactor, the firing angle of thyristor is to be controlled in the range of
(a) 0° to 90°
(b) 0° to 180°
(c) 90° to 180°
(d) 90° to 270°

27. The knee voltage (cut in voltage) of Si diode is
(a) 0.2 V
(b) 0.7 V
(c) 0.8 V
(d) 1.0 V

28. When the diode is forward biased, it is equivalent to
(a) An off switch
(b) An On switch
(c) A high resistance
(d) None of these

29. Avalanche breakdown in a diode occurs when
(a) Potential barrier is reduced to zero.
(b) Forward current exceeds certain value.
(c) Reverse bias exceeds a certain value.
(d) None of these

30. When a reverse bias is applied to a diode, it will
(a) Raise the potential barrier
(b) Lower the potential barrier
(c) Increases the majority-carrier a current greatly
(d) None of these above

31. The snubber circuit is used in thyristor circuits for
(a) Triggering
(b) dv/dt protection
(c) di/dt protection
(d) phase shifting

32. In a 3-phase bridge rectifier circuit for HVDC transmission, the ratio of output dc voltage to input ac voltage is
(a) 0.5
(b) 1.0
(c) 1.71
(d) 2

33. The quality of output ac voltage of a cycloconverter is improved with
(a) Increase in output voltage at reduced frequency
(b) Increase in output voltage at increased frequency
(c) decrease in output voltage at reduced frequency
(d) decrease in output voltage at increased frequency

34. In a three phase voltage source inverter operating in square wave mode, the output line voltage is free from
(a) 3rd harmonic
(b) 7th harmonic
(c) 11th harmonic
(d) 13th harmonic

35. Which of the following configurations is used for both motoring and generative braking?
(a) First quadrant chopper
(b) Second quadrant chopper
(c) Two quadrant chopper
(d) Four quadrant chopper

36. Sodium vapor lamps are preferred over incandescent bulb because of
(a) higher tolerance to voltage fluctuations
(b) higher intensity of illumination
(c) longer life
(d) none of these

37. In case of fluorescent lamp if only the ends of the lamp remain lighted it indicates
(a) a short circuited starter
(b) a defective choke
(c) a defective tube
(d) under voltage of supply

38. The light output of fluorescent lamp is around
(a) 10 lumens/watt
(b) 20 lumens/watt
(c) 70 lumens/watt
(d) 200 lumens/watt

39. Solar Constant is
(a) 140 Wm-2
(b) 1.4 Wm-2
(c) 1.4 kWm-2
(d) 1.4 MWm-2

40. Which of these is not a renewable source of energy
(a) The sun
(b) Natural gas
(c) Wind
(d) Ocean tidal energy

41. A solar cell converts
(a) heat energy into electrical energy
(b) solar energy into electrical energy
(c) heat energy into light energy
(d) solar energy into light energy

42. The voltage of a single solar cell is
(a) 0.5 V
(b) 1 V
(c) 1.1 V
(d) 5 W

43. The output of a solar cell is of the order of
(a) 0.1 W
(b) 0.5 W
(c) 1 W
(d) 5 W

44. The maximum theoretical efficiencies of solar sales could be around
(a) 99%
(b) 60%
(c) 48%
(d) 1%.

45. Thermal storage of energy is possible in the form of
(a) sensible heat
(b) latent heat
(c) chemical reaction
(d) any of the above.

46. The major disadvantage, with solar cells for power generation is
(a) lack of availability
(b) large area requirement
(c) variable power
(d) high cost

47. Which of the following area is preferred for solar thermal electric plants?
(a) mountain tops
(b) hot arid zones
(c) coastal areas
(d) high rainfall zones.

48. Windmill towers are generally more productive if they are
(a) higher, to minimize turbulence and maximize wind speed
(b) lower, to minimize turbulence and maximized wind speed
(c) lower, to increase heat convection from the ground
(d) higher, to increase heat convection from the ground

49. A major disadvantage of solar power is ___
(a) its cost effectiveness compared to other types of power
(b) its efficiency level compared to other types of power
(c) the variation in sunshine around the world
(d) lack of knowledge on long-term economic impacts

50. The source of energy of the sun is
(a) nuclear fission
(b) chemical reaction
(c) nuclear fusion
(d) photo electric effect