EEE 251 Possible Examination Questions and Answers- Get In Here

EEE 251 Possible Examination Questions and Answers

 You are advised to take time to practice these questions before entering the examination hall. 

Note that none of these questions were copied from the school past-questions or from any of the lecturer. Therefore, you should also read your book. Do not wholly depend on this questions. But it will go a long way to help you.

1. The no-load current drawn by the transformer is usually what percent of the full-load current ? 

(a) 0.2 to 0.5 per cent 

(b) 2 to 5 per cent 

(c) 12 to 15 percent 

(d) 20 to 30 per cent 

Ans: b

2. The dielectric strength of transformer oil is expected to be 

(a) lkV 

(b) 33 kV 

(c) 100 kV 

(d) 330 kV 

Ans: b

3. Sumpner’s test is conducted on trans-formers to determine 

(a) temperature 

(b) stray losses 

(c) all-day efficiency 

(d) none of the above 

Ans: a

4. Efficiency of a power transformer is of the order of 

(a) 100 per cent 

(b) 98 per cent 

(c) 50 per cent 

(d) 25 per cent 

Ans: b

5. In a given transformer for given applied voltage, losses which remain constant irrespective of load changes are 

(a) friction and windage losses 

(b) copper losses 

(c) hysteresis and eddy current losses 

(d) none of the above 

Ans: c

6. A common method of cooling a power transformer is 

(a) natural air cooling 

(b) air blast cooling 

(c) oil cooling 

(d) any of the above 

Ans: c

7. The no load current in a transformer lags behind the applied voltage by an angle of about 

(a) 180° 

(b) 120″ 

(c) 90° 

(d) 75°

Ans: d

8. In the transformer, the function of a conservator is to 

(a) provide fresh air for cooling the transformer 

(b) supply cooling oil to the transformer in time of need 

(c) protect the transformer from damage when oil expends due to heating 

(d) none of the above 

Ans: c

9. Power transformers are designed to have maximum efficiency at 

(a) nearly full load 

(b) 70% full load 

(c) 50% full load 

(d) no load 

Ans: a

10. The maximum efficiency of a distribution transformer is 

(a) at no load 

(b) at 50% full load 

(c) at 80% full load 

(d) at full load 

Ans: b

11. Transformer breaths in when 

(a) load on it increases 

(b) load on it decreases 

(c) load remains constant 

(d) none of the above 

Ans: b

12. Greater the secondary leakage flux 

(a) less will be the secondary induced e.m.f. 

(b) less will be the primary induced e.m.f. 

(c) less will be the primary terminal voltage 

(d) none of the above 

Ans: a

13. The purpose of providing iron core in a step-up transformer is 

(a) to provide coupling between primary and secondary 

(b) to increase the magnitude of mutual flux 

(c) to decrease the magnitude of magnetizing current 

(d) to provide all above features 

Ans: c

14. The power transformer is a constant 

(a) voltage device 

(b) current device 

(c) power device 

(d) main flux device 

Ans: d

15. Two transformers operating in parallel will share the load depending upon their 

(a) leakage reactance 

(b) per unit impedance 

(c) efficiencies 

(d) ratings 

Ans: b

16. If the percentage impedances of the two transformers working in parallel are different, then 

(a) transformers will be overheated 

(b) power factors of both the transformers will be same 

(c) parallel operation will be not possible 

(d) parallel operation will still be possible, but the power factors at which the two transformers operate will be different from the power factor of the common load 

Ans: d

17. In a transformer the tappings are generally provided on 

(a) primary side 

(b) secondary side 

(c) low voltage side 

(d) high voltage side 

Ans: c

18. The thickness of laminations used in a transformer is usually 

(a) 0.4 mm to 0.5 mm 

(b) 4 mm to 5 mm 

(c) 14 mm to 15 mm 

(d) 25 mm to 40 mm 

Ans: a

19. In a transformer the resistance between its primary and secondary is 

(a) zero 

(b) 1 ohm 

(c) 1000 ohms 

(d) infinite 

Ans: d

20. Gas is usually not liberated due to dissociation of transformer oil unless the oil temperature exceeds 

(a) 50°C 

(b) 80°C 

(c) 100°C

(d) 150°C 

Ans: d

21. The main reason for generation of harmonics in a transformer could be 

(a) fluctuating load 

(b) poor insulation 

(c) mechanical vibrations 

(d) saturation of core 

Ans: d

22. Star/star transformers work satisfactorily when 

(a) load is unbalanced only 

(b) load is balanced only 

(c) on balanced as well as unbalanced loads 

(d) none of the above 

Ans: b

23. Delta/star transformer works satisfactorily when 

(a) load is balanced only 

(b) load is unbalanced only 

(c) on balanced as well as unbalanced loads 

(d) none of the above 

Ans: c

24. Buchholz’s relay gives warning and protection against 

(a) electrical fault inside the transformer itself 

(b) electrical fault outside the transformer in outgoing feeder 

(c) for both outside and inside faults 

(d) none of the above 

Ans: a

25. The magnetising current of a transformer is usually small because it has 

(a) small air gap 

(b) large leakage flux 

(c) laminated silicon steel core 

(d) fewer rotating parts 

Ans: a

26. The leakage flux in a transformer depends upon 

(a) load current 

(b) load current and voltage 

(c) load current, voltage and frequency 

(d) load current, voltage, frequency and power factor 

Ans: a

27. Noise level test in a transformer is a 

(a) special test 

(b) routine test 

(c) type test 

(d) none of the above 

Ans: c

28. Which of the following is not a routine test on transformers ? 

(a) Core insulation voltage test 

(b) Impedance test 

(c) Radio interference test 

(d) Polarity test 

Ans: c

29. A transformer can have zero voltage regulation at 

(a) leading power factor 

(b) lagging power factor 

(c) unity power factor 

(d) zero power factor 

Ans: a

30. Helical coils can be used on 

(a) low voltage side of high kVA transformers 

(b) High-frequency transformers 

(c) high voltage side of small capacity transformers 

(d) high voltage side of high kVA rating transformers 

Ans: a

31. The core used in high frequency transformer is usually 

(a) copper core 

(b) cost iron core 

(c) air core 

(d) mild steel core 

Ans: c

32. During short circuit test iron losses are negligible because 

(a) the current on secondary side is negligible 

(b) the voltage on secondary side does not vary 

(c) the voltage applied on primary side is low 

(d) full-load current is not supplied to the transformer 

Ans: c

33. In an actual transformer the iron loss remains practically constant from noload to fullload because (a) value of transformation ratio remains constant 

(b) permeability of transformer core remains constant 

(c) core flux remains practically constant 

(d) primary voltage remains constant 

(e) secondary voltage remains constant 

Ans: c

34. If the supply frequency to the transformer is increased,”the iron loss will 

(a) not change 

(b) decrease 

(c) increase 

(d) any of the above 

Ans: c

35. Iron loss of a transformer can be measured by 

(a) low power factor wattmeter 

(b) unity power factor wattmeter 

(c) frequency meter 

(d) any type of wattmeter 

Ans: a

36. For the parallel operation of single phase transformers it is necessary that they should have 

(a) same efficiency 

(b) same polarity 

(c) same kVA rating 

(d) same number of turns on the secondary side. 

Ans: b

37. The transformer oil should have _____ volatility and _____ viscosity. 

(a) low,low 

(b) high,high 

(c) low,high 

(d) high,low 

Ans: a

38. The size of a transformer core will depend on 

(a) frequency 

(b) area of the core 

(c) flux density of the core material 

(d) (a) and (b) both 

Ans: d

39. A shell-type transformer has 

(a) high eddy current losses 

(b) reduced magnetic leakage 

(c) negligibly hysteresis losses 

(d) none of the above 

Ans: b

40. For given applied voltage, with the increase in frequency of the applied voltage 

(a) eddy current loss will decrease 

(b) eddy current loss will increase 

(c) eddy current loss will remain unchanged 

(d) none of the above 

Ans: c

41. Losses which occur in rotating electric machines and do not occur in trans formers are 

(a) friction and windage losses 

(b) magnetic losses 

(c) hysteresis and eddy current losses 

(d) copper losses 

Ans: a

42. Which of the following protection is normally not provided on small distribution transformers ? (a) Overfluxing protection 

(b) Buchholz relay 

(c) Overcurrent protection 

(d) All of the above 

Ans: b

43. If a transformer is continuously operated the maximum temperature rise will occur in 

(a) core 

(b) windings 

(c) tank 

(d) any of the above 

Ans: b

44. The maximum load that a power transformer can carry is limited by its 

(a) temperature rise 

(b) dielectric strength of oil 

(c) voltage ratio 

(d) copper loss 

Ans: c

45. The voltage transformation ratio of a transformer is equal to the ratio of 

(a) primary turns to secondary turns 

(b) secondary current to primary current 

(c) secondary induced e.m.f. to primary induced e.m.f. 

(d) secondary terminal voltage to primary applied voltage 

Ans: c

46. Minimum voltage regulation occurs when the power factor of the load is 

(a) unity 

(b) lagging 

(c) leading 

(d) zero 

Ans: c

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