Transformers

1. Which of the following does not change in a transformer?

1. Current
2. Voltage
3. Frequency
4. All of the above

2. In a transformer the energy is conveyed from primary to secondary

1. though cooling coil
2. through air
3. by the flux
4. none of the above

3. A transformer core is laminated to

1. reduce hysteresis loss
2. reduce eddy current losses
3. reduce copper losses
4. reduce all the above losses

4. Which loss is not common between a transformer and rotating machines?

1. Eddy current loss
2. Copper loss
3. Windage loss
4. Hysteresis loss.

5. The degree of mechanical vibrations produced by the laminations of a transformer depends on

1. tightness of clamping
2. guage of laminations
3. size of laminations
4. all of the above.

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

1. 0.2 to 0.5 per cent
2. 2 to 5 per cent
3. 12 to 15 per cent
4. 20 to 30 per cent

7. In case there are burrs on the edges of the laminations of the transformer, it is likely to result in

1. vibrations
2. noise
3. higher eddy current loss
4. higher hysteresis loss

8. The path of a magnetic flux in a transformer should have

1. high resistance
2. high reluctance
3. low resistance
4. low reluctance.

9. No-load test on a transformer is carried out to determine

1. copper loss
2. magnetising current
3. magnetising current and loss
4. efficiency of the transformer

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

1. 1 kV
2. 33 kV
3. 100 kV
4. 330 kV

11. Sumpner’s test is conducted on transformers to determine

1. temperature
2. stray losses
3. all-day efficiency
4. none of the above

12. The permissible flux density in case of cold rolled grain oriental steel is around

1. 1.7 Wb/m²
2. 2.7 Wb/m²
3. 3.7 Wb/m²
4. 4.7 Wb/m²

13. During the short-circuit test on a small transformer the frequency is increased from 50 Hz to 200 Hz. The copper losses will increase by a factor of

1. 16
2. 4
3. 4
4. ¼ .

14. The efficiency of a transformer will be maximum when

1. copper losses = hysteresis losses
2. hysteresis losses = eddy current losses
3. eddy current losses = copper losses
4. copper losses = iron losses

15. No-load current in a transformer

1. lags behind the voltage by about 75⁰
2. leads the voltage by about 75⁰
3. lags behind the voltage by about 15⁰
4. leads the voltage by about 15⁰

16. The purpose of providing the iron core in a transformer is to

1. provide support to windings
2. reduce hysteresis loss
3. decrease the reluctance of the magnetic path
4. reduce eddy current losses.

17. Which of the following is not a part of transformer installation?

1. Conservator
2. Breather
3. Buchholz relay
4. Exciter

18. Which conducting short-circuit test on a transformer the following side is short-circuited

1. high voltage side
2. low voltage side
3. primary side
4. secondary side

19. In a transformer the toppings are generally provided on

1. primary side
2. secondary side
3. low voltage side
4. high voltage side

20. The use of higher flux density in the transformer design

1. reduces weight per kVA
2. reduces iron losses
3. reduces copper losses
4. increases part load efficiency

21. The chemical used in breather for transformer should have the quality of

1. ionizing air
2. absorbing moisture
3. cleansing the transformer oil
4. cooling the transformer oil

22. The chemical used in breather is

1. asbestos fibre
2. silica sand
3. sodium chloride
4. silica gel

23. If a pump motor is run on 2/3^rd of its supply voltage, it will

1. continue to deliver same power
2. burn
3. stall
4. continue to run at lower speed

24. An ideal transformer has infinite values of primary and secondary inductances. The statement is

1. true
2. false

25. An ideal transformer ratings are usually expressed in terms of

1. volts
2. amperes
3. kW
4. kVA

26. The noise resulting from vibrations of laminations set by magnetic force, is termed as

1. magnetostriction
2. boo
3. hum
4. zoom

27. Hysteresis loss in a transformer varies as (B_max= maximum flux density)

1. B_max
2. B_max^1.6
3. B_max²
4. B_max^2.4

28. Material used for construction of transformer core is usually

1. wood
2. copper
3. aluminium
4. silicon steel

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

1. 0.4 mm to 0.5 mm
2. 4 mm to 5 mm
3. 14 mm to 15 mm
4. 25 mm to 40 mm

30. The function of conservator in a transformer is

1. to protect against internal fault
2. to reduce copper as well as core losses
3. to cool the transformer oil
4. to take case of the expansion and contraction of transformer oil due to variation of temperature of surroundings.

31. The higher voltage for transmitting electrical power in India is

1. 33 kV
2. 66 kV
3. 132 kV
4. 400 kV

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

1. zero
2. 1 Ω
3. 1000 Ω
4. Infinite

33. A transformer oil must be free from

1. sludge
2. odour
3. gases
4. moisture

34. A Buchholz relay can be installed on

1. auto-transformers
2. air-cooled transformers
3. welding transformers
4. oil-cooled transformers

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

1. 50⁰C
2. 80⁰C
3. 100⁰C
4. 150⁰C

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

1. fluctuating load
2. poor insulation
3. mechanical vibrations
4. saturation of core

37. Distribution transformers are generally designed for maximum efficiency around

1. 90% load
2. zero load
3. 25% load
4. 50% load

38. Which of the following properties is not necessarily desirable in the material for transformer core?

1. Mechanical strength
2. Low hysteresis loss
3. High thermal conductivity
4. High permeability

39. Helical coils can be used on

1. low voltage side of high kVA transformers
2. high frequency transformers
3. high voltage side of small capacity transformers
4. high voltage side of high kVA rating transformers

40. High frequency transformers sometimes make use of ferrite cores because it has

1. high specific gravity
2. high resistance
3. high hysteresis
4. low permeability

41. Cross over windings are used on

1. low voltage side of high kVA rating transformers
2. current transformers
3. high voltage side of high kVA rating transformers
4. high voltage side of low kVA rating transformers

42. While conducting short-circuit test on a transformer the following side is short-circuited

1. h.v.side
2. l.v.side
3. primary side
4. secondary side

43. A substance is placed in strong magnetic field. Which of the following will have maximum influence on the capacity of the substance to get magnetized?

44. Harmonics in transformer result in

1. Increase core losses
2. Increase I²R losses
3. Magnetic interference with communication circuits
4. all of the above

45. Tertiary winding is provided in transformers having

1. mesh/star winding
2. mesh/mesh winding
3. star/star winding
4. any of the above

46. The core used in high frequency transformer is usually

1. copper core
2. cost iron core
3. air core
4. mild steel core

47. The full-load copper loss of a transformer is 1600 W. At half load the copper loss will be

1. 6400 W
2. 1600 W
3. 800 W
4. 400 W

48. The value of flux involved in the e.m.f. equation of a transformer is

1. average value
2. r.m.s. value
3. maximum value
4. instantaneous value

49.  Silicon steel used in laminations mainly reduces

1. hysteresis loss
2. eddy current losses
3. copper losses
4. all the the above

50. Which winding of the transformer has less cross-sectional area?

1. Primary winding
2. Secondary winding
3. Low voltage winding
4. High voltage winding

51. Power transformers are generally designed to have maximum efficiency around

1. no-load
2. half-load
3. near full-load
4. 10% overload

52. Which of the following is the main advantage of an auto-transformer over a two winding transformer?

1. Hysteresis losses are reduced
2. Saving in winding material
3. Copper losses are negligible
4. Eddy losses are totally eliminated

53. During short-circuit test iron losses are negligible because

1. the current on secondary side is negligible
2. the voltage on secondary side does not vary
3. the voltage applied on primary side is low
4. full-load current is not supplied to the transformer

54. Two transformers are connected in parallel. These transformers do not have equal percentage impedance. This is likely to result in.

1. short-circuiting of the secondaries
2. power factor of one of the transformers is leading while that of the other lagging.
3. transformers having higher copper losses will have negligible core losses
4. loading of the transformers not in proportion to their kVA ratings

55. The changes in volume of transformers cooling oil due to variation of atmospheric temperature during day and night is taken care of by which part of transformer?

1. Conservator
2. Breather
3. Bushings
4. Buchholz relay

56.The transformer laminations are insulated from each other by

1. mica strip
2. thin coat of varnish
3. paper
4. any of the above

57.Which type of winding is used in 3-phase shell-type transformer?

1. Circular type
2. Sandwich type
3. Cylindrical type
4. Rectangular type

58. During open circuit test of a transformer

1. primary is supplied rated voltage
2. primary is supplied full-load current
3. primary is supplied current at reduced voltage
4. primary is supplied rated kVA

59.Open circuit test on transformers is conducted to determine

1. hysteresis losses
2. copper losses
3. core losses
4. eddy current losses

60. Short circuit test on transformers is conducted to determine

1. hysteresis losses
2. copper losses
3. core losses
4. eddy current losses

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

1. same efficiency
2. same polarity
3. same kVA rating
4. same number of turns on the secondary side.

62.The transformer oil should have ……. voltatility and …. viscosity

1. low ….. low
2. high ….. high
3. low ….. high
4. high ….. low

63.The function of breather in a transformer is

1. to provide oxygen inside the tank
2. to cool the coils during reduced load
3. to filter the transformer cooling oil
4. to arrest flow of moisture when outside air enters the transformer.

64. The secondary winding of which of the following transformers is always kept closed?

1. Step-up transformer
2. Sept-down transformer
3. Potential transformer
4. Current transformer

65. The size of a  transformer core will depend on

1. frequency
2. area of the core
3. flux density of the core material
4. (a) and (b) both

66. Natural air cooling is generally restricted for transformers up to

1. 1.5 MVA
2. 5 MVA
3. 15 MVA
4. 50 MVA

67. A shell-type transformer has

1. high eddy current losses
2. reduced magnetic leakage
3. negligible hysteresis losses
4. none of the above.

68. A transformer can have regulation  closer to zero

1. on full-load
2. on overload
3. on leading power factor
4. on zero power factor

69. A transformer transforms

1. voltage
2. current
3. current and voltage
4. power

70. Which of the following is not the standard voltage for power supply in India

1. 11 kV
2. 33 kV
3. 66 kV
4. 122 kV

Electrical Machines-II

1.      a) Explain in detail the principle of flux in transformer is always constant at all loads.

b) Derive the emf equation of transformer? Draw the NO-Load phasor diagram? (6+9)

2.      a) Define voltage regulation of a transformer. Deduce the expression for the voltage regulation.

b) Derive the equivalent circuit diagram of single phase transformer? (7+8)

3.      a) The number of turns on the primary and secondary windings of a single phase transformer is 350 and 35 respectively. If the primary is connected to a 2.2 kV 50 HZ supply. Determine the secondary voltage.

b) Describe the procedure for conducting sumpners test on pair of transformers? (5+10)

4.      What are the different losses occurring in a transformer on load? How can these losses be determined experimentally? (15)

5.      A 5 KVA, 1000/200V, 50Hz single phase transformer gave the following test results:

Open-circuit test: (l.v side): 200V, 1.2 A, 90 W

Short-circuit test (h.v Side): 50 V, 5 A, 110 W

Compute the parameters of the approximate equivalent circuit referred to l.v side. Also draw the exact equivalent circuit referred to l.v side. (15)

6.      A three phase transformer is used to step down the supply voltage from 10000 V to 440 V. If the output capacity of the transformer is 132 kVA, find the secondary and primary currents of the transformer. (15)

7.       a) Why does the magnitude of the magnetic field remain constant, and also what is the speed of rotation of the magnetic field, so formed? Also what is meant by the term ‘Synchronous Speed’.

b) Derive the relationship between mechanical power developed, rotor input and rotor copper loss. (6+9)

8.  Explain the speed control of induction motor form stator side? (15)