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Question Bank in Transformers

1. REE Board Exam April 1997

A single-phase transformer is rated 4,160-V primary and 250-V secondary. It has 1,500 turns in the primary and voltage regulation of 8%. What should be the number of turns in the secondary winding? A. 88 turns C. 90 turns B. 92 turns D. 86 turns

2. EE Board Exam April 1991

A certain transformer has a ratio of 8 is to 1. The primary winding resistance is 0.00283 Ω per turn and has 1,155 more turns than the secondary winding. If the copper loss in the primary is 20% less than the secondary, find the resistance of the secondary winding. A. 0.073 ohm C. 0.068 ohm B. 0.090 ohm D. 0.084 ohm

3. EE Board Exam October 1991

A 10-kVA; 2,400/240 V single-phase transformer has the following resistances and leakage reactances. Find the primary voltage required to produce 240-V at the secondary terminals at fullload when the load power factor is 0.8 lagging. r1 = 3 ohms x1 = 15 ohms r2 = 0.03 ohm x2 = 0.15 ohm A. 2,450 volts C. 2,496 volts B. 2,575 volts D. 2,632 volts

4. EE Board Exam April 1983

A single-phase transformer rated 75 kVA, 8000 volts primary and 240 volts secondary is given the short circuit test. With the secondary terminals short circuited, 440 volts (60 Hz) is impressed on the primary, which then draws rated current and 1125 watts. Solve for the percentage impedance of the transformer. A. 5.29% C. 5.50% B. 4.93% D. 5.13%

5. EE Board Exam April 1993

A short circuit test was performed upon a 10-kVA, 2300/230 volt transformer with the following results: Esc = 137 volts; Psc = 192 W; Isc = 4.34 A. Calculate in primary terms the reactance of the transformer. A. 31.56 ohms B. 29.88 ohms C. 10.19 ohms D. 12.45 ohms

6. EE Board Exam October 1984

A single-phase, 25 kVA, 4800/240 volt transformer was given the short circuit test. With the secondary terminals short-circuited, 93.75 volts was applied on the primary so that the ammeter reads rated current and the wattmeter reads 162.74 watts, all on the primary side. Solve for the percentage resistance of the transformer

A. 1.95% B. 3.39%

C. D.

3.02% 2.23%

7. EE Board Exam April 1990

The following data were obtained when a short circuit test was performed upon a 100 kVA, 2400/240 volts distribution transformer: Esc = 72 volts; Isc = 41.6 A; Psc = 1,180 W. All instruments are on the high side during the short circuit test. Calculate the equivalent resistance and reactance of the transformer. A. 0.682 ohm, 1.59 ohms B. 0.682 ohm, 1.73 ohms C. 0.752 ohm, 1.73 ohms D. 0.752 ohm, 1.59 ohms

8. REE Board Exam March 1998

A 13.8 kV/480 V, 10 MVA three-phase transformer has 5% impedance. What is the impedance in ohms referred to the primary? A. 0.952 ohm B. 0.03 ohm C. 5.125 ohms D. 9.01 ohms

9. EE Board Exam April 1990

B. The following data were obtained when a short circuit test was performed upon a 100 kVA, 2400/240 volts distribution transformer: Esc = 72 volts; Isc = 41.6 A; Psc = 1,180 W. All instruments are on the high side during the short circuit test. Calculate the percent regulation at a power factor of 0.75 lagging. A. 2.54% C. 2.62% B. 2.86% D. 2.71%

10. EE Board Exam October 1992

A 200-kVA transformer with impedance of 5% and the power factor of the load is 85%. The primary voltage is 6,000 volts while the copper loss is 5 kW. Find the percentage regulation. A. 5.03% C. 4.43% B. 5.21% D. 4.03%

11. EE Board Exam April 1993

A short circuit test was performed upon a 10-kVA, 2300/230 volt transformer with the following results: Esc = 137 volts; Psc = 192 W; Isc = 4.34 A. Calculate the percent regulation at a lagging power factor of 0.707. A. 5.28% C. 5.37% B. 5.55% D. 5.72%

12. EE Board Exam October 1992

In a 440 V, 50 c/s transformer, the total iron loss is 2500 W. When the supplied p.d. is 220 V at 25 c/s, the corresponding loss is 850 W. Calculate the eddy current loss at normal frequency and p.d. A. 2337 W C. 2010 W B. 2165 W D. 2216 W

13. EE Board Exam April 1990

A 50 kVA, 2300/230 volts, 60 cycle transformer is tested in the laboratory so that its characteristics maybe determined. The standard test requires an open circuit test and short circuit test. Open Circuit Test: I = 6.5 A E = 230 V P = 187 W Short Circuit Test I = 21.7 A E = 115 V P = 570 W Determine the efficiency of the transformer at half load, unity pf. A. 97.55% C. 98.45% B. 98.70% D. 97.80%

14. EE Board Exam April 1992

Given that the full load copper losses are exactly twice the iron losses in a 50 kVA transformer, and that the quarter load efficiency is 96.5%, calculate the full load efficiency at unity power factor. A. 97.48% C. 97.64% B. 98.12% D. 96.25%

15. EE Board Exam April 1995

The core loss of a 5 kVA single –phase transformer with normal voltage applied to the primary is 75 watts. The maximum efficiency occurs at 60% of full-load kVA. What is the fullload efficiency of the transformer at 0.80 power factor? A. 95.16% C. 93.38% B. 89.52% D. 91.36%

16. EE Board Exam October 1980, October 1982

A 25-kVA distribution transformer operates at 120% load, 0.3 pf lagging for 1 hour; art full-load, 0.9 lagging pf for 3 hours; 50% load at unity pf for 8 hours; and 10% load at 0.8 leading pf for the rest of the day. If core loss is 150 watts and full load copper loss is 500 W, what is the all-day efficiency of the transformer? A. 94.23% C. 97.14% B. 92.86% D. 96.68%

17. EE Board Exam October 1990

Find the all-day efficiency of a transformer having maximum efficiency of 98% at 15-kVA at unity power factor and loaded as follows: 12 hours 2-kW at 0.5 pf lagging 6 hours 12-kW at 0.8 pf lagging 6 hours at no load A. 95.3% C. 94.6% B. 91.7% D. 93.3%

18. EE Board Exam October 1991

A 30-kVA, 2,400/240 volts, 60-Hz transformer has a full-load power of unity over the period of 24 hrs. The maximum efficiency is 95% and it occurs at full-load. Calculate the all-day efficiency if loaded as follows: 6-hours at full-load

6 hours at quarter load A. 91.53% C. 90.37% B. 92.45% D. 93.86% 19. EE Board Exam April 1995 Ideal transformer assumptions do not include A. no leakage flux B. no saturation of core C. zero resistance of the winding D. zero reactance of the winding

20. EE Board Exam October 1993

A transformer is designed to carry 30 times its rated current for 1 second. The length of time that a current of 20 times the rated current can be allowed to flow is A. 0.67 sec C. 1.5 sec B. 2 sec D. 2.25 sec

21. REE Board Exam October 1998

Which of the following given below is the eddy current loss not proportional to? A. square of the thickness B. square of the frequency C. square of the load current of the laminated core D. square of the flux density

22. REE Board Exam October 1997

A small single transformer has 10.2 watts no load loss. The core has a volume of 750 cu. cm. The maximum flux density is 10,000 gauss and the -4 hysteresis constant of core is 5 x 10 . Using the Steinmetz law to find the hysteresis, determine the eddy current loss. A. 4.55 watts C. 3.55 watts B. 5.55 watts D. 2.55 watts

23. EE Board Exam April 1992

When the emf in the two windings of a transformer are opposite in direction, the polarity is A. positive C. negative B. additive D. subtractive

24. EE Board Exam October 1991

The copper loss of the transformer will A. not be affected by a change in the power factor B. decrease with a lowering of the power factor C. increase with an increase of the power factor D. increase with the lowering of the power factor

25. EE Board Exam June 1990

Power lost due to alternating magnetization and demagnetization of the core material A. copper loss B. eddy current loss C. hysteresis loss D. rheostat loss

26. EE Board Exam June 1990

In the open circuit test of the transformer the wattmeter reading is equal to A. eddy current loss B. current loss C. hysteresis loss D. core loss

27. EE Board Exam October 1991

A 30-kVA; 2,400/240 volts, 60-Hz transformer has a full-load power of unity over the period of 24 hrs. The maximum efficiency is 95% and it occurs at full-load. Calculate the all-day efficiency if loaded 6-hours at full-load. A. 78.90% C. 89.67% B. 88.38% D. 92.45%

28. EE Board Exam April 1991

A certain transformer has a ratio of 8:1. The primary winding resistance is 0.00283 Ω and has 1,165 more turns than the secondary winding. If the copper loss in the primary is 20% less than the secondary, calculate the resistance of the secondary winding. -5 A. 5.53 x 10 ohm -4 B. 2.56 x 10 ohm -7 C. 6.78 x 10 ohm -5 D. 4.18 x 10 ohm

29. EE Board Exam October 1992

A 200-kVA transformer has an impedance of 15% and the power factor of the load is 85%. The primary voltage is 6,000 volts while the copper loss is 15 kW. Find the percentage voltage regulation. A. 13.49% C. 8.91% B. 15.78% D. 23.67%

30. EE Board Exam June 1990

Transformer connection that can transform threephase system to two-phase system and vice versa A. Scott or T-connection B. open delta C. autotransformer connection D. wye-delta connection

31. EE Board Exam June 1990

Which transformer has only one winding? A. distribution transformer B. power transformer C. autotransformer D. isolated winding transformer

32. EE Board Exam April 1990

When the primary and secondary windings take the form of a common ring which is encircled by two or more rings of magnetic materials distributed around its periphery, the transformer is termed as A. grounding transformer B. regulating transformer C. core type transformer

D. shell type transformer

33. A transformer transforms A. B. C. D.

frequency voltage current voltage and current

34. Which of the following is not a basic element of a transformer? A. Core B. primary winding C. secondary winding D. mutual flux

35. In an ideal transformer A. B. C. D.

windings have no resistance core has no losses core has infinite permeability all of the above

36. The main purpose of using core in a transformer is to A. decrease iron losses B. prevent eddy current loss C. eliminate magnetic hysteresis D. decrease reluctance of the common magnetic circuit

37. Transformer cores are laminated in order to A. B. C. D.

simplify its construction minimize eddy current loss reduce cost reduce hysteresis

38. A transformer having 1000 primary turns is

connected to a 250-V a.c supply. For a secondary voltage of 400 V, the number of secondary turns should be A. 1600 C. 400 B. 250 D. 1250

39. The primary and secondary induce e.m.fs. E1 and E2 in a two-winding transformer are always A. equal in magnitude B. anti-phase with each other C. in-phase with each other D. determined by load on transformer secondary

40. A step-up transformer increases A. voltage B. current

C. D.

power frequency

41. The primary and secondary windings of an

ordinary 2-winding transformer always have A. different number of turns B. same size of copper wire C. a common magnetic circuit D. separate magnetic circuits

42. In a transformer, the leakage flux of each winding is proportional to the current in that winding because A. Ohm’s law applies to magnetic circuits B. leakage paths do not saturate C. the two windings are electrically isolated D. mutual flux is confined to the core

43. In a two-winding transformer, the e.m.f. per turn

in secondary winding is always ____ the induced e.m.f. power turn in primary. A. equal to K times B. equal to 1/K times C. equal to D. greater than

44. In relation to a transformer, the ratio 20:1

indicates that A. there are 20 turns on primary one turn on secondary th B. secondary voltage is 1/20 of primary voltage C. primary current is 20 times greater than the secondary current D. for every 20 turns on primary, there is one turn on secondary

45. In performing the short circuit test of a

transformer A. high voltage side is usually short circuited B. low voltage side is usually short circuited C. any side is short circuited with preference D. none of the above

46. The equivalent resistance of the primary of a

transformer having K = 5 and R1 = 0.1 ohm when referred to secondary becomes ____ ohm. A. 0.5 C. 0.004 B. 0.02 D. 2.5

47. A transformer has negative voltage regulation when its power factor is A. zero B. unity

C. D.

leading lagging

48. The primary reason why open-circuit test is

performed on the transformer is that it A. draws sufficiently large no-load current for convenient reading B. requires least voltage to perform the test C. needs minimum power input D. involves less core loss

49. No-load test on a transformer is carried out to determine A. copper loss B. magnetizing current C. magnetizing current and no-load loss D. efficiency of the transformer

50. The main purpose of performing open-circuit test on a transformer is to measure its A. Cu loss B. core loss C. total loss D. insulation resistance

51. During short-circuit test, the iron lose of a

transformer is negligible because A. the entire input is just sufficient to meet Cu losses only B. flux produced is a small fraction of the normal flux C. iron core becomes fully saturated D. supply frequency is held constant

52. The iron loss of a transformer at 400 Hz is 10 W.

Assuming that eddy current and hysteresis losses vary as the square of flux density, the iron loss of the transformer at rated voltage but 50 Hz would be ____ watt. A. 80 C. 1.25 B. 640 D. 100

53. In operating a 400 Hz transformer at 50 Hz A. B. C. D.

only voltage is reduced in the same proportion as the frequency only kVA rating is reduced in the same proportion as the frequency both voltage and kVA rating are reduced in the same proportion as the frequency none of the above

54. The voltage applied to the h.v. side of a

transformer during short-circuit test is 2% of its rated voltage. The core loss will be ____ percent of the rated core loss. A. 4 C. 0.25 B. 0.4 D. 0.04

55. Transformers are rated in kVA instead of kW

because A. load power factor is often not know B. kVA is fixed whereas kW depends on load p.f. C. total transformer loss depends on voltampere D. it has become customary

56. When a 400 Hz transformer is operated its kVA rating is A. reduced to 1/8 B. increased 8 times C. unaffected D. increased 64 times

57. At relatively light loads, transformer efficiency is low because A. secondary output is low B. transformer losses are high C. fixed loss is high in proportion to the output

D.

Cu loss is small

58. A 200 kVA transformer has an iron loss of 1 kW and full-load Cu loss of 2 kW. Its load kVA corresponding to maximum efficiency is ____ kVA. A. 100 C. 50 B. 200 D. 141.4

59. If Cu loss of a transformer at 7/8th full-load is 4900 W, then its full-load Cu loss would be ____ watt. A. 5600 C. 375 B. 6400 D. 429

60. The ordinary efficiency of a given transformer is maximum when A. it runs at half full-load B. it runs at full-load C. its Cu loss equals iron loss D. it runs slightly overload

61. The output current corresponding to maximum efficiency for a transformer having core loss of 100 W and equivalent resistance referred to secondary of 0.25 is ____ ampere. A. 20 C. 5 B. 25 D. 400

62. The maximum efficiency of a 100-kVA

transformer having iron loss of 900 kW and F.L. Cu loss of 1600 W occurs at ____ kVA. A. 56.3 C. 75 B. 133.3 D. 177.7

63. The all-day efficiency of a transformer depends primarily on A. its copper loss B. the amount of load C. the duration of load D. both B and C

64. The marked increase in kVA capacity produced by connecting a 2 winding transformer as an autotransformer is due to A. increase in turn ratio B. increase in secondary voltage C. increase in transformer efficiency D. establishment of conductive link between primary and secondary

65. The kVA rating of an ordinary 2-winding

.

transformer is increased when connected as an autotransformer because A. transformation ratio is increased B. secondary voltage is increased C. energy is transferred both inductively and conductivity D. secondary current is increased

66. The saving in Cu achieved by converting 2-

winding transformer into an autotransformer is determined by A. voltage transformation ratio B. load on the secondary C. magnetic quality of core material D. size of the transformer core

67. An autotransformer having a transformation ratio of 0.8 supplies a load of 3 kW. The power transferred conductively from primary to secondary is ____ kW. A. 0.6 C. 1.5 B. 2.4 D. 0.27

68. The essential condition for parallel operation of

two 1-φ transformer is that they should have the same A. polarity B. kVA rating C. voltage D. percentage impedance

69. If the impedance triangles of two transformers

operating in parallel are not identical in shape and size, the two transformers will A. share the load unequally B. get heated unequally C. have a circulatory secondary current even when unloaded D. run with different power factors

70. Two transformers A and B having equal outputs

and voltage ratios but unequal percentage impedances of 4 and 2 are operating in parallel. Transformer A will be running over-load by ____ percent. A. 50 C. 33 B. 66 D. 25

Unsolved problems 71. EE Board Exam October 1991, April 1994 A 1000 kVA, 6,600/400 volts 60 cycle single phase core type transformer has the following average daily load: Full-load at 0.8 pf for 8 hours Half-load at 0.707 pf for 10 hours No-load for 6 hours Calculate the ratio of full-load copper loss to the iron loss for the transformer to be most economical for the above loading. A. 2.05 C. 2.17 B. 3.09 D. 2.28 THREE PHASE TRANSFORMER CONNECTIONS 72. EE Board Exam June 1990 What should be the KVA rating of each two transformers in an open-delta bank when the three-phase balanced load is 290.4 kVA? A. 155.60 kVA B. 173.40 kVA C. 150.25 kVA

D. 167.66 kVA

73. EE Board Exam April 1981

A V-V or open delta connected transformer bank, composed of two identical units serves a balanced 3-phase load of 16 kVA at 230 volts. Solve for the minimum size in kVA of the transformers needed. A. 8 kVA C. 16 kVA B. 9.24 kVA D. 8.25 kVA 74. EE Board Exam April 1986 A V-V (open delta) connected transformer bank serves a balanced 3-phase load of 40 kVA at 230 V and 0.866 pf lagging. Solve for the minimum kVA of each transformer needed to accommodate the load without overloading. A. 25 C. 23 B. 24 D. 20

75. EE Board Exam April 1985

Two single-phase distribution transformers are banked in V-V (open delta). If each transformer is rated 37.5 kVA, solve for the maximum 3-phase load that the bank can carry without overloading, considering that the load has a power factor of 0.85 lagging. A. 52.5 kW C. 53.6 kW B. 49.5 kW D. 55.2 kW

76. EE Board Exam April 1980

Two single-phase distribution transformers connected in open delta will supply power to a 200 hp, 3-phase induction motor, operating at 0.70 power factor and 0.90 efficiency. Solve for the minimum size in kVA of each transformer needed to supply the power without being overloaded. A. 136.73 kVA B. 124.45 kVA C. 130.38 kVA D. 129.48 kVA

77. EE Board Exam October 1986

The Utility Company supplies two single-phase transformers banked in open delta. If each transformer is rated 75 kVA capacity, what is the maximum 3-phase load in kilowatts that the bank can carry without suffering overload considering that the load has a lagging power factor of 0.80. A. 104 kW C. 130 kW B. 112 kW D. 114 kW

78. EE Board Exam October 1980

Two identical 1-phase transformers connected in open delta will serve a 3-phase motor lad of 250 hp, 0.70 pf lagging, 0.90 efficiency. Solve for the minimum size in kVA of each transformer required to serve the load without overloading. A. 150.33 C. 160.43 B. 170.91 D. 185.13

79. EE Board Exam October 1985, April 1986

An open-delta connected transformer bank serves a balanced 3-phase load of 40 kVA at 230 V and 0.866 pf lagging. Solve for the real power and p at which each transformer is operating. A. 11.54 kW at 0.5 pf and 23.09 kW at unity pf B. 12.22 kW at 0.5 pf and 22.42 at unity pf C. 11.54 kW at 0.866 pf and 23.09 kW at 0.707 pf D. 12.22 kW at 0.866 pf and 22.429 kW at 0.707 pf

80. EE Board Exam April 1983

Two single-phase transformers are connected in V-V (open delta) and serving a delta connected impedance load with each impedance equal to ohms. If the transformer voltages impressed on the transformer are: V V

Solve for the kVA delivered by each transformer. A. 7.28 and 5.20 B. 5.87 and 6.61 C. 6.84 and 5.64 D. 6.24 and 6.24

81. EE Board Exam June 1990

Two single-phase transformers in an open delta bank served a three-phase balanced load of 290.4 kVA. If a third transformer is added for delta operation, what percent increase in load does this represent? A. 73.2% C. 80.5% B. 84.6% D. 79.4%

82. EE Board Exam October 1983

Three single-phase transformers each rated 75 kVA are banked in delta and supplying a 3-phase load drawing 160 kVA at 0.8 lagging power factor. If one transformer is removed for repairs, solve for the amount of overloading of the remaining units. A. 30 kVA C. 20 kVA B. 10 kVA D. 25 kVA

83. EE Board Exam April 1992

A 6600-volt, 60 Hz, 2-phase network is connected to a three-phase, 4-wire system by a Scottconnected transformer combination. If there are 1500 turns on the 2-phase side, calculate the number of turns on the 3-phase side required for the main and teaser transformers. Voltage per phase on the 3-phase side is 240 V. A. 94 turns and 83 turns B. 93 turns and 84 turns C. 95 turns and 82 turns D. 92 turns and 85 turns

84. EE Board Exam April 1985

A balanced 3-phase transformer bank has deltaconnected primary and secondary windings. The primary to secondary winding voltage ratio is

2400/277. Solve for the primary line current if the low-side (secondary) load draws a line current of 1000 A. A. 200 A C. 150 A B. 115 A D. 175 A

85. EE Board Exam October 1991

What should be the turns ratio of a three-phase transformer to transform 10,000 kVA from 230 kV to 4160 V if the transformer is to be connected Δ – Y? A. 94 C. 96 B. 98 D. 97

86. REE Board Exam October 1998

The potential transformer of a line has a ratio of 132,000/66. These are connected wye-delta. The voltmeter connected at the secondary reads 64 volts. What is the voltage of the line? A. 221.7 kV C. 112.5 kV B. 135.2 kV D. 132 kV

87. REE Board Exam October 1997

A power transformer rated 50,000 kVA, 34.5 kV/13.8 kV is connected wye-grounded primary and delta on the secondary. Determine the full load phase current on the secondary side. A. 2092 A C. 1449 A B. 1725 A D. 1208 A

88.

REE Board Exam October 1998 A 69/13.8 kV, 10 MVA three-phase transformer is connected wye-grounded delta. What is the kV rating of the secondary winding? A. 7.97 C. 15 B. 23.9 D. 13.8

89. REE Board Exam October 1997

The PTs of a 220 kV transmission line have a ratio of 132.76 kV/66.375 V are connected wyewye. A voltmeter connected line to ground reads 66 volts, what is the transmission line voltage? A. 228.63 kV C. 223.15 kV B. 220.10 kV D. 225.25 kV

90. REE Board Exam October 1997

A power transformer rated 50,000 kVA, 34.5 kV/13.8 kV is connected wye-wye. What are the line currents at full load? A. 525.5 A, 1725.4 A B. 836.7 A, 2092 A C. 483.1 A, 1207.7 A D. 1449.3 A, 3623.2 A

91. EE Board Exam October 1985

Two transformers are connected in parallel to supply a common load of 175 kVA. Transformer A is rated 100 kVA with equivalent impedance of 9.6 ohms while transformer B is rated 75 kVA with equivalent impedance of 12.5 ohms. Find the kVA load of each transformer. A. 78, 97 C. 80, 95

B. 76, 99

D.

75, 100

92. EE Board Exam October 1981

Two distribution transformers are connected in parallel supplying a load of 125 kVA. If transformer A is rated 50 kVA, with 14 ohms equivalent reactance and transformer B is rated 75 kVA with 11 ohms equivalent reactance. Solve for the loading of each transformer. A. 55 kVA, 70 kVA B. 50 kVA, 75 kVA C. 58 kVA, 67 kVA D. 52 kVA, 73 kVA

93. REE Board Exam March 1998

A 125 kVA distribution transformer with 4% impedance is connected in parallel with another transformer rated 75 kVA and with 35% impedance. Both have same voltage ratio. (Neglect resistance of each transformer.) The total load is 140 kVA at 85% power factor. How much load does each carry? A. 80.5 kVA, 59.5 kVA B. 77.78 kVA, 62.22 kVA C. 80 kVA, 60 kVA D. 87.50 kVA, 52.5 kVA

94. EE Board Exam April 1993

MERALCO has two single-phase transformers with equal turns ratio and ratings are operated in parallel to supply a load of 280 kW at 0.8 lagging pf. Transformer A has a resistance of 2% and a reactance of 8%. Transformer B has a resistance of 1% resistance and a reactance of 6%. Determine the power delivered by the transformers A and B to the load. A. A = 123 kW, B = 157 kW B. A = 125 kW, B = 155 kW C. A = 122 kW, B = 158 kW D. A = 127 kW, B = 153 kW

95. REE Board Exam April 1994

Two single phase transformers with equal ratings and turns ratio are operated in parallel to supply a load of 180 kW at a lagging pf of 0.90. Transformer A has a resistance of 1% and a reactance of 6%. Transformer B has a resistance of 2% and a reactance of 5%. Calculate the power delivered by each transformer. A. 83 kW, 97 kW B. 79 kW, 92 kW C. 88 kW, 92 kW D. 80 kW, 100 kW

96. REE Board Exam March 1998

A power customer draws power at 220 volts from a transformer on a pole. Current transformers with ratio of 200/5 are use to meter the electrical usage. What is the multiplier of the kW hr and demand meters? A. 40 C. 200 B. 100 D. 80

97. REE Board Exam October 1997

At a 155 kV substation, the PT ratio is 1000 and the CT ratio is 1200/5. The potential going into the wattmeter is 115 volts. What is the MW indicated when the wattmeter reads 800 watts? A. 192 MW C. 19.2 MW B. 150 MW D. 15 MW

98. REE Board Exam March 1998

A 50 MVA, 33 kV/11 kW, three-phase, wye-delta connected transformer has a 3% impedance. What is the %impedance at 100 MVA base and 34.5 kV base? A. 1.639% C. 5.74% B. 5.49% D. 6.56%

99. REE Board Exam September 2001, October

1998 The loads of a wye-connected transformer are: Ia = 10 cis (-30°) A Ib = 12 cis 215° A Ic = 15 cis 82° A What is the neutral current? A. 1.04 cis 72.8° A B. 0.92 cis 62.5° A C. 2.21 cis -30° A D. 3.11 cis 72.8° A

100.

REE Board Exam October 1998 Its primary function is to provide ground source A. zigzag transformer B. sectionalizer C. surge arrester D. tertiary transformer

101.

REE Board Exam October 1997 What is the main use of a zigzag transformer? A. to step down voltage B. to provide path to zero sequence currents C. to stabilize transformer sequence currents D. to step down current

102.

REE Board Exam October 1997 A power transformer is rated at 50,000 kVA, 34.5 kV/13.8 kV is connected wye-wye, what are the line currents at full-load? A. 525.2 amperes, 1,725 amperes B. 836.7 amperes, 2,092 amperes C. 483.1 amperes, 1,207.7 amperes D. 1,449.3 amperes, 3,623.2 amperes

103.

REE Board Exam October 1996 A three-phase transformer has a primary voltage rating of 34.5 kV, wye-connected with two 2.5% taps above and two 2.5% taps below the rated

voltage. The secondary winding is rated 460 volts and is delta connected. If it has 100 turns, what will be the effective number of turns in the primary winding if voltage tap us set at minus 2.5%? A. 7,133 turns C. 4,222 turns B. 7,312 turns D. 4,333 turns

104.

EE Board Exam April 1988 Two single-phase transformers each rated 75kVA are connected in V-V or open-delta, to serve a three-phase load of 120 kW at 0.80 power factor lagging. To prevent the overloading of transformers, determine the size of the capacitor in kVAR. A. 50 C. 30 B. 40 D. 20

105.

ECE Board Exam April 2001 A transformer composed of two or more coils which are wound around a non-metallic core A. Metallic core transformer B. Free core transformer C. Iron core transformer D. Air core transformer

106.

ECE Board Exam April 1999 An autotransformer contains how many coils? A. None C. Two B. One D. Three

107.

ECE Board Exam April 2000 Find unloaded output of a power supply having a transformer regulation of 11.10% and 900 V load output. A. 99.99 V C. 999.9 V B. 90 V D. 900 V

108.

ECE Board Exam April 1998 A transformer consists of the following? A. An inductance and resistance B. A parallel resonant circuit C. A capacitor and an inductor D. Two coils wound on a common core

109.

ECE Board Exam April 2001 How much is the secondary voltage of a power transformer whose parameters are as follows, primary voltage is 240 Volts, 140 turns in the primary and 7 turns in the secondary. A. 12 V C. 6 V B. 24 V D. 36 V

110.

ECE Board Exam April 2000 It is a transformer with two or more windings on a laminated iron core used to supply stepped up or

stepped down values of voltages to various circuits of an equipment. A. RF transformer B. Power transformer C. Autotransformer D. AF transformer

111.

ECE Board Exam April 1999 A transformer with 100 turns in the primary winding and 25 turns in the secondary winding is which of the following? A. A step-up transformer B. An isolation transformer C. A step-down transformer D. An autotransformer

112.

ECE Board Exam November 2001 Find the primary voltage of a power transformer under the following conditions 3 turns for primary, 6 turns for secondary and 120 volts for secondary voltage. A. 8 volts C. 6 volts B. 60 volts D. 12 volts

113.

ECE Board Exam April 1999 The current that flows in the primary of a transformer with the secondary open (or no load is attached). A. Source current B. Exciting current C. Load current D. Coupling current

114.

ECE Board Exam April 1996 What type of transformer that is used to protect technicians and operates from deadly electrical shock? A. Absorber transformer B. Step down transformer C. Step up transformer D. Isolation transformer

115.

ECE Board Exam November 2000 Components of the transformer which receives energy from the primary winding and delivers it to the load. A. Enclosure B. Core C. Secondary winding D. Primary winding

116.

ECE Board Exam November 1998 Which of the following does not contribute to losses in a transformer? A. Leakage resistance B. Self-inductance

C. Stray capacitance D. Iron losses

117.

ECE Board Exam April 2001 A transformer composed of two or more coils which are wound around a non-metallic core A. Metallic core transformer B. Free core transformer C. Air core transformer D. Iron core transformer

118.

A transformer will

work on ____. A. a.c. only B. d.c. only C. a.c. as well as d.c. D. none of the above

119.

The primary and secondary of a transformer are ____ coupled. A. B. C. D.

electrically magnetically electrically & magnetically none of the above

120.

A transformer is an efficient device because it ____. A. is a static device B. uses capacitive coupling C. uses inductive coupling D. uses electric coupling

121.

A transformer transfers electrical energy from primary to secondary usually with a change in ____. A. frequency C. voltage B. power D. time period

122.

The voltage per turn of the primary of a transformer is ____ the voltage per turn of the secondary. A. more than B. less than C. the same as D. none of these

123.

The winding of the transformer with greater number of turns will be ____. A. high-voltage winding B. low-voltage winding C. either high or low voltage winding D. none of the above

124.

A transformer does not possess ____ changing property.

A. impedance B. voltage

C. D.

current power

133. 125.

The iron-core is used to ____ of the transformer. A. increase the weight B. provide tight magnetic coupling C. reduce core losses D. none of the above

126.

If a transformer core has air gaps, then ____. A. reluctance of magnetic path is decreased B. hysteresis loss is decreased C. magnetizing current is greatly increased D. eddy current is increased

127.

The maximum flux produced in the core of a transformer is ____.

A. directly proportional to supply frequency B. inversely proportional to supply frequency C. inversely proportional to primary voltage D. none of the above

128.

The flux in the core of a single-phase transformer is ____. A. B. C. D.

purely alternating one purely rotating one partly alternating and partly rotating none of the above

129.

A transformer is so designed that primary and secondary have ____. A. high leakage flux B. large resistance C. tight magnetic coupling D. good electric coupling

130.

When the primary of a transformer is connected to a d.c. supply, ____. A. primary draws small current B. primary leakage reactance is increased C. core losses are increased D. primary may burn out

131.

An ideal transformer is one which ____. A. has no losses and leakage reactance B. does not work C. has the same number of primary and secondary turns D. none of the above

132.

The no-load primary current Io is about ____ of full-load primary current. A. 3 – 5% C. 30 – 40% B. 15 – 30% D. above 40%

The no-load p.f. of a transformer is small because ____

A. B. C. D.

iron losses component of Io is large magnetizing component of Io is large magnetizing component of Io is small none of the above

134.

The no-load input power to a transformer is practically equal to ____ loss in the transformer. A. iron B. copper C. eddy current D. none of these

135.

Two things which are same for primary and secondary of a transformer are ____. A. ampere-turns and voltage per turn B. resistance and leakage reactances C. currents and induced voltages D. none of the above

136.

A transformer operates poorly at very low frequencies because ____. A. permeability of core is increased B. magnetizing current is abnormally high C. primary reactance is too much increased D. none of the above

137.

If a power transformer is operated at very high frequencies, then ____. A. primary reactance is too much increased B. primary will draw large power C. core losses will be excessive D. none of the above

138.

The primary leakage flux links ____. A. primary winding only B. secondary winding only C. both primary and secondary winding D. none of the above

139.

The effect of leakage flux in a transformer is to ____.

A. B. C. D.

140.

increase copper losses decrease copper losses cause voltage drop in the windings none of the above

Leakage flux in a transformer occurs because ____. A. iron core has high permeability B. air is not good magnetic insulator C. applied voltage is sinusoidal D. transformer is not an efficient device

141.

The mutual flux in a transformer remains constant at all loads because ____. A. applied voltage and frequency are constant B. leakage flux is small C. iron-core is used D. losses are small

142.

Cores of large transformer are built up to nearly circular crosssection in order to reduce ____. A. leakage reactance B. iron losses C. eddy current loss D. copper loss

Unsolved problems

143.

REE Board Exam September 2002 A 20 kV/7.87 kV autotransformer has 200 A current in the common winding. What is the secondary line current? A. 143.52 A C. 56.48 A B. 200 A D. 329 A

144.

REE Board Exam April 2004 Instrument transformers are used in indicating and metering and with protective devices, they are used for A. measuring C. relaying B. detecting D. sensing

145.

REE Board Exam October 2000 The CT ratio and PT ratio used to protect a line are 240 and 2000 respectively. If the impedance of each line is 10 Ω, what is the relay impedance to protect the line from fault? A. 83.33 Ω C. 48,000 Ω B. 1.2 Ω D. 12 Ω

146.

REE Board Exam October 2000 Two single-phase transformers operate in parallel. Transformer 1 is rated 100 kVA with 4% reactance while transformer 2 is rated 75 kVA with 3% reactance. The transformers have secondary voltages of 220 volts and same voltage ratio. If the common load is 125 kVA at 80% p.f. lagging, determine the kVA sharing of each transformer. A. 71.43 kVa and 53.57 kVA B. 75 kVa and 50 kVA C. 62.5 kVA and 62.5 kVA D. 82.5 kVA and 42.5 kVA

147.

April 2004

REE Board Exam

A transformer rated 2,000 kVA, 34,500/240 volts has 5.75% impedance. What is the per unit impedance? A. 0.0635 C. 0.0575 B. 0.0656 D. 34.2

148.

REE Board Exam April 2001 A 69 kV/13.8 kV, 7.5 MVA transformer has 8% impedance. What is its impedance at 100 MVA base? A. 0.6% C. 800% B. 8% D. 106.7%

149.

REE Board Exam October 1997 A 3-phase transformer is rated 30 MVA, 115Δ/13.8Y kV and 2% impedance, the resistance is negligible compared with the reactance. Determine the percent (%) voltage drop at the secondary at 80% load and at 75% power factor. A. 4.1% C. 1.05% B. 2.26% D. 3.15%

150.

Which of the following connection is best suited for 3-phase, 4wire service? A. – C. –Y B. Y – Y D. Y –

151.

In a three-phase Y – Y transformer connection, neutral is fundamental to the A. suppression of harmonies B. passage of unbalanced currents due to loads C. provision of dual electric service D. balancing of phase voltages with respect to line voltages

152.

As compared to – bank, the capacity of the V – V bank of transformers is ____ percent. A. 57.7 C. 50 B. 66.7 D. 86.6

153.

If three transformers in a – are delivering their rated load and one transformer is removed, then overload and each of the remaining transformer is ____ percent. A. 66.7 C. 73.2 B. 173.2 D. 58

154.

When a V – V system is converted into a – system, increase in a capacity of the system is ____ percent.

A. 86.6 B. 66.7

C. D.

73.2 50

155.

For supplying is a balanced 3- load of 40-kVA, rating of each transformer in V – V bank, should be nearly ____ kVA. A. 20 C. 34.6 B. 23 D. 25

156.

When a closedbank is converted into an open- bank. Each of the two remaining transformer supplies ____ percent of the original load. A. 66.7 C. 50 B. 57.7 D. 73.2

157.

If the load p.f. is 0.886, then the average p.f of the V-bank is

A. 0.886 B. 0.75

C. D.

0.51 0.65

158.

A T – T connection has a higher ratio of utilization that a V – V connection only when A. identical transformers are used B. load power factor is leading C. load power factor is unity D. non-identical transformer are used

159.

The biggest advantage of T – T connection over the V – V connection for 3-phase power transformation is that it provides A. a set of balanced voltages under load B. a true 3-phase, 4 wire system C. a higher ratio of utilization D. more voltages

160.

One of the following statements concerning parallel operation of transformer, the one which is not correct is A. transformers have equal voltage ratings B. transformers must have same ratio of transformation C. transformers must be operated at the same frequency D. transformers must have equal

161.

Statement. An auto-transformer is more efficient in transferring energy from primary to secondary circuit. Reason. Because it does so both inductively and conductively. Key A. statement is false, reason is correct and relevant B. statement is correct, reason is correct but irrelevant C. both statement and reason are correct and are connected to each other as cause and effect D. both statement and reason are false

162.

Out of the following given choices for polyphase transformer connections which one will you select for threeto-two phase conversion? A. Scott B. star/star C. double Scott D. star/double-delta

163.

AT–T transformer cannot be paralleled with ____ transformer. A. V – V C. Y – Y B. Y – D. –

164.

Instrument transformers are used in a.c. circuits for extending the range of A. ammeters B. voltmeters C. wattmeters D. all of the above

165.

Before removing the ammeter from the current transformer, its secondary must be short-circuited in order to avoid A. excessive heating of the core B. high secondary emf C. increase in iron losses D. all of the above

166.

transformer is made of A. silicon steel B. annealed copper C. seasoned copper D. aluminum

The core of a

167.

The core of a transformer is assembled with laminated sheets to reduce A. hysteresis loss B. eddy-current loss C. magnetic noise D. magnetizing current

168.

The use of grainoriented laminated sheets in building a transformer core A. reduces magnetizing current B. reduces eddy-current loss in the core C. reduces hysteresis loss D. increases the no-load power factor angle

169.

Cooling of transformers is necessary to A. increase the efficiency B. dissipate the heat generated in the windings C. reduce the losses

D. reduce humming

170.

The emf induced in the windings of a transformer will A. lag the core flux by 90 degrees B. be in-phase with the core flux C. be out-of-phase with the core flux D. be independent of the core flux

171.

A 100-kVA, 1100/440-V, 50-Hz single-phase transformer has 100 turns on the secondary winding. The number of turns in the primary will be A. 550 C. 2750 B. 275 D. 5500

172.

The emf induced in the secondary winding of a 50-Hz single-phase transformer having 100 turns on its secondary is 222 V. The maximum flux density in the core is 2 0.1 Wb/m . The cross-sectional area of the core is 2 2 A. 0.1 m C. 1 m 2 2 B. 0.01 m D. 0.001 m

173.

The copper-loss and core-loss of a transformer of a transformer at various loads are as shown below. At what load will the efficiency of the transformer be a maximum? A. Load = 50 kVA, Core loss = 320 W, Copper Loss = 500 W B. Load = 40 kVA, Core loss = 320 W, Copper Loss = 320 W C. Load = 30 kVA, Core loss = 320 W, Copper Loss = 180 W D. Load = 20 kVA, Core loss = 320 W, Copper Loss = 80 W

174.

A transformer when supplying a load maintained 11 kV across terminals. When the load was switched off, the terminal voltage became 11550 V. What is the voltage regulation of the load? A. 11.55% C. 5% B. 5.5% D. 55%

175.

Power lost in the open-circuit and shiort0-circuit tests on a transformer gives approximately an account of the following losses: A. Core-losses and copper losses respectively B. Copper-losses and core-losses respectively C. Eddy-current loss and hysteresis loss respectively D. Hysteresis loss and eddy-current loss respectively

176.

Maximum efficiency of a transformer occurs when A. hysteresis loss and eddy current loss are minimum

B. the sum of hysteresis loss and eddy current loss is equal to copper loss in the windings C. power factor of the load is leading D. hysteresis loss is equal to eddy-current loss

177.

The all-day efficiency of a transformer is the ratio of

A. B. C. D.

kWh output and kWh input per day kWh output and kWh input in a day output power and input power input power and output power

178.

The ratio of the primary to secondary voltage of a transformer is 2:1. The saving in terms of weight of copper required if an autotransformer is used instead of a two-winding transformer will be A. 50% C. 66.67% B. 33.33% D. 97%

179.

For satisfactory parallel operation of two single-phase transformers a number of conditions are to be fulfilled. A number of conditions are written below. Indicate which of these is not required to be fulfilled. A. kVA ratings of the two transformers should be equal B. Transformers should be properly connected with regard to their polarity C. Voltage ratings of the primary windings should be suitable for supply system voltage and frequency. The turns ratio of the transformer should be equal. D. The percentage impedance of the two transformers should be equal.

180.

An additional condition for parallel operation of three-phase transformers over single-phase transformers is that A. the transformers should belong to the same vector group B. ratios of winding resistances to resistances for the transformers should be equal C. the transformers should have the same kva ratings D. the transformers should not belong to the same vector group

181.

The maximum flux density in the core of a 3000/240-V, 50 Hz singlephase distribution transformer is 1.25 Tesla. If the voltage induced per turn is 8 volts, determine the 2 cross-sectional core in cm . A. 292 B. 375 C. 288 D. 362

182.

The net cross section of a transformer is 20 square inches. The maximum flux density is 11 kilogausses. The frequency is 60 Hz. If there are 1,000 turns in the primary, determine the primary induced emf. A. B. C. D.

3780 V 3945 V 3590 V 3850 V

183.

_____current are wasteful currents which flow in cores of transformers and produced heat. A. Residual C. Sneak B. Eddy D. Magnetizing

184.

Calculate the total magnetic flux in a 60-cycle transformer in which the induced emf per turn of the winding is equal to 2 V. A. 7.50 mWb B. 7.02 mWb C. 7.60 mWb D. 7.42 mWb

185.

The low side of a certain welding transformer has 2 turns and delivers 300 A. If the high voltage side has 30 turns, how much current lows through it? A. B. C. D.

10 A 12 A 20 A 15 A

static induction dynamic induction mutual induction self-induction

used to change the value of A. frequency B. voltage

A transformer operates A. always at unity power factor B. has its own power factor C. at power factor below a particular value D. at power factor depending on the power factor of the load

191.

The laminations are made from A. low carbon steel B. silicon steel sheet C. nickel alloy steel stampings D. chrome steel sheets

192.

The steel for construction of transformer core is made so as to made A. low permeability and high hysteresis B. low permeability and low hysteresis C. high permeability and low hysteresis D. high permeability and high hysteresis

193.

The special silicon steel is used for laminations because A. B. C. D.

hysteresis losses are reduced eddy current losses are reduced both of the above losses are reduced none of the above

A. B. C. D.

electric circuit magnetic circuit winding wire gauge none of the above

195. Transformer is C. D.

power power factor

188.

The path of the magnetic flux in a transformer has A, low reactance B. high reactance C. high conductivity D. low resistance

189.

190.

What is common in the two windings of a transformer?

The principle of working of working of the transformer is

187.

electrically electromagnetically

194.

186.

A. B. C. D.

C. D.

Electric power is transformed from one coil to the other in a transformer A. physically B. magnetically

The main function of the iron core in a transformer is to A. provide strength to the windings B. to decrease the hysteresis losses C. decrease the reluctance of the magnetic path D. reduce eddy current losses

196.

Preferably, the resistance between the primary and the secondary of a transformer should be A. as low as possible B. as high as possible C. low or high depending upon whether it is step up or step down respectively D. high or low depending upon whether it is step up or step down respectively

197.

assumptions do not include

Ideal transformer

A. B. C. D.

zero reactance of windings zero resistance of windings no leakage flux no saturation of flux

198.

The reactance of a transformer is determined by its A. leakage flux B. common core flux C. size of core D. permeability of the material of the core

199.

The efficiency of the transformer is normally in the range of A. B. C. D.

50% to 70% 60% to 75% 80% to 90% 90% to 98%

The resistance of the low voltage winding of a transformer

B. C. D.

is equal to the resistance of its high voltage side is more than the resistance of its high voltage side is less than the resistance of its high voltage side either B or C

201.

The basic property of the transformer is that is changes the voltage level of an a.c. signal A. without changing the power B. without changing its shape C. without changing its frequency D. without changing power, frequency or shape

202.

The induced emf in the transformer secondary will depend upon A. B. C. D.

maximum flux in core and frequency only frequency, flux and number of turns in the secondary frequency of the supply only number of turns in the secondary only

203.

At every instant the direction of secondary current in a transformer must be such as to oppose any change in flux. This is in accordance with A. Faraday’s law B. Joule’s law C. Lenz’s law D. Coulomb’s law

204.

transformation ratio is A. E1/E2 B. N1/N2

The voltage

E2/E1 N2/N1

205.

The magnitude of the mutual flux in a transformer is A. low at low loads and high at high loads B. high at low loads and low at high loads C. same at all levels D. varies at low loads and constant at high loads

206.

The concentric windings are used in core type transformer with A. B. C. D.

200.

A.

C. D.

LT winding placed next to core HT winding placed next to core LT winding on the outer side HT winding on the outer side

207.

Cross-over windings are used for A. high voltage winding of small rating transformers B. low voltage winding of small rating transformers C. high voltage winding of large rating transformers D. none of the above

208.

In an ideal transformer on no-load, the primary applied voltage is balanced by A. the secondary voltage B. the drop across resistance and reactances C. the secondary induced emf D. the primary induced emf

209.

Transformers are

rated in A. kW B. kV C. kWh D. kVA

210.

What type of core is used for a high frequency transformer? A. B. C. D.

211.

air core closed iron core aluminum core open iron core

If in a transformer the secondary winding are doubled and at the same time the primary voltage is reduced to half, then secondary voltage will A. be halved B. not change C. be four times as much D. be reduced to a quarter

B. 375

212.

The no-load current of a transformer in terms of full-load current is usually A. 1 to 3% B. 3 to 9% C. 9 to 12% D. 12 to 20%

213.

Which is the common method of cooling a power transformer? A. B. C. D.

362

220.

The net cross section of a transformer is 20 square inches. The maximum flux density is 11 kilogausses. The frequency is 60 Hz. If there are 1,000 turns in the primary, determine the primary induced emf.

A. 3780 V B. 3945 V

C. D.

3590 V 3850 V

221.

The low side of a certain welding transformer has 2 turns and delivers 300 A. If the high voltage side has 30 turns, how much current flows through it?

air cooling air blast cooling oil cooling natural cooling

214.

The use of higher flux density in transformer design A. decreases the weight per kVA B. increases the weight per kVA C. increases the weight per kW D. decreases the weight per kW

215.

What is the efficiency of transformer compared with that of electrical motors of the same power? A. much smaller B. somewhat smaller C. much higher D. about the same

216.

The transformer oil used in transformer provide A. cooling and lubrication B. insulation and lubrication C. insulation and cooling D. insulation, cooling and lubrication

217.

In any transformer the voltage per turn in primary and secondary remains A. always different B. always same C. always in the ratio of a D. sometime same

218.

In transformer the purpose of breather is to A. extract moisture of the air B. to take insulating oil from conservator C. to provide cooling to the winding D. to provide insulation to the winding

219.

The maximum flux density in the core of a 3000/240-V, 50 Hz singlephase distribution transformer is 1.25 Tesla. If the voltage induced per turn is 8 volts, determine the 2 cross-sectional area of the core in cm .

A. 292

D.

C.

288

A. 10 A B. 12 A

C. D.

20 A 15 A

222.

The eddy current and hysteresis losses of a 4,400-V, 60-Hz singlephase distribution transformer are 280 and 560 watts, respectively. Determine the core loss when the transformer is connected across a 4,600-volt, 50-Hz source. A. 977 W C. 994 W B. 983 W D. 967 W

223.

A 20-kVA, 500-V load is to be supplied by an ideal step-up autotransformer from a 400-V source. Find the current in the common winding. A. 30 A B. 40 A C. 50 A D. none of these

224.

Calculate the total magnetic flux in a 60-cycle transformer in which the induced emf per turn of the winding is equal to 2 V. A. 7.50 mWb C. 7.50 mWb B. 7.02 mWb D. 7.42 mWb

225.

The current densities in the primary and secondary windings of a step don transformer are 1250 and 1500 A per square inch, respectively. The ratio of transformation is 10:1 and the mean length per turn of the primary is 15% greater than that of the secondary. If the primary winding resistance is 20 ohms, calculate the resistance of the secondary windings. A. 0.202 ohm C. 0.209 ohm B. 0.219 ohm D. 0.220 ohm

226.

A 10-kVA, 2000/400 V, single-phase transformer has primary and secondary winding impedances of 5.0 + j10 and 0.2 + j0.45 ohms, respectively. Calculate the voltage across the secondary

terminals at full load, 80% power actor lagging, when the primary is supplied with 2,000 volts. A. 400 V C. 385 V B. 394 V D. 379 V

227.

An open delta bank, consisting of two-single phase transformers is operating with balanced three-phase load of 50 kVA, 440 V at 0.8 p lagging and a single phase load of 10 kW resistive connected across c-a leg. Determine the minimum ratings of the transformers. Assume a phase sequence of a-bc. A. 38.80 kVA, 34.06 kVA B. 34.32 kVA, 33.40 kVA C. 35.48 kVA, 36.34 kVA D. 30.83 kVA, 39.52 kVA

228.

An open delta, three-phase transformer consisting of two singlephase transformers is operating with a balanced three-phase load of 50 kVA, 440-V at a lagging pf of 0.8. Determine the operating pf of each transformer. A. 0.393 lagging, 0.993 lagging B. 0.383 lagging, 0.887 lagging C. 0.383 lagging, 0.993 lagging D. 0.393 lagging, 0.887 lagging

229.

Which of the following given below is the eddy current loss not proportional to A. square of the thickness B. square of the frequency C. square of the load current of the laminated core D. square of the flux density

230.

A transformer will have negative voltage regulation when its load is

A. capacitive B. resistive

C. D.

inductive both B and C

231.

EE Board Exam April 1990 Copper loss of a transformer are determined by means of a A. polarity test B. short circuit test C. open circuit test D. inductive circuit test

232.

The secondary of a transformer is connected wye-grounded. The voltage from line to neutral is 132 volts. If the load current is 40 amperes, calculate the load in kVA.

A. 5.28 kVA B. 9.15 kVA

C. D.

20.11 kVA 15.84 kVA

233.

The current that flows through the common winding of a 20kV/7.97 kV autotransformer is 200 amperes. Determine the secondary current. A. 330 amperes C. 200 amperes B. 220 amperes D. 150 amperes

234.

The secondary current of a 20 kV/7.97-kV autotransformer is 100 amperes. Determine the current that flows through the common winding. A. 60 amperes C. 70 amperes B. 100 amperes D. 50 amperes

235.

Two transformer are connected in parallel supplying a load of 2,500 kVA. The first transformer has an impedance of 3% and a rating of 1,250 kVA while the second has an impedance of 3% and a rating of 1,500 kVA. Calculate the load shared by the second transformer. A. 1453.19 kVA C. 1363.64 kVA B. 1792.51 kVA D. 1227.53 kVA

236.

A transformer has a core loss of 64 W and a copper loss of 144 W when it is carrying 20% overload current. The load at which this transformer will operate at the maximum efficiency is ____ percent.

A. 80 B. 66

C. D.

237.

120 75

A 100 ohms load has a pf of 0.8. A 1 k generator supplies power to the load through a transformer. Best matching will be produced when transformer ratio is approximately A. 3:1 C. 10:1 B. 8:1 D. 2:1

238. A. B.

C. D.

239. A. B.

C. D.

View more...
1. REE Board Exam April 1997

A single-phase transformer is rated 4,160-V primary and 250-V secondary. It has 1,500 turns in the primary and voltage regulation of 8%. What should be the number of turns in the secondary winding? A. 88 turns C. 90 turns B. 92 turns D. 86 turns

2. EE Board Exam April 1991

A certain transformer has a ratio of 8 is to 1. The primary winding resistance is 0.00283 Ω per turn and has 1,155 more turns than the secondary winding. If the copper loss in the primary is 20% less than the secondary, find the resistance of the secondary winding. A. 0.073 ohm C. 0.068 ohm B. 0.090 ohm D. 0.084 ohm

3. EE Board Exam October 1991

A 10-kVA; 2,400/240 V single-phase transformer has the following resistances and leakage reactances. Find the primary voltage required to produce 240-V at the secondary terminals at fullload when the load power factor is 0.8 lagging. r1 = 3 ohms x1 = 15 ohms r2 = 0.03 ohm x2 = 0.15 ohm A. 2,450 volts C. 2,496 volts B. 2,575 volts D. 2,632 volts

4. EE Board Exam April 1983

A single-phase transformer rated 75 kVA, 8000 volts primary and 240 volts secondary is given the short circuit test. With the secondary terminals short circuited, 440 volts (60 Hz) is impressed on the primary, which then draws rated current and 1125 watts. Solve for the percentage impedance of the transformer. A. 5.29% C. 5.50% B. 4.93% D. 5.13%

5. EE Board Exam April 1993

A short circuit test was performed upon a 10-kVA, 2300/230 volt transformer with the following results: Esc = 137 volts; Psc = 192 W; Isc = 4.34 A. Calculate in primary terms the reactance of the transformer. A. 31.56 ohms B. 29.88 ohms C. 10.19 ohms D. 12.45 ohms

6. EE Board Exam October 1984

A single-phase, 25 kVA, 4800/240 volt transformer was given the short circuit test. With the secondary terminals short-circuited, 93.75 volts was applied on the primary so that the ammeter reads rated current and the wattmeter reads 162.74 watts, all on the primary side. Solve for the percentage resistance of the transformer

A. 1.95% B. 3.39%

C. D.

3.02% 2.23%

7. EE Board Exam April 1990

The following data were obtained when a short circuit test was performed upon a 100 kVA, 2400/240 volts distribution transformer: Esc = 72 volts; Isc = 41.6 A; Psc = 1,180 W. All instruments are on the high side during the short circuit test. Calculate the equivalent resistance and reactance of the transformer. A. 0.682 ohm, 1.59 ohms B. 0.682 ohm, 1.73 ohms C. 0.752 ohm, 1.73 ohms D. 0.752 ohm, 1.59 ohms

8. REE Board Exam March 1998

A 13.8 kV/480 V, 10 MVA three-phase transformer has 5% impedance. What is the impedance in ohms referred to the primary? A. 0.952 ohm B. 0.03 ohm C. 5.125 ohms D. 9.01 ohms

9. EE Board Exam April 1990

B. The following data were obtained when a short circuit test was performed upon a 100 kVA, 2400/240 volts distribution transformer: Esc = 72 volts; Isc = 41.6 A; Psc = 1,180 W. All instruments are on the high side during the short circuit test. Calculate the percent regulation at a power factor of 0.75 lagging. A. 2.54% C. 2.62% B. 2.86% D. 2.71%

10. EE Board Exam October 1992

A 200-kVA transformer with impedance of 5% and the power factor of the load is 85%. The primary voltage is 6,000 volts while the copper loss is 5 kW. Find the percentage regulation. A. 5.03% C. 4.43% B. 5.21% D. 4.03%

11. EE Board Exam April 1993

A short circuit test was performed upon a 10-kVA, 2300/230 volt transformer with the following results: Esc = 137 volts; Psc = 192 W; Isc = 4.34 A. Calculate the percent regulation at a lagging power factor of 0.707. A. 5.28% C. 5.37% B. 5.55% D. 5.72%

12. EE Board Exam October 1992

In a 440 V, 50 c/s transformer, the total iron loss is 2500 W. When the supplied p.d. is 220 V at 25 c/s, the corresponding loss is 850 W. Calculate the eddy current loss at normal frequency and p.d. A. 2337 W C. 2010 W B. 2165 W D. 2216 W

13. EE Board Exam April 1990

A 50 kVA, 2300/230 volts, 60 cycle transformer is tested in the laboratory so that its characteristics maybe determined. The standard test requires an open circuit test and short circuit test. Open Circuit Test: I = 6.5 A E = 230 V P = 187 W Short Circuit Test I = 21.7 A E = 115 V P = 570 W Determine the efficiency of the transformer at half load, unity pf. A. 97.55% C. 98.45% B. 98.70% D. 97.80%

14. EE Board Exam April 1992

Given that the full load copper losses are exactly twice the iron losses in a 50 kVA transformer, and that the quarter load efficiency is 96.5%, calculate the full load efficiency at unity power factor. A. 97.48% C. 97.64% B. 98.12% D. 96.25%

15. EE Board Exam April 1995

The core loss of a 5 kVA single –phase transformer with normal voltage applied to the primary is 75 watts. The maximum efficiency occurs at 60% of full-load kVA. What is the fullload efficiency of the transformer at 0.80 power factor? A. 95.16% C. 93.38% B. 89.52% D. 91.36%

16. EE Board Exam October 1980, October 1982

A 25-kVA distribution transformer operates at 120% load, 0.3 pf lagging for 1 hour; art full-load, 0.9 lagging pf for 3 hours; 50% load at unity pf for 8 hours; and 10% load at 0.8 leading pf for the rest of the day. If core loss is 150 watts and full load copper loss is 500 W, what is the all-day efficiency of the transformer? A. 94.23% C. 97.14% B. 92.86% D. 96.68%

17. EE Board Exam October 1990

Find the all-day efficiency of a transformer having maximum efficiency of 98% at 15-kVA at unity power factor and loaded as follows: 12 hours 2-kW at 0.5 pf lagging 6 hours 12-kW at 0.8 pf lagging 6 hours at no load A. 95.3% C. 94.6% B. 91.7% D. 93.3%

18. EE Board Exam October 1991

A 30-kVA, 2,400/240 volts, 60-Hz transformer has a full-load power of unity over the period of 24 hrs. The maximum efficiency is 95% and it occurs at full-load. Calculate the all-day efficiency if loaded as follows: 6-hours at full-load

6 hours at quarter load A. 91.53% C. 90.37% B. 92.45% D. 93.86% 19. EE Board Exam April 1995 Ideal transformer assumptions do not include A. no leakage flux B. no saturation of core C. zero resistance of the winding D. zero reactance of the winding

20. EE Board Exam October 1993

A transformer is designed to carry 30 times its rated current for 1 second. The length of time that a current of 20 times the rated current can be allowed to flow is A. 0.67 sec C. 1.5 sec B. 2 sec D. 2.25 sec

21. REE Board Exam October 1998

Which of the following given below is the eddy current loss not proportional to? A. square of the thickness B. square of the frequency C. square of the load current of the laminated core D. square of the flux density

22. REE Board Exam October 1997

A small single transformer has 10.2 watts no load loss. The core has a volume of 750 cu. cm. The maximum flux density is 10,000 gauss and the -4 hysteresis constant of core is 5 x 10 . Using the Steinmetz law to find the hysteresis, determine the eddy current loss. A. 4.55 watts C. 3.55 watts B. 5.55 watts D. 2.55 watts

23. EE Board Exam April 1992

When the emf in the two windings of a transformer are opposite in direction, the polarity is A. positive C. negative B. additive D. subtractive

24. EE Board Exam October 1991

The copper loss of the transformer will A. not be affected by a change in the power factor B. decrease with a lowering of the power factor C. increase with an increase of the power factor D. increase with the lowering of the power factor

25. EE Board Exam June 1990

Power lost due to alternating magnetization and demagnetization of the core material A. copper loss B. eddy current loss C. hysteresis loss D. rheostat loss

26. EE Board Exam June 1990

In the open circuit test of the transformer the wattmeter reading is equal to A. eddy current loss B. current loss C. hysteresis loss D. core loss

27. EE Board Exam October 1991

A 30-kVA; 2,400/240 volts, 60-Hz transformer has a full-load power of unity over the period of 24 hrs. The maximum efficiency is 95% and it occurs at full-load. Calculate the all-day efficiency if loaded 6-hours at full-load. A. 78.90% C. 89.67% B. 88.38% D. 92.45%

28. EE Board Exam April 1991

A certain transformer has a ratio of 8:1. The primary winding resistance is 0.00283 Ω and has 1,165 more turns than the secondary winding. If the copper loss in the primary is 20% less than the secondary, calculate the resistance of the secondary winding. -5 A. 5.53 x 10 ohm -4 B. 2.56 x 10 ohm -7 C. 6.78 x 10 ohm -5 D. 4.18 x 10 ohm

29. EE Board Exam October 1992

A 200-kVA transformer has an impedance of 15% and the power factor of the load is 85%. The primary voltage is 6,000 volts while the copper loss is 15 kW. Find the percentage voltage regulation. A. 13.49% C. 8.91% B. 15.78% D. 23.67%

30. EE Board Exam June 1990

Transformer connection that can transform threephase system to two-phase system and vice versa A. Scott or T-connection B. open delta C. autotransformer connection D. wye-delta connection

31. EE Board Exam June 1990

Which transformer has only one winding? A. distribution transformer B. power transformer C. autotransformer D. isolated winding transformer

32. EE Board Exam April 1990

When the primary and secondary windings take the form of a common ring which is encircled by two or more rings of magnetic materials distributed around its periphery, the transformer is termed as A. grounding transformer B. regulating transformer C. core type transformer

D. shell type transformer

33. A transformer transforms A. B. C. D.

frequency voltage current voltage and current

34. Which of the following is not a basic element of a transformer? A. Core B. primary winding C. secondary winding D. mutual flux

35. In an ideal transformer A. B. C. D.

windings have no resistance core has no losses core has infinite permeability all of the above

36. The main purpose of using core in a transformer is to A. decrease iron losses B. prevent eddy current loss C. eliminate magnetic hysteresis D. decrease reluctance of the common magnetic circuit

37. Transformer cores are laminated in order to A. B. C. D.

simplify its construction minimize eddy current loss reduce cost reduce hysteresis

38. A transformer having 1000 primary turns is

connected to a 250-V a.c supply. For a secondary voltage of 400 V, the number of secondary turns should be A. 1600 C. 400 B. 250 D. 1250

39. The primary and secondary induce e.m.fs. E1 and E2 in a two-winding transformer are always A. equal in magnitude B. anti-phase with each other C. in-phase with each other D. determined by load on transformer secondary

40. A step-up transformer increases A. voltage B. current

C. D.

power frequency

41. The primary and secondary windings of an

ordinary 2-winding transformer always have A. different number of turns B. same size of copper wire C. a common magnetic circuit D. separate magnetic circuits

42. In a transformer, the leakage flux of each winding is proportional to the current in that winding because A. Ohm’s law applies to magnetic circuits B. leakage paths do not saturate C. the two windings are electrically isolated D. mutual flux is confined to the core

43. In a two-winding transformer, the e.m.f. per turn

in secondary winding is always ____ the induced e.m.f. power turn in primary. A. equal to K times B. equal to 1/K times C. equal to D. greater than

44. In relation to a transformer, the ratio 20:1

indicates that A. there are 20 turns on primary one turn on secondary th B. secondary voltage is 1/20 of primary voltage C. primary current is 20 times greater than the secondary current D. for every 20 turns on primary, there is one turn on secondary

45. In performing the short circuit test of a

transformer A. high voltage side is usually short circuited B. low voltage side is usually short circuited C. any side is short circuited with preference D. none of the above

46. The equivalent resistance of the primary of a

transformer having K = 5 and R1 = 0.1 ohm when referred to secondary becomes ____ ohm. A. 0.5 C. 0.004 B. 0.02 D. 2.5

47. A transformer has negative voltage regulation when its power factor is A. zero B. unity

C. D.

leading lagging

48. The primary reason why open-circuit test is

performed on the transformer is that it A. draws sufficiently large no-load current for convenient reading B. requires least voltage to perform the test C. needs minimum power input D. involves less core loss

49. No-load test on a transformer is carried out to determine A. copper loss B. magnetizing current C. magnetizing current and no-load loss D. efficiency of the transformer

50. The main purpose of performing open-circuit test on a transformer is to measure its A. Cu loss B. core loss C. total loss D. insulation resistance

51. During short-circuit test, the iron lose of a

transformer is negligible because A. the entire input is just sufficient to meet Cu losses only B. flux produced is a small fraction of the normal flux C. iron core becomes fully saturated D. supply frequency is held constant

52. The iron loss of a transformer at 400 Hz is 10 W.

Assuming that eddy current and hysteresis losses vary as the square of flux density, the iron loss of the transformer at rated voltage but 50 Hz would be ____ watt. A. 80 C. 1.25 B. 640 D. 100

53. In operating a 400 Hz transformer at 50 Hz A. B. C. D.

only voltage is reduced in the same proportion as the frequency only kVA rating is reduced in the same proportion as the frequency both voltage and kVA rating are reduced in the same proportion as the frequency none of the above

54. The voltage applied to the h.v. side of a

transformer during short-circuit test is 2% of its rated voltage. The core loss will be ____ percent of the rated core loss. A. 4 C. 0.25 B. 0.4 D. 0.04

55. Transformers are rated in kVA instead of kW

because A. load power factor is often not know B. kVA is fixed whereas kW depends on load p.f. C. total transformer loss depends on voltampere D. it has become customary

56. When a 400 Hz transformer is operated its kVA rating is A. reduced to 1/8 B. increased 8 times C. unaffected D. increased 64 times

57. At relatively light loads, transformer efficiency is low because A. secondary output is low B. transformer losses are high C. fixed loss is high in proportion to the output

D.

Cu loss is small

58. A 200 kVA transformer has an iron loss of 1 kW and full-load Cu loss of 2 kW. Its load kVA corresponding to maximum efficiency is ____ kVA. A. 100 C. 50 B. 200 D. 141.4

59. If Cu loss of a transformer at 7/8th full-load is 4900 W, then its full-load Cu loss would be ____ watt. A. 5600 C. 375 B. 6400 D. 429

60. The ordinary efficiency of a given transformer is maximum when A. it runs at half full-load B. it runs at full-load C. its Cu loss equals iron loss D. it runs slightly overload

61. The output current corresponding to maximum efficiency for a transformer having core loss of 100 W and equivalent resistance referred to secondary of 0.25 is ____ ampere. A. 20 C. 5 B. 25 D. 400

62. The maximum efficiency of a 100-kVA

transformer having iron loss of 900 kW and F.L. Cu loss of 1600 W occurs at ____ kVA. A. 56.3 C. 75 B. 133.3 D. 177.7

63. The all-day efficiency of a transformer depends primarily on A. its copper loss B. the amount of load C. the duration of load D. both B and C

64. The marked increase in kVA capacity produced by connecting a 2 winding transformer as an autotransformer is due to A. increase in turn ratio B. increase in secondary voltage C. increase in transformer efficiency D. establishment of conductive link between primary and secondary

65. The kVA rating of an ordinary 2-winding

.

transformer is increased when connected as an autotransformer because A. transformation ratio is increased B. secondary voltage is increased C. energy is transferred both inductively and conductivity D. secondary current is increased

66. The saving in Cu achieved by converting 2-

winding transformer into an autotransformer is determined by A. voltage transformation ratio B. load on the secondary C. magnetic quality of core material D. size of the transformer core

67. An autotransformer having a transformation ratio of 0.8 supplies a load of 3 kW. The power transferred conductively from primary to secondary is ____ kW. A. 0.6 C. 1.5 B. 2.4 D. 0.27

68. The essential condition for parallel operation of

two 1-φ transformer is that they should have the same A. polarity B. kVA rating C. voltage D. percentage impedance

69. If the impedance triangles of two transformers

operating in parallel are not identical in shape and size, the two transformers will A. share the load unequally B. get heated unequally C. have a circulatory secondary current even when unloaded D. run with different power factors

70. Two transformers A and B having equal outputs

and voltage ratios but unequal percentage impedances of 4 and 2 are operating in parallel. Transformer A will be running over-load by ____ percent. A. 50 C. 33 B. 66 D. 25

Unsolved problems 71. EE Board Exam October 1991, April 1994 A 1000 kVA, 6,600/400 volts 60 cycle single phase core type transformer has the following average daily load: Full-load at 0.8 pf for 8 hours Half-load at 0.707 pf for 10 hours No-load for 6 hours Calculate the ratio of full-load copper loss to the iron loss for the transformer to be most economical for the above loading. A. 2.05 C. 2.17 B. 3.09 D. 2.28 THREE PHASE TRANSFORMER CONNECTIONS 72. EE Board Exam June 1990 What should be the KVA rating of each two transformers in an open-delta bank when the three-phase balanced load is 290.4 kVA? A. 155.60 kVA B. 173.40 kVA C. 150.25 kVA

D. 167.66 kVA

73. EE Board Exam April 1981

A V-V or open delta connected transformer bank, composed of two identical units serves a balanced 3-phase load of 16 kVA at 230 volts. Solve for the minimum size in kVA of the transformers needed. A. 8 kVA C. 16 kVA B. 9.24 kVA D. 8.25 kVA 74. EE Board Exam April 1986 A V-V (open delta) connected transformer bank serves a balanced 3-phase load of 40 kVA at 230 V and 0.866 pf lagging. Solve for the minimum kVA of each transformer needed to accommodate the load without overloading. A. 25 C. 23 B. 24 D. 20

75. EE Board Exam April 1985

Two single-phase distribution transformers are banked in V-V (open delta). If each transformer is rated 37.5 kVA, solve for the maximum 3-phase load that the bank can carry without overloading, considering that the load has a power factor of 0.85 lagging. A. 52.5 kW C. 53.6 kW B. 49.5 kW D. 55.2 kW

76. EE Board Exam April 1980

Two single-phase distribution transformers connected in open delta will supply power to a 200 hp, 3-phase induction motor, operating at 0.70 power factor and 0.90 efficiency. Solve for the minimum size in kVA of each transformer needed to supply the power without being overloaded. A. 136.73 kVA B. 124.45 kVA C. 130.38 kVA D. 129.48 kVA

77. EE Board Exam October 1986

The Utility Company supplies two single-phase transformers banked in open delta. If each transformer is rated 75 kVA capacity, what is the maximum 3-phase load in kilowatts that the bank can carry without suffering overload considering that the load has a lagging power factor of 0.80. A. 104 kW C. 130 kW B. 112 kW D. 114 kW

78. EE Board Exam October 1980

Two identical 1-phase transformers connected in open delta will serve a 3-phase motor lad of 250 hp, 0.70 pf lagging, 0.90 efficiency. Solve for the minimum size in kVA of each transformer required to serve the load without overloading. A. 150.33 C. 160.43 B. 170.91 D. 185.13

79. EE Board Exam October 1985, April 1986

An open-delta connected transformer bank serves a balanced 3-phase load of 40 kVA at 230 V and 0.866 pf lagging. Solve for the real power and p at which each transformer is operating. A. 11.54 kW at 0.5 pf and 23.09 kW at unity pf B. 12.22 kW at 0.5 pf and 22.42 at unity pf C. 11.54 kW at 0.866 pf and 23.09 kW at 0.707 pf D. 12.22 kW at 0.866 pf and 22.429 kW at 0.707 pf

80. EE Board Exam April 1983

Two single-phase transformers are connected in V-V (open delta) and serving a delta connected impedance load with each impedance equal to ohms. If the transformer voltages impressed on the transformer are: V V

Solve for the kVA delivered by each transformer. A. 7.28 and 5.20 B. 5.87 and 6.61 C. 6.84 and 5.64 D. 6.24 and 6.24

81. EE Board Exam June 1990

Two single-phase transformers in an open delta bank served a three-phase balanced load of 290.4 kVA. If a third transformer is added for delta operation, what percent increase in load does this represent? A. 73.2% C. 80.5% B. 84.6% D. 79.4%

82. EE Board Exam October 1983

Three single-phase transformers each rated 75 kVA are banked in delta and supplying a 3-phase load drawing 160 kVA at 0.8 lagging power factor. If one transformer is removed for repairs, solve for the amount of overloading of the remaining units. A. 30 kVA C. 20 kVA B. 10 kVA D. 25 kVA

83. EE Board Exam April 1992

A 6600-volt, 60 Hz, 2-phase network is connected to a three-phase, 4-wire system by a Scottconnected transformer combination. If there are 1500 turns on the 2-phase side, calculate the number of turns on the 3-phase side required for the main and teaser transformers. Voltage per phase on the 3-phase side is 240 V. A. 94 turns and 83 turns B. 93 turns and 84 turns C. 95 turns and 82 turns D. 92 turns and 85 turns

84. EE Board Exam April 1985

A balanced 3-phase transformer bank has deltaconnected primary and secondary windings. The primary to secondary winding voltage ratio is

2400/277. Solve for the primary line current if the low-side (secondary) load draws a line current of 1000 A. A. 200 A C. 150 A B. 115 A D. 175 A

85. EE Board Exam October 1991

What should be the turns ratio of a three-phase transformer to transform 10,000 kVA from 230 kV to 4160 V if the transformer is to be connected Δ – Y? A. 94 C. 96 B. 98 D. 97

86. REE Board Exam October 1998

The potential transformer of a line has a ratio of 132,000/66. These are connected wye-delta. The voltmeter connected at the secondary reads 64 volts. What is the voltage of the line? A. 221.7 kV C. 112.5 kV B. 135.2 kV D. 132 kV

87. REE Board Exam October 1997

A power transformer rated 50,000 kVA, 34.5 kV/13.8 kV is connected wye-grounded primary and delta on the secondary. Determine the full load phase current on the secondary side. A. 2092 A C. 1449 A B. 1725 A D. 1208 A

88.

REE Board Exam October 1998 A 69/13.8 kV, 10 MVA three-phase transformer is connected wye-grounded delta. What is the kV rating of the secondary winding? A. 7.97 C. 15 B. 23.9 D. 13.8

89. REE Board Exam October 1997

The PTs of a 220 kV transmission line have a ratio of 132.76 kV/66.375 V are connected wyewye. A voltmeter connected line to ground reads 66 volts, what is the transmission line voltage? A. 228.63 kV C. 223.15 kV B. 220.10 kV D. 225.25 kV

90. REE Board Exam October 1997

A power transformer rated 50,000 kVA, 34.5 kV/13.8 kV is connected wye-wye. What are the line currents at full load? A. 525.5 A, 1725.4 A B. 836.7 A, 2092 A C. 483.1 A, 1207.7 A D. 1449.3 A, 3623.2 A

91. EE Board Exam October 1985

Two transformers are connected in parallel to supply a common load of 175 kVA. Transformer A is rated 100 kVA with equivalent impedance of 9.6 ohms while transformer B is rated 75 kVA with equivalent impedance of 12.5 ohms. Find the kVA load of each transformer. A. 78, 97 C. 80, 95

B. 76, 99

D.

75, 100

92. EE Board Exam October 1981

Two distribution transformers are connected in parallel supplying a load of 125 kVA. If transformer A is rated 50 kVA, with 14 ohms equivalent reactance and transformer B is rated 75 kVA with 11 ohms equivalent reactance. Solve for the loading of each transformer. A. 55 kVA, 70 kVA B. 50 kVA, 75 kVA C. 58 kVA, 67 kVA D. 52 kVA, 73 kVA

93. REE Board Exam March 1998

A 125 kVA distribution transformer with 4% impedance is connected in parallel with another transformer rated 75 kVA and with 35% impedance. Both have same voltage ratio. (Neglect resistance of each transformer.) The total load is 140 kVA at 85% power factor. How much load does each carry? A. 80.5 kVA, 59.5 kVA B. 77.78 kVA, 62.22 kVA C. 80 kVA, 60 kVA D. 87.50 kVA, 52.5 kVA

94. EE Board Exam April 1993

MERALCO has two single-phase transformers with equal turns ratio and ratings are operated in parallel to supply a load of 280 kW at 0.8 lagging pf. Transformer A has a resistance of 2% and a reactance of 8%. Transformer B has a resistance of 1% resistance and a reactance of 6%. Determine the power delivered by the transformers A and B to the load. A. A = 123 kW, B = 157 kW B. A = 125 kW, B = 155 kW C. A = 122 kW, B = 158 kW D. A = 127 kW, B = 153 kW

95. REE Board Exam April 1994

Two single phase transformers with equal ratings and turns ratio are operated in parallel to supply a load of 180 kW at a lagging pf of 0.90. Transformer A has a resistance of 1% and a reactance of 6%. Transformer B has a resistance of 2% and a reactance of 5%. Calculate the power delivered by each transformer. A. 83 kW, 97 kW B. 79 kW, 92 kW C. 88 kW, 92 kW D. 80 kW, 100 kW

96. REE Board Exam March 1998

A power customer draws power at 220 volts from a transformer on a pole. Current transformers with ratio of 200/5 are use to meter the electrical usage. What is the multiplier of the kW hr and demand meters? A. 40 C. 200 B. 100 D. 80

97. REE Board Exam October 1997

At a 155 kV substation, the PT ratio is 1000 and the CT ratio is 1200/5. The potential going into the wattmeter is 115 volts. What is the MW indicated when the wattmeter reads 800 watts? A. 192 MW C. 19.2 MW B. 150 MW D. 15 MW

98. REE Board Exam March 1998

A 50 MVA, 33 kV/11 kW, three-phase, wye-delta connected transformer has a 3% impedance. What is the %impedance at 100 MVA base and 34.5 kV base? A. 1.639% C. 5.74% B. 5.49% D. 6.56%

99. REE Board Exam September 2001, October

1998 The loads of a wye-connected transformer are: Ia = 10 cis (-30°) A Ib = 12 cis 215° A Ic = 15 cis 82° A What is the neutral current? A. 1.04 cis 72.8° A B. 0.92 cis 62.5° A C. 2.21 cis -30° A D. 3.11 cis 72.8° A

100.

REE Board Exam October 1998 Its primary function is to provide ground source A. zigzag transformer B. sectionalizer C. surge arrester D. tertiary transformer

101.

REE Board Exam October 1997 What is the main use of a zigzag transformer? A. to step down voltage B. to provide path to zero sequence currents C. to stabilize transformer sequence currents D. to step down current

102.

REE Board Exam October 1997 A power transformer is rated at 50,000 kVA, 34.5 kV/13.8 kV is connected wye-wye, what are the line currents at full-load? A. 525.2 amperes, 1,725 amperes B. 836.7 amperes, 2,092 amperes C. 483.1 amperes, 1,207.7 amperes D. 1,449.3 amperes, 3,623.2 amperes

103.

REE Board Exam October 1996 A three-phase transformer has a primary voltage rating of 34.5 kV, wye-connected with two 2.5% taps above and two 2.5% taps below the rated

voltage. The secondary winding is rated 460 volts and is delta connected. If it has 100 turns, what will be the effective number of turns in the primary winding if voltage tap us set at minus 2.5%? A. 7,133 turns C. 4,222 turns B. 7,312 turns D. 4,333 turns

104.

EE Board Exam April 1988 Two single-phase transformers each rated 75kVA are connected in V-V or open-delta, to serve a three-phase load of 120 kW at 0.80 power factor lagging. To prevent the overloading of transformers, determine the size of the capacitor in kVAR. A. 50 C. 30 B. 40 D. 20

105.

ECE Board Exam April 2001 A transformer composed of two or more coils which are wound around a non-metallic core A. Metallic core transformer B. Free core transformer C. Iron core transformer D. Air core transformer

106.

ECE Board Exam April 1999 An autotransformer contains how many coils? A. None C. Two B. One D. Three

107.

ECE Board Exam April 2000 Find unloaded output of a power supply having a transformer regulation of 11.10% and 900 V load output. A. 99.99 V C. 999.9 V B. 90 V D. 900 V

108.

ECE Board Exam April 1998 A transformer consists of the following? A. An inductance and resistance B. A parallel resonant circuit C. A capacitor and an inductor D. Two coils wound on a common core

109.

ECE Board Exam April 2001 How much is the secondary voltage of a power transformer whose parameters are as follows, primary voltage is 240 Volts, 140 turns in the primary and 7 turns in the secondary. A. 12 V C. 6 V B. 24 V D. 36 V

110.

ECE Board Exam April 2000 It is a transformer with two or more windings on a laminated iron core used to supply stepped up or

stepped down values of voltages to various circuits of an equipment. A. RF transformer B. Power transformer C. Autotransformer D. AF transformer

111.

ECE Board Exam April 1999 A transformer with 100 turns in the primary winding and 25 turns in the secondary winding is which of the following? A. A step-up transformer B. An isolation transformer C. A step-down transformer D. An autotransformer

112.

ECE Board Exam November 2001 Find the primary voltage of a power transformer under the following conditions 3 turns for primary, 6 turns for secondary and 120 volts for secondary voltage. A. 8 volts C. 6 volts B. 60 volts D. 12 volts

113.

ECE Board Exam April 1999 The current that flows in the primary of a transformer with the secondary open (or no load is attached). A. Source current B. Exciting current C. Load current D. Coupling current

114.

ECE Board Exam April 1996 What type of transformer that is used to protect technicians and operates from deadly electrical shock? A. Absorber transformer B. Step down transformer C. Step up transformer D. Isolation transformer

115.

ECE Board Exam November 2000 Components of the transformer which receives energy from the primary winding and delivers it to the load. A. Enclosure B. Core C. Secondary winding D. Primary winding

116.

ECE Board Exam November 1998 Which of the following does not contribute to losses in a transformer? A. Leakage resistance B. Self-inductance

C. Stray capacitance D. Iron losses

117.

ECE Board Exam April 2001 A transformer composed of two or more coils which are wound around a non-metallic core A. Metallic core transformer B. Free core transformer C. Air core transformer D. Iron core transformer

118.

A transformer will

work on ____. A. a.c. only B. d.c. only C. a.c. as well as d.c. D. none of the above

119.

The primary and secondary of a transformer are ____ coupled. A. B. C. D.

electrically magnetically electrically & magnetically none of the above

120.

A transformer is an efficient device because it ____. A. is a static device B. uses capacitive coupling C. uses inductive coupling D. uses electric coupling

121.

A transformer transfers electrical energy from primary to secondary usually with a change in ____. A. frequency C. voltage B. power D. time period

122.

The voltage per turn of the primary of a transformer is ____ the voltage per turn of the secondary. A. more than B. less than C. the same as D. none of these

123.

The winding of the transformer with greater number of turns will be ____. A. high-voltage winding B. low-voltage winding C. either high or low voltage winding D. none of the above

124.

A transformer does not possess ____ changing property.

A. impedance B. voltage

C. D.

current power

133. 125.

The iron-core is used to ____ of the transformer. A. increase the weight B. provide tight magnetic coupling C. reduce core losses D. none of the above

126.

If a transformer core has air gaps, then ____. A. reluctance of magnetic path is decreased B. hysteresis loss is decreased C. magnetizing current is greatly increased D. eddy current is increased

127.

The maximum flux produced in the core of a transformer is ____.

A. directly proportional to supply frequency B. inversely proportional to supply frequency C. inversely proportional to primary voltage D. none of the above

128.

The flux in the core of a single-phase transformer is ____. A. B. C. D.

purely alternating one purely rotating one partly alternating and partly rotating none of the above

129.

A transformer is so designed that primary and secondary have ____. A. high leakage flux B. large resistance C. tight magnetic coupling D. good electric coupling

130.

When the primary of a transformer is connected to a d.c. supply, ____. A. primary draws small current B. primary leakage reactance is increased C. core losses are increased D. primary may burn out

131.

An ideal transformer is one which ____. A. has no losses and leakage reactance B. does not work C. has the same number of primary and secondary turns D. none of the above

132.

The no-load primary current Io is about ____ of full-load primary current. A. 3 – 5% C. 30 – 40% B. 15 – 30% D. above 40%

The no-load p.f. of a transformer is small because ____

A. B. C. D.

iron losses component of Io is large magnetizing component of Io is large magnetizing component of Io is small none of the above

134.

The no-load input power to a transformer is practically equal to ____ loss in the transformer. A. iron B. copper C. eddy current D. none of these

135.

Two things which are same for primary and secondary of a transformer are ____. A. ampere-turns and voltage per turn B. resistance and leakage reactances C. currents and induced voltages D. none of the above

136.

A transformer operates poorly at very low frequencies because ____. A. permeability of core is increased B. magnetizing current is abnormally high C. primary reactance is too much increased D. none of the above

137.

If a power transformer is operated at very high frequencies, then ____. A. primary reactance is too much increased B. primary will draw large power C. core losses will be excessive D. none of the above

138.

The primary leakage flux links ____. A. primary winding only B. secondary winding only C. both primary and secondary winding D. none of the above

139.

The effect of leakage flux in a transformer is to ____.

A. B. C. D.

140.

increase copper losses decrease copper losses cause voltage drop in the windings none of the above

Leakage flux in a transformer occurs because ____. A. iron core has high permeability B. air is not good magnetic insulator C. applied voltage is sinusoidal D. transformer is not an efficient device

141.

The mutual flux in a transformer remains constant at all loads because ____. A. applied voltage and frequency are constant B. leakage flux is small C. iron-core is used D. losses are small

142.

Cores of large transformer are built up to nearly circular crosssection in order to reduce ____. A. leakage reactance B. iron losses C. eddy current loss D. copper loss

Unsolved problems

143.

REE Board Exam September 2002 A 20 kV/7.87 kV autotransformer has 200 A current in the common winding. What is the secondary line current? A. 143.52 A C. 56.48 A B. 200 A D. 329 A

144.

REE Board Exam April 2004 Instrument transformers are used in indicating and metering and with protective devices, they are used for A. measuring C. relaying B. detecting D. sensing

145.

REE Board Exam October 2000 The CT ratio and PT ratio used to protect a line are 240 and 2000 respectively. If the impedance of each line is 10 Ω, what is the relay impedance to protect the line from fault? A. 83.33 Ω C. 48,000 Ω B. 1.2 Ω D. 12 Ω

146.

REE Board Exam October 2000 Two single-phase transformers operate in parallel. Transformer 1 is rated 100 kVA with 4% reactance while transformer 2 is rated 75 kVA with 3% reactance. The transformers have secondary voltages of 220 volts and same voltage ratio. If the common load is 125 kVA at 80% p.f. lagging, determine the kVA sharing of each transformer. A. 71.43 kVa and 53.57 kVA B. 75 kVa and 50 kVA C. 62.5 kVA and 62.5 kVA D. 82.5 kVA and 42.5 kVA

147.

April 2004

REE Board Exam

A transformer rated 2,000 kVA, 34,500/240 volts has 5.75% impedance. What is the per unit impedance? A. 0.0635 C. 0.0575 B. 0.0656 D. 34.2

148.

REE Board Exam April 2001 A 69 kV/13.8 kV, 7.5 MVA transformer has 8% impedance. What is its impedance at 100 MVA base? A. 0.6% C. 800% B. 8% D. 106.7%

149.

REE Board Exam October 1997 A 3-phase transformer is rated 30 MVA, 115Δ/13.8Y kV and 2% impedance, the resistance is negligible compared with the reactance. Determine the percent (%) voltage drop at the secondary at 80% load and at 75% power factor. A. 4.1% C. 1.05% B. 2.26% D. 3.15%

150.

Which of the following connection is best suited for 3-phase, 4wire service? A. – C. –Y B. Y – Y D. Y –

151.

In a three-phase Y – Y transformer connection, neutral is fundamental to the A. suppression of harmonies B. passage of unbalanced currents due to loads C. provision of dual electric service D. balancing of phase voltages with respect to line voltages

152.

As compared to – bank, the capacity of the V – V bank of transformers is ____ percent. A. 57.7 C. 50 B. 66.7 D. 86.6

153.

If three transformers in a – are delivering their rated load and one transformer is removed, then overload and each of the remaining transformer is ____ percent. A. 66.7 C. 73.2 B. 173.2 D. 58

154.

When a V – V system is converted into a – system, increase in a capacity of the system is ____ percent.

A. 86.6 B. 66.7

C. D.

73.2 50

155.

For supplying is a balanced 3- load of 40-kVA, rating of each transformer in V – V bank, should be nearly ____ kVA. A. 20 C. 34.6 B. 23 D. 25

156.

When a closedbank is converted into an open- bank. Each of the two remaining transformer supplies ____ percent of the original load. A. 66.7 C. 50 B. 57.7 D. 73.2

157.

If the load p.f. is 0.886, then the average p.f of the V-bank is

A. 0.886 B. 0.75

C. D.

0.51 0.65

158.

A T – T connection has a higher ratio of utilization that a V – V connection only when A. identical transformers are used B. load power factor is leading C. load power factor is unity D. non-identical transformer are used

159.

The biggest advantage of T – T connection over the V – V connection for 3-phase power transformation is that it provides A. a set of balanced voltages under load B. a true 3-phase, 4 wire system C. a higher ratio of utilization D. more voltages

160.

One of the following statements concerning parallel operation of transformer, the one which is not correct is A. transformers have equal voltage ratings B. transformers must have same ratio of transformation C. transformers must be operated at the same frequency D. transformers must have equal

161.

Statement. An auto-transformer is more efficient in transferring energy from primary to secondary circuit. Reason. Because it does so both inductively and conductively. Key A. statement is false, reason is correct and relevant B. statement is correct, reason is correct but irrelevant C. both statement and reason are correct and are connected to each other as cause and effect D. both statement and reason are false

162.

Out of the following given choices for polyphase transformer connections which one will you select for threeto-two phase conversion? A. Scott B. star/star C. double Scott D. star/double-delta

163.

AT–T transformer cannot be paralleled with ____ transformer. A. V – V C. Y – Y B. Y – D. –

164.

Instrument transformers are used in a.c. circuits for extending the range of A. ammeters B. voltmeters C. wattmeters D. all of the above

165.

Before removing the ammeter from the current transformer, its secondary must be short-circuited in order to avoid A. excessive heating of the core B. high secondary emf C. increase in iron losses D. all of the above

166.

transformer is made of A. silicon steel B. annealed copper C. seasoned copper D. aluminum

The core of a

167.

The core of a transformer is assembled with laminated sheets to reduce A. hysteresis loss B. eddy-current loss C. magnetic noise D. magnetizing current

168.

The use of grainoriented laminated sheets in building a transformer core A. reduces magnetizing current B. reduces eddy-current loss in the core C. reduces hysteresis loss D. increases the no-load power factor angle

169.

Cooling of transformers is necessary to A. increase the efficiency B. dissipate the heat generated in the windings C. reduce the losses

D. reduce humming

170.

The emf induced in the windings of a transformer will A. lag the core flux by 90 degrees B. be in-phase with the core flux C. be out-of-phase with the core flux D. be independent of the core flux

171.

A 100-kVA, 1100/440-V, 50-Hz single-phase transformer has 100 turns on the secondary winding. The number of turns in the primary will be A. 550 C. 2750 B. 275 D. 5500

172.

The emf induced in the secondary winding of a 50-Hz single-phase transformer having 100 turns on its secondary is 222 V. The maximum flux density in the core is 2 0.1 Wb/m . The cross-sectional area of the core is 2 2 A. 0.1 m C. 1 m 2 2 B. 0.01 m D. 0.001 m

173.

The copper-loss and core-loss of a transformer of a transformer at various loads are as shown below. At what load will the efficiency of the transformer be a maximum? A. Load = 50 kVA, Core loss = 320 W, Copper Loss = 500 W B. Load = 40 kVA, Core loss = 320 W, Copper Loss = 320 W C. Load = 30 kVA, Core loss = 320 W, Copper Loss = 180 W D. Load = 20 kVA, Core loss = 320 W, Copper Loss = 80 W

174.

A transformer when supplying a load maintained 11 kV across terminals. When the load was switched off, the terminal voltage became 11550 V. What is the voltage regulation of the load? A. 11.55% C. 5% B. 5.5% D. 55%

175.

Power lost in the open-circuit and shiort0-circuit tests on a transformer gives approximately an account of the following losses: A. Core-losses and copper losses respectively B. Copper-losses and core-losses respectively C. Eddy-current loss and hysteresis loss respectively D. Hysteresis loss and eddy-current loss respectively

176.

Maximum efficiency of a transformer occurs when A. hysteresis loss and eddy current loss are minimum

B. the sum of hysteresis loss and eddy current loss is equal to copper loss in the windings C. power factor of the load is leading D. hysteresis loss is equal to eddy-current loss

177.

The all-day efficiency of a transformer is the ratio of

A. B. C. D.

kWh output and kWh input per day kWh output and kWh input in a day output power and input power input power and output power

178.

The ratio of the primary to secondary voltage of a transformer is 2:1. The saving in terms of weight of copper required if an autotransformer is used instead of a two-winding transformer will be A. 50% C. 66.67% B. 33.33% D. 97%

179.

For satisfactory parallel operation of two single-phase transformers a number of conditions are to be fulfilled. A number of conditions are written below. Indicate which of these is not required to be fulfilled. A. kVA ratings of the two transformers should be equal B. Transformers should be properly connected with regard to their polarity C. Voltage ratings of the primary windings should be suitable for supply system voltage and frequency. The turns ratio of the transformer should be equal. D. The percentage impedance of the two transformers should be equal.

180.

An additional condition for parallel operation of three-phase transformers over single-phase transformers is that A. the transformers should belong to the same vector group B. ratios of winding resistances to resistances for the transformers should be equal C. the transformers should have the same kva ratings D. the transformers should not belong to the same vector group

181.

The maximum flux density in the core of a 3000/240-V, 50 Hz singlephase distribution transformer is 1.25 Tesla. If the voltage induced per turn is 8 volts, determine the 2 cross-sectional core in cm . A. 292 B. 375 C. 288 D. 362

182.

The net cross section of a transformer is 20 square inches. The maximum flux density is 11 kilogausses. The frequency is 60 Hz. If there are 1,000 turns in the primary, determine the primary induced emf. A. B. C. D.

3780 V 3945 V 3590 V 3850 V

183.

_____current are wasteful currents which flow in cores of transformers and produced heat. A. Residual C. Sneak B. Eddy D. Magnetizing

184.

Calculate the total magnetic flux in a 60-cycle transformer in which the induced emf per turn of the winding is equal to 2 V. A. 7.50 mWb B. 7.02 mWb C. 7.60 mWb D. 7.42 mWb

185.

The low side of a certain welding transformer has 2 turns and delivers 300 A. If the high voltage side has 30 turns, how much current lows through it? A. B. C. D.

10 A 12 A 20 A 15 A

static induction dynamic induction mutual induction self-induction

used to change the value of A. frequency B. voltage

A transformer operates A. always at unity power factor B. has its own power factor C. at power factor below a particular value D. at power factor depending on the power factor of the load

191.

The laminations are made from A. low carbon steel B. silicon steel sheet C. nickel alloy steel stampings D. chrome steel sheets

192.

The steel for construction of transformer core is made so as to made A. low permeability and high hysteresis B. low permeability and low hysteresis C. high permeability and low hysteresis D. high permeability and high hysteresis

193.

The special silicon steel is used for laminations because A. B. C. D.

hysteresis losses are reduced eddy current losses are reduced both of the above losses are reduced none of the above

A. B. C. D.

electric circuit magnetic circuit winding wire gauge none of the above

195. Transformer is C. D.

power power factor

188.

The path of the magnetic flux in a transformer has A, low reactance B. high reactance C. high conductivity D. low resistance

189.

190.

What is common in the two windings of a transformer?

The principle of working of working of the transformer is

187.

electrically electromagnetically

194.

186.

A. B. C. D.

C. D.

Electric power is transformed from one coil to the other in a transformer A. physically B. magnetically

The main function of the iron core in a transformer is to A. provide strength to the windings B. to decrease the hysteresis losses C. decrease the reluctance of the magnetic path D. reduce eddy current losses

196.

Preferably, the resistance between the primary and the secondary of a transformer should be A. as low as possible B. as high as possible C. low or high depending upon whether it is step up or step down respectively D. high or low depending upon whether it is step up or step down respectively

197.

assumptions do not include

Ideal transformer

A. B. C. D.

zero reactance of windings zero resistance of windings no leakage flux no saturation of flux

198.

The reactance of a transformer is determined by its A. leakage flux B. common core flux C. size of core D. permeability of the material of the core

199.

The efficiency of the transformer is normally in the range of A. B. C. D.

50% to 70% 60% to 75% 80% to 90% 90% to 98%

The resistance of the low voltage winding of a transformer

B. C. D.

is equal to the resistance of its high voltage side is more than the resistance of its high voltage side is less than the resistance of its high voltage side either B or C

201.

The basic property of the transformer is that is changes the voltage level of an a.c. signal A. without changing the power B. without changing its shape C. without changing its frequency D. without changing power, frequency or shape

202.

The induced emf in the transformer secondary will depend upon A. B. C. D.

maximum flux in core and frequency only frequency, flux and number of turns in the secondary frequency of the supply only number of turns in the secondary only

203.

At every instant the direction of secondary current in a transformer must be such as to oppose any change in flux. This is in accordance with A. Faraday’s law B. Joule’s law C. Lenz’s law D. Coulomb’s law

204.

transformation ratio is A. E1/E2 B. N1/N2

The voltage

E2/E1 N2/N1

205.

The magnitude of the mutual flux in a transformer is A. low at low loads and high at high loads B. high at low loads and low at high loads C. same at all levels D. varies at low loads and constant at high loads

206.

The concentric windings are used in core type transformer with A. B. C. D.

200.

A.

C. D.

LT winding placed next to core HT winding placed next to core LT winding on the outer side HT winding on the outer side

207.

Cross-over windings are used for A. high voltage winding of small rating transformers B. low voltage winding of small rating transformers C. high voltage winding of large rating transformers D. none of the above

208.

In an ideal transformer on no-load, the primary applied voltage is balanced by A. the secondary voltage B. the drop across resistance and reactances C. the secondary induced emf D. the primary induced emf

209.

Transformers are

rated in A. kW B. kV C. kWh D. kVA

210.

What type of core is used for a high frequency transformer? A. B. C. D.

211.

air core closed iron core aluminum core open iron core

If in a transformer the secondary winding are doubled and at the same time the primary voltage is reduced to half, then secondary voltage will A. be halved B. not change C. be four times as much D. be reduced to a quarter

B. 375

212.

The no-load current of a transformer in terms of full-load current is usually A. 1 to 3% B. 3 to 9% C. 9 to 12% D. 12 to 20%

213.

Which is the common method of cooling a power transformer? A. B. C. D.

362

220.

The net cross section of a transformer is 20 square inches. The maximum flux density is 11 kilogausses. The frequency is 60 Hz. If there are 1,000 turns in the primary, determine the primary induced emf.

A. 3780 V B. 3945 V

C. D.

3590 V 3850 V

221.

The low side of a certain welding transformer has 2 turns and delivers 300 A. If the high voltage side has 30 turns, how much current flows through it?

air cooling air blast cooling oil cooling natural cooling

214.

The use of higher flux density in transformer design A. decreases the weight per kVA B. increases the weight per kVA C. increases the weight per kW D. decreases the weight per kW

215.

What is the efficiency of transformer compared with that of electrical motors of the same power? A. much smaller B. somewhat smaller C. much higher D. about the same

216.

The transformer oil used in transformer provide A. cooling and lubrication B. insulation and lubrication C. insulation and cooling D. insulation, cooling and lubrication

217.

In any transformer the voltage per turn in primary and secondary remains A. always different B. always same C. always in the ratio of a D. sometime same

218.

In transformer the purpose of breather is to A. extract moisture of the air B. to take insulating oil from conservator C. to provide cooling to the winding D. to provide insulation to the winding

219.

The maximum flux density in the core of a 3000/240-V, 50 Hz singlephase distribution transformer is 1.25 Tesla. If the voltage induced per turn is 8 volts, determine the 2 cross-sectional area of the core in cm .

A. 292

D.

C.

288

A. 10 A B. 12 A

C. D.

20 A 15 A

222.

The eddy current and hysteresis losses of a 4,400-V, 60-Hz singlephase distribution transformer are 280 and 560 watts, respectively. Determine the core loss when the transformer is connected across a 4,600-volt, 50-Hz source. A. 977 W C. 994 W B. 983 W D. 967 W

223.

A 20-kVA, 500-V load is to be supplied by an ideal step-up autotransformer from a 400-V source. Find the current in the common winding. A. 30 A B. 40 A C. 50 A D. none of these

224.

Calculate the total magnetic flux in a 60-cycle transformer in which the induced emf per turn of the winding is equal to 2 V. A. 7.50 mWb C. 7.50 mWb B. 7.02 mWb D. 7.42 mWb

225.

The current densities in the primary and secondary windings of a step don transformer are 1250 and 1500 A per square inch, respectively. The ratio of transformation is 10:1 and the mean length per turn of the primary is 15% greater than that of the secondary. If the primary winding resistance is 20 ohms, calculate the resistance of the secondary windings. A. 0.202 ohm C. 0.209 ohm B. 0.219 ohm D. 0.220 ohm

226.

A 10-kVA, 2000/400 V, single-phase transformer has primary and secondary winding impedances of 5.0 + j10 and 0.2 + j0.45 ohms, respectively. Calculate the voltage across the secondary

terminals at full load, 80% power actor lagging, when the primary is supplied with 2,000 volts. A. 400 V C. 385 V B. 394 V D. 379 V

227.

An open delta bank, consisting of two-single phase transformers is operating with balanced three-phase load of 50 kVA, 440 V at 0.8 p lagging and a single phase load of 10 kW resistive connected across c-a leg. Determine the minimum ratings of the transformers. Assume a phase sequence of a-bc. A. 38.80 kVA, 34.06 kVA B. 34.32 kVA, 33.40 kVA C. 35.48 kVA, 36.34 kVA D. 30.83 kVA, 39.52 kVA

228.

An open delta, three-phase transformer consisting of two singlephase transformers is operating with a balanced three-phase load of 50 kVA, 440-V at a lagging pf of 0.8. Determine the operating pf of each transformer. A. 0.393 lagging, 0.993 lagging B. 0.383 lagging, 0.887 lagging C. 0.383 lagging, 0.993 lagging D. 0.393 lagging, 0.887 lagging

229.

Which of the following given below is the eddy current loss not proportional to A. square of the thickness B. square of the frequency C. square of the load current of the laminated core D. square of the flux density

230.

A transformer will have negative voltage regulation when its load is

A. capacitive B. resistive

C. D.

inductive both B and C

231.

EE Board Exam April 1990 Copper loss of a transformer are determined by means of a A. polarity test B. short circuit test C. open circuit test D. inductive circuit test

232.

The secondary of a transformer is connected wye-grounded. The voltage from line to neutral is 132 volts. If the load current is 40 amperes, calculate the load in kVA.

A. 5.28 kVA B. 9.15 kVA

C. D.

20.11 kVA 15.84 kVA

233.

The current that flows through the common winding of a 20kV/7.97 kV autotransformer is 200 amperes. Determine the secondary current. A. 330 amperes C. 200 amperes B. 220 amperes D. 150 amperes

234.

The secondary current of a 20 kV/7.97-kV autotransformer is 100 amperes. Determine the current that flows through the common winding. A. 60 amperes C. 70 amperes B. 100 amperes D. 50 amperes

235.

Two transformer are connected in parallel supplying a load of 2,500 kVA. The first transformer has an impedance of 3% and a rating of 1,250 kVA while the second has an impedance of 3% and a rating of 1,500 kVA. Calculate the load shared by the second transformer. A. 1453.19 kVA C. 1363.64 kVA B. 1792.51 kVA D. 1227.53 kVA

236.

A transformer has a core loss of 64 W and a copper loss of 144 W when it is carrying 20% overload current. The load at which this transformer will operate at the maximum efficiency is ____ percent.

A. 80 B. 66

C. D.

237.

120 75

A 100 ohms load has a pf of 0.8. A 1 k generator supplies power to the load through a transformer. Best matching will be produced when transformer ratio is approximately A. 3:1 C. 10:1 B. 8:1 D. 2:1

238. A. B.

C. D.

239. A. B.

C. D.

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