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# MCQ in Transformers Part 2 | REE Board Exam

This is the Multiple Choice Questions Part 2 of the Series in Transformers as one of the Electrical Engineering topic. In Preparation for the REE Board Exam make sure to expose yourself and familiarize in each and every questions compiled here taken from various sources including but not limited to past Board Exam Questions in Electrical Engineering field, Electrical Engineering Books, Journals and other Electrical Engineering References.

#### Continue Practice Exam Test Questions Part 2 of the Series

Choose the letter of the best answer in each questions.

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

Solution:

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

B. 640

C. 1.25

D. 100

Solution:

53. In operating a 400 Hz transformer at 50 Hz

A. only voltage is reduced in the same proportion as the frequency

B. only kVA rating is reduced in the same proportion as the frequency

C. both voltage and kVA rating are reduced in the same proportion as the frequency

D. none of these

Solution:

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

B. 0.4

C. 0.25

D. 0.04

Solution:

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

Solution:

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

Solution:

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

Solution:

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

B. 200

C. 50

D. 141.4

Solution:

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

B. 6400

C. 375

D. 429

Solution:

60. The ordinary efficiency of a given transformer is maximum when

A. it runs at half full-load

C. its Cu loss equals iron loss

Solution:

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

B. 25

C. 5

D. 400

Solution:

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

B. 133.3

C. 75

D. 177.7

Solution:

63. The all-day efficiency of a transformer depends primarily on

A. its copper loss

D. both B and C

Solution:

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

Solution:

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

Solution:

66. The saving in Cu achieved by converting 2-winding transformer into an autotransformer is determined by

A. voltage transformation ratio

C. magnetic quality of core material

D. size of the transformer core

Solution:

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

B. 2.4

C. 1.5

D. 0.27

Solution:

68. The essential condition for parallel operation of two 1-ฯ transformer is that they should have the same

A. polarity

B. kVA rating

C. voltage

D. percentage impedance

Solution:

69. If the impedance triangles of two transformers operating in parallel are not identical in shape and size, the two transformers will

B. get heated unequally

C. have a circulatory secondary current even when unloaded

D. run with different power factors

Solution:

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

B. 66

C. 33

D. 25

Solution:

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

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

B. 3.09

C. 2.17

D. 2.28

Solution:

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

Solution:

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

B. 9.24 kVA

C. 16 kVA

D. 8.25 kVA

Solution:

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

B. 24

C. 23

D. 20

Solution:

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

B. 49.5 kW

C. 53.6 kW

D. 55.2 kW

Solution:

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

Solution:

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

B. 112 kW

C. 130 kW

D. 114 kW

Solution:

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

B. 170.91

C. 160.43

D. 185.13

Solution:

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

Solution:

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

Solution:

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%

B. 84.6%

C. 80.5%

D. 79.4%

Solution:

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

B. 10 kVA

C. 20 kVA

D. 25 kVA

Solution:

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 Scott-connected 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

Solution:

84. EE Board Exam April 1985

A balanced 3-phase transformer bank has delta-connected 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

B. 115 A

C. 150 A

D. 175 A

Solution:

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 ฮ โ Y?

A. 94

B. 98

C. 96

D. 97

Solution:

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

B. 135.2 kV

C. 112.5 kV

D. 132 kV

Solution:

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

B. 1725 A

C. 1449 A

D. 1208 A

Solution:

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

B. 23.9

C. 15

D. 13.8

Solution:

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

B. 220.10 kV

C. 223.15 kV

D. 225.25 kV

Solution:

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

Solution:

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

B. 76, 99

C. 80, 95

D. 75, 100

Solution:

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

Solution:

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

Solution:

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

Solution:

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

Solution:

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

B. 100

C. 200

D. 80

Solution:

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

B. 150 MW

C. 19.2 MW

D. 15 MW

Solution:

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%

B. 5.49%

C. 5.74%

D. 6.56%

Solution:

99. REE Board Exam September 2001, October 1998

The loads of a wye-connected transformer are:

Ia = 10 cis (-30ยฐ) A

Ib = 12 cis 215ยฐ A

Ic = 15 cis 82ยฐ A

What is the neutral current?

A. 1.04 cis 72.8ยฐ A

B. 0.92 cis 62.5ยฐ A

C. 2.21 cis -30ยฐ A

D. 3.11 cis 72.8ยฐ A

Solution:

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

Solution:

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