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Objective questions and answers or MCQ of Electrical Machines page-6


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101. The efficiency of the transformer is independent of power factor.

a) true
b) false

Ans: (b)

102. The condition for the maximum efficiency of the transformer is that

a) Copper losses are half of the iron losses
b) Copper losses are equal to iron losses
c) Copper losses are negligible in comparison to iron losses
d) Iron losses are zero

Ans: (b)

103. If the iron loss and full load copper losses are given then the load at which two losses would be equal ( i.e. corresponding to maximum efficiency ) is given by

a) full load × (iron loss)/(f.l.cu-loss)
b) full load × ((iron loss)2)/(f.l.cu-loss)
c)full load × √((iron loss)/(f.l.cu-loss))
d) full load × √((f.l.cu-loss)/(iron loss))

Ans: (c)

104. The distribution transformers are designed to keep core losses minimum and copper losses are relatively less important because

a) The primary of such transformers is energized for all twenty-four hours and core losses occur throughout the day whereas copper losses will occur only when secondary is supplying load
b) Core losses are always more than copper losses
c) Core losses may destroy the insulation
d) Core losses will heat up the oil of the transformer rapidly

Ans: (a)

105. The tapping in the transformer are always provided in the low voltage side

a) true
b)false

Ans: (b)

106.Enumerate the conditions for successful parallel operation of the single phase transformers

a) The percentage impedance should be equal
b) The transformers should be properly connected with regard in polarity
c) The primary winding of the transformers should be suitable for the supply system voltage and frequency
d) The voltage ratings of transformers of primary and secondary winding should be identical

Ans: (a)

107. What will happen if the transformers working in parallel are not connected with regard to polarity?

a) Incorrect polarity will result in dead short-circuit
b) The transformers will not share load in proportion to their KVA rating
c) The power factors of two transformers will be different from the power factor of common load
d) None of the above

Ans: (a)

108. What will happen if the percentage impedances of the two transformers working in parallel are different?

a) Parallel operation will not be possible
b) Parallel operation will still be possible, but the power factors at which the two
transformers operate will be different from the power factor of the common load
c) Transformers will be over heated
d) Power factors of both of the transformers will be same

Ans: (b)

109. Which of the following connections is most suitable and economical for small, high-voltage transformers?

a) Delta-delta connection
b) Star-star connection
c)Star-delta connection
d)Delta-star connection

Ans: (b)

110. Delta-delta connection is economical for large, low-voltage transformers in which insulation problems is not urgent.

a) true
b) false

Ans: (a)

111. The average power factor at which open-delta bank of single phase transformers operates is less than that of load.

a) true
b) false

Ans: (a)

112. Scott connection is used

a) To accomplish three phase to three phase transformation only
b) To accomplish three phase to two phase transformation only
c) To accomplish three phase to three phase and three phase to two phase transformation
d) None of the above

Ans: (c)

113. Under balanced load conditions, main transformer rating in the Scott connection is

a) 10% greater than teaser transformer
b) 15% greater than teaser transformer
c) 57.7% greater than teaser transformer
d) 66.6% greater than teaser transformer

Ans: (b)

114. If the load is balanced on one side of the transformer in the Scott connection, the load is balanced on the other side as well.

a) true
b) false

Ans: (a)

115. If K is the transformation ratio of main transformer in the Scott connection then the transformation ratio of the teaser will be

a) K/√3
b) √(3/2K)
c) 2K/√3
d) √(K/2)

Ans: (c)


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