Sunday, September 13, 2020

Chapter 7: Transformer- Single and Three phase-Page 9

 Chapter 7: Transformer- Single and Three phase

81. An auto-transformer is preferred in cases where
A. load is fluctuating
B. power factor is varying
C. ratio of transformation is low
D. ratio of transformation is high

... Answer is (C)



82. Buchholz relay works on the principle of
A. production of eddy currents whenever load changes are rapid
B. ionization of transformers oil
C. magnetic hum due to sudden variation in load
D. generation of gas due to breakdown of the insulation.

... Answer is (D)


83. Which of the following transformers is smallest?
A. 1 kVA, 50 Hz
B. 1 kVA, 200 Hz
C. 1 kVA, 400 Hz 

D. I kVA, 600 Hz

... Answer is (D)



84. In parallel operation of two single phase transformers, if the impedance triangles of the transformers are not identical in shape and size.
A. power factors at which the transformers operate will be same but different from power factor of common load
B. power factors at which the transformers operate and power factor of common load all will be same.
C. power factor of one transformer and power factor of common load will be same.
D. power factors at which the transfomers operate will be different from one another and again these will be different from power factor of common load.

  ... Answer is (D)


85. A 100 kVA transformer has copper loss of 1000 W at full load and iron loss of 1000 watts. At half the full load and 0.8 Power factor (lagging), the efficiency of the transformer will be nearly
Answer is (B)


86. Eddy current losses in a transformer may be reduced by
A. decreasing the thickness of laminations
B. increasing the thickness of laminations
C. decreasing the air gap in the magnetic circuit.
D. using wire of higher gauge for winding the transformer.

  ... Answer is (A)



87. The magnetic flux in a transformer follows a path of
A. high reluctance
B. low reluctance
C. high conductivity
D. low conductivity

... Answer is (B)


88. In a transformer, the primary and the secondary voltage are
A. 60° out of phase
B. 90
° out of phase
C. 180° out of phase
D. always in phase

... Answer is (C)


89. Steel used in the construction of transformer core has loss
A. low permeability and low hysteresis
B. low permeability and high hysteresis loss
C. high permeability and high-hysteresis loss
D. high permeability and low hysteresis loss.

  ... Answer is (D)


90.  The iron losses of a transformer may be calculated by knowing the weight of
A. yokes alone
B. cores alone
C. cores and yokes
D. copper winding alone.

  ... Answer is (C)



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