Sunday, September 13, 2020

Chapter 7: Transformer- Single and Three phase-Page 8

 Chapter 7: Transformer- Single and Three phase

71. an a 50 Hz transformer be used for 25 Hz, if the input voltage is maintained constant at the rated vale corresponding to 50 Hz?
A. Yes. Since the voltage is constant, current levels will not changed.
B. No. Flux will be doubled which will drive the care to excessive saturation
C. No. Owing to decreased reactance of transformer, input current will be doubled at load
D. Yes. At constant voltage, insulation will not be overstressed.

... Answer is (B)



72. Transformer B : primary Δ; secondary Δ In a transformer fed from a fundamental frequency voltage source, the source of harmonics is the
A. overload
B. poor insulation
C. iron loss
D. saturation of core

... Answer is (B)


73. A 40 kVA transformer has a core loss of 400 W and a full-load copper loss of 800 W The proportion of full-load at maximum efficiency is

... Answer is (C)



74. A single-phase transformer has a rating of 15 kVA, 600/120 V. It is reconnected as an auto-transformer to supply at 720 V from a 600V primary source. The maximum load it can supply is
A. 90 kVA
B. 18 kVA
C. 15 kVA
D. 12 kVA

  ... Answer is (A)


75. A single-Phase induction regulator is a constant input transformer to obtain smooth variation of the output voltage by varying the
A. ratio of turns between primary and secondary windings
B. frequency
C. flux density in the core
D. angle between the magnetic axes of the primary and secondary windings.
Answer is (D)


76. An auto-transformer having a transformation ratio of 0.8 supplies a load of 10 kW. The power transforred inductively from the primary to the secondary is
A. 10 kW
B. 8 kW
C. 2 kW
D. zero

  ... Answer is (C)



77. The secondary of a current transformer is always short circuited under operating conditions because
A. it protects the primary circuits
B. it is safe to human beings
C. it avoids core saturation voltage induction
D. none of the above

... Answer is (C)


78. While conducting short-circuit test on a transformer the following side is short-circuited
A. H.V. side
B. L.V. side
C. primary side
D. none of the above

... Answer is (B)


79. What is a typical use of an auto transformer?
A. Isolating transformer
B. Toy transformer
C. Control Transformer
D. Variable transformer

  ... Answer is (D)


80.  The insulating properties of transformer oil are adversely affected by the presence of
A. oxygen
B. copper
C. moisture
D. ozone

  ... Answer is (C)



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