Tuesday, September 1, 2020

Chapter 7: Transformer- Single and Three phase- Page 1

 Chapter 7: Transformer- Single and Three phase

1. A transformer transforms
A. frequency
B. only voltage
C. only current
D. power

... Answer is (D)



2. An ideal transformer is one which
A. has a common core for its primary and secondary windings
B. has no losses and magnetic leakage
C. has core of stainless steel and windings of pure copper metal
D. has interleaved primary and secondary windings

... Answer is (B)


3. Coupling between the two windings of a transformer can be increased by
A. increasing the number of turns in the two windings
B. increasing the primary voltage
C. interleaving the windings on a commoncore of low reluctance
D. reducing the insulation of the two windings

... Answer is (C)



4. In an ideal transformer, the no-load primary current Io
A. lags behind VI by 90°
B. is in phase with VI
C. leads VI by 90°
D. lags V1 by an angle lying between 0° and 90

  ... Answer is (A)


5. The primary and secondary induced emfs E1 and E2 in a two-winding transformer are always
A. equal in magnitude
B. antiphase with each other
C. in-phase with each other determined by load on transformer secondary
D. None of the above
Answer is (C)


6. Transformer cores are built up from laminations rather than from solid metal so that
A. oil penetrates the core more easily
B. eddy current loss is reduced
C. less insulation is required for the windings
D. turn ratio is higher than voltage ratio

  ... Answer is (B)



7. Iron loss in an actual transformer remains practically constant from no-load to full load because
A. core flux remains practically constant
B. primary voltage remains constant
C. value of transformation ratio remains constant
D. premeability of transformer core remains constant

... Answer is (A)


8. The primary reason why open-circuit test is performed on the lowest-voltage winding of 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. has become a universal custom

... Answer is (A)


9. During short-circuit test, the iron loss of a transformer is negligible because
A. the entire input is just sufficient to meet Cu losses only
B. voltage applied across the h.v. side is a small fraction of the rated voltage and so is the flux.
C. iron core becomes fully saturated
D. supply frequency is held constant

  ... Answer is (B)


10.  The commercial efficiency of a transformer while on open-circuit is
A. zero
B. maximum
C. indeterminate
D. incalculable

  ... Answer is (A)



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