78. Can 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.
79. Two 3-phase transformers are to be connected for parallel operation. Which one of the following arrangements is impossible?
A. Transformer A: primary Y ; secondary Y Transformer B : primary A ; secondary A
B. Transformer A: Primary A ; secondary Y Transformer B : primary A ; secondary A
C. Transformer A: primary Y ; secondary A Transformer B : primary A ; secondary A
D. Transformer A : primary A ; secondary A
80. Transformer B : primary A; secondary A In a transformer fed from a fundamental frequency voltage source, the source of harmonics is the
B. poor insulation
C. iron loss
D. saturation of core
81. 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
82. 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
83. Three 3-phase transformers each rates at 75 MVA, 132 kV/11 kV, have the following different methods of cooling :1. Self oil cooled.2. Forced oil cooled3. Forced air cooled The correct sequence in ascending order in terms of the weights of these transformers is
A. 1, 2, 3
B. 2, 3, 1
C. 3, 1, 2
D. 3, 2, 1
84. 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
C. flux density in the core
D. angle between the magnetic axes of the primary and secondary windings.
Page 12 of 45