274. The percentage resistance and percentage reactance of a 10 kVA, 400 V/ 200 V, 3-phase transformer are 2% and 10% respectively. If the constant losses in the machine are 1%, the maximum possible percentage efficiency of the transformer is
A.98.32
B.97.25
C.96.85
D.96.12

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275.  A 400 V/100 V, 10 kVA two-winding transformer is reconnected as an auto-transformer across a suitable voltage source. The maximum rating of such an arrangement could be
A. 50 kVA
B. 15 kVA
C. 12.5 kVA
D. 8.75 kVA

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276.  In a constant voltage transformer (CVT), the output voltage remains constant due to
A. capacitor
B. input inductor
C. saturation
D. tapped windings

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277. 1n the protection of transformers, harmonic restraint is used to guard against
A. magnetizing inrush current
B. unbalanced operation
C. lightning
D. switching over-voltages

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278. The main reason for connecting a pulse transformer at the output stage of a thyristor triggering circuit is to
A. amplify the power of the triggering pulse
B. provide electrical isolation
C. reduce the turn on time ofthe thyristor
D. avoid spurious triggering of the thyristor due to noise

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279.  A 3-phase transformer has rating of 20 MVA, 220 kV (star) ? 33 kV (delta) with leakage reactance of 12%. The transformer reactance (in ohms) referred to each phase of the L.V. delta-connected side is
A.23.5
B.19.6
C.18.5
D.8.7

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280.  A lkVA, 230 V/100V, single phase, 50 Hz transformer having negligible winding resistance and leakage inductance is operating under saturation, while 250 V, 50 Hz sinusoidal supply is connected to the high voltage winding. A resistive load is connected to the low voltage winding which draws rated current. Which one of the following quantities will not be sinusoidal?
A. Voltage induced across the low voltage winding
B. Core flux
C. Load current
D. Current drawn from the source

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