288.  A transformer designed for operation on 60 Hz supply is worked on 50 Hz supply system without changing its voltage and current ratings. When compared with full load efficiency at 60 Hz, the transformer efficiency on full load at 50 Hz will
A. increase marginally
B. increase by a factor of 1.2
C. remain unaltered
D. decrease marginally

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289.  A transformer has a core loss of 200 W and a full load copper loss of 800W The maximum efficiency ofthe transformer will occur at
A. 0.5 times full load current
B. 0.6 times full load current
C. 0.7 times full load current
D. 0.8 times full load current

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290.  Open-circuit (oc) and short-circuit (sc) tests on an AC generator are conducted under which of the following conditions? 1. oc test at nominal flux and sc test at nominal current.2. oc test at reduced flux and sc test at reduced current.3. oc test at zero armature current and sc test at reduced flux.4. oc test at zero current and sc test at nominal flux. Select the correct answer using the codes given below :Codes :
A. 1 and 3
B. 2 and 3
C. 1 and 4
D. 2 and 4

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291. The given figure represents a trans former with two windings 1 and 2 wound on the core as shown. By applying a volt age Vi across winding 1, a voltage V2 is induced across winding 2. In an idealised condition V2 would lag Vi by ?> V2
A. Zero degrees
B. 90 degrees
C. 180 degrees
D. 270 degrees

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292.  The leakage flux in a transformer depends upon the
A. applied voltage
B. frequency
C. load current
D. mutual flux

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293.  The voltage regulation of a large transformer is mainly influenced by
A. no-load current and load power factor
B. winding resistances and load power factor
C. leakage fluxes and load power factor
D. windingresistances and core loss

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294.  The series equivalent impedance of a single-phase transformer referred to its secondary side is Ze2 = Re2 + jXe2. The voltage regulation of this transformer will be zero when the power factor of the load is Xe2
A. unity
B. 7 I leading
C. Ze2 lagging
D. Ze2 leading

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