Transformers

71. The chemical used in breather is

1. asbestos fiber
2. silica sand
3. sodium chloride
4. silica gel

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72. The core used in high frequency transformer is usually

1. copper core
2. cost iron core
3. air core
4. mild steel core

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73. The degree of mechanical vibrations produced by the laminations of a transformer depends on

1. tightness of clamping
2. gauge of laminations
3. size of laminations
4. all of the above

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74. The dielectric strength of transformer oil is expected to be

1. lkV
2. 33 kV
3. 100 kV
4. 330 kV

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75. The efficiencies of transformers compared with that of electric motors of the same power are

1. about the same
2. much smaller
3. much higher
4. somewhat smaller

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76. The efficiency of a transformer will be maximum when

1. copper losses = hysteresis losses
2. hysteresis losses = eddy current losses
3. eddy current losses = copper losses
4. copper losses = iron losses

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77. The efficiency of a transformer, under heavy loads, is comparatively low because

1. copper loss becomes high in proportion to the output
2. iron loss is increased considerably
3. voltage drop both in primary and secondary becomes large
4. secondary output is much less as compared to primary input

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78. The efficiency of two identical transformers under load conditions can be determined by

1. short-circuit test
2. back-to-back test
3. open circuit test
4. any of the above

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79. The full-load copper loss of a trans¬former is 1600 W. At half-load, the copper loss will be

1. 6400 W
2. 1600 W
3. 800 W
4. 400 W

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80. The function of breather in a transformer is

1. to provide oxygen inside the tank
2. to cool the coils during reduced load
3. to cool the transformer oil
4. to arrest flow of moisture when outside air enters the transformer

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