Transformers

21. Efficiency of a power transformer is of the order of

1. 100 per cent
2. 98 per cent
3. 50 per cent
4. 25 per cent

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22. For a transformer, operating at constant load current, maximum efficiency will occur at

1. 0.8 leading power factor
2. 0.8 lagging power factor
3. zero power factor
4. unity power factor

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23. For given applied voltage, with the increase in frequency of the applied voltage

1. eddy current loss will decrease
2. eddy current loss will increase
3. eddy current loss will remain unchanged
4. none of the above

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24. For the parallel operation of single phase transformers it is necessary that they should have

1. same efficiency
2. same polarity
3. same kVA rating
4. same number of turns on the secondary side.

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25. Gas is usually not liberated due to dissociation of transformer oil unless the oil temperature exceeds

1. 50°C
2. 80°C
3. 100°C
4. 150°C

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26. Greater the secondary leakage flux

1. less will be the secondary induced e.m.f.
2. less will be the primary induced e.m.f.
3. less will be the primary terminal voltage
4. none of the above

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27. Harmonics in transformer result in

1. increased core losses
2. increased I2R losses
3. magnetic interference with communication circuits
4. all of the above

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28. Helical coils can be used on

1. low voltage side of high kVA transformers
2. high frequency transformers
3. high voltage side of small capacity transformers
4. high voltage side of high kVA rating transformers

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29. Hysteresis loss in a transformer varies as CBmax = maximum flux density)

1. Bmax
2. Bmax1-6
3. Bmax1-83
4. B max

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30. If a transformer is continuously operated the maximum temperature rise will occur in

1. core
2. windings
3. tank
4. any of the above

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