EEE 251 Possible Examination Questions and Answers
You are advised to take time to practice these questions before entering the examination hall.
Note that none of these questions were copied from the school past-questions or from any of the lecturer. Therefore, you should also read your book. Do not wholly depend on this questions. But it will go a long way to help you.
1. The no-load current drawn by the transformer is
usually what percent of the full-load current ?
(a) 0.2 to 0.5 per cent
(b) 2 to 5 per cent
(c) 12 to 15 percent
(d) 20 to 30 per cent
Ans: b
2. The dielectric strength of transformer oil is
expected to be
(a) lkV
(b) 33 kV
(c) 100 kV
(d) 330 kV
Ans: b
3. Sumpner’s test is conducted on trans-formers to
determine
(a) temperature
(b) stray losses
(c) all-day efficiency
(d) none of the above
Ans: a
4. Efficiency of a power transformer is of the order
of
(a) 100 per cent
(b) 98 per cent
(c) 50 per cent
(d) 25 per cent
Ans: b
5. In a given transformer for given applied voltage,
losses which remain constant irrespective of load changes are
(a) friction and windage losses
(b) copper losses
(c) hysteresis and eddy current losses
(d) none of the above
Ans: c
6. A common method of cooling a power transformer
is
(a) natural air cooling
(b) air blast cooling
(c) oil cooling
(d) any of the above
Ans: c
7. The no load current in a transformer lags behind
the applied voltage by an angle of about
(a) 180°
(b) 120″
(c) 90°
(d) 75°
Ans: d
8. In the transformer, the function of a conservator
is to
(a) provide fresh air for cooling the
transformer
(b) supply cooling oil to the transformer in time of
need
(c) protect the transformer from damage when oil
expends due to heating
(d) none of the above
Ans: c
9. Power transformers are designed to have maximum
efficiency at
(a) nearly full load
(b) 70% full load
(c) 50% full load
(d) no load
Ans: a
10. The maximum efficiency of a distribution
transformer is
(a) at no load
(b) at 50% full load
(c) at 80% full load
(d) at full load
Ans: b
11. Transformer breaths in when
(a) load on it increases
(b) load on it decreases
(c) load remains constant
(d) none of the above
Ans: b
12. Greater the secondary leakage flux
(a) less will be the secondary induced e.m.f.
(b) less will be the primary induced e.m.f.
(c) less will be the primary terminal voltage
(d) none of the above
Ans: a
13. The purpose of providing iron core in a step-up
transformer is
(a) to provide coupling between primary and
secondary
(b) to increase the magnitude of mutual flux
(c) to decrease the magnitude of magnetizing
current
(d) to provide all above features
Ans: c
14. The power transformer is a constant
(a) voltage device
(b) current device
(c) power device
(d) main flux device
Ans: d
15. Two transformers operating in parallel will
share the load depending upon their
(a) leakage reactance
(b) per unit impedance
(c) efficiencies
(d) ratings
Ans: b
16. If the percentage impedances of the two
transformers working in parallel are different, then
(a) transformers will be overheated
(b) power factors of both the transformers will be
same
(c) parallel operation will be not possible
(d) parallel operation will still be possible, but
the power factors at which the two transformers operate will be different from
the power factor of the common load
Ans: d
17. In a transformer the tappings are generally
provided on
(a) primary side
(b) secondary side
(c) low voltage side
(d) high voltage side
Ans: c
18. The thickness of laminations used in a
transformer is usually
(a) 0.4 mm to 0.5 mm
(b) 4 mm to 5 mm
(c) 14 mm to 15 mm
(d) 25 mm to 40 mm
Ans: a
19. In a transformer the resistance between its
primary and secondary is
(a) zero
(b) 1 ohm
(c) 1000 ohms
(d) infinite
Ans: d
20. Gas is usually not liberated due to dissociation
of transformer oil unless the oil temperature exceeds
(a) 50°C
(b) 80°C
(c) 100°C
(d) 150°C
Ans: d
21. The main reason for generation of harmonics in a
transformer could be
(a) fluctuating load
(b) poor insulation
(c) mechanical vibrations
(d) saturation of core
Ans: d
22. Star/star transformers work satisfactorily
when
(a) load is unbalanced only
(b) load is balanced only
(c) on balanced as well as unbalanced loads
(d) none of the above
Ans: b
23. Delta/star transformer works satisfactorily
when
(a) load is balanced only
(b) load is unbalanced only
(c) on balanced as well as unbalanced loads
(d) none of the above
Ans: c
24. Buchholz’s relay gives warning and protection
against
(a) electrical fault inside the transformer
itself
(b) electrical fault outside the transformer in
outgoing feeder
(c) for both outside and inside faults
(d) none of the above
Ans: a
25. The magnetising current of a transformer is
usually small because it has
(a) small air gap
(b) large leakage flux
(c) laminated silicon steel core
(d) fewer rotating parts
Ans: a
26. The leakage flux in a transformer depends
upon
(a) load current
(b) load current and voltage
(c) load current, voltage and frequency
(d) load current, voltage, frequency and power
factor
Ans: a
27. Noise level test in a transformer is a
(a) special test
(b) routine test
(c) type test
(d) none of the above
Ans: c
28. Which of the following is not a routine test on
transformers ?
(a) Core insulation voltage test
(b) Impedance test
(c) Radio interference test
(d) Polarity test
Ans: c
29. A transformer can have zero voltage regulation
at
(a) leading power factor
(b) lagging power factor
(c) unity power factor
(d) zero power factor
Ans: a
30. Helical coils can be used on
(a) low voltage side of high kVA transformers
(b) High-frequency transformers
(c) high voltage side of small capacity
transformers
(d) high voltage side of high kVA rating
transformers
Ans: a
31. The core used in high frequency transformer is
usually
(a) copper core
(b) cost iron core
(c) air core
(d) mild steel core
Ans: c
32. During short circuit test iron losses are
negligible because
(a) the current on secondary side is
negligible
(b) the voltage on secondary side does not
vary
(c) the voltage applied on primary side is low
(d) full-load current is not supplied to the
transformer
Ans: c
33. In an actual transformer the iron loss remains
practically constant from noload to fullload because (a) value of
transformation ratio remains constant
(b) permeability of transformer core remains
constant
(c) core flux remains practically constant
(d) primary voltage remains constant
(e) secondary voltage remains constant
Ans: c
34. If the supply frequency to the transformer is
increased,”the iron loss will
(a) not change
(b) decrease
(c) increase
(d) any of the above
Ans: c
35. Iron loss of a transformer can be measured
by
(a) low power factor wattmeter
(b) unity power factor wattmeter
(c) frequency meter
(d) any type of wattmeter
Ans: a
36. For the parallel operation of single phase
transformers it is necessary that they should have
(a) same efficiency
(b) same polarity
(c) same kVA rating
(d) same number of turns on the secondary
side.
Ans: b
37. The transformer oil should have _____ volatility
and _____ viscosity.
(a) low,low
(b) high,high
(c) low,high
(d) high,low
Ans: a
38. The size of a transformer core will depend
on
(a) frequency
(b) area of the core
(c) flux density of the core material
(d) (a) and (b) both
Ans: d
39. A shell-type transformer has
(a) high eddy current losses
(b) reduced magnetic leakage
(c) negligibly hysteresis losses
(d) none of the above
Ans: b
40. For given applied voltage, with the increase in
frequency of the applied voltage
(a) eddy current loss will decrease
(b) eddy current loss will increase
(c) eddy current loss will remain unchanged
(d) none of the above
Ans: c
41. Losses which occur in rotating electric machines
and do not occur in trans formers are
(a) friction and windage losses
(b) magnetic losses
(c) hysteresis and eddy current losses
(d) copper losses
Ans: a
42. Which of the following protection is normally
not provided on small distribution transformers ? (a) Overfluxing
protection
(b) Buchholz relay
(c) Overcurrent protection
(d) All of the above
Ans: b
43. If a transformer is continuously operated the
maximum temperature rise will occur in
(a) core
(b) windings
(c) tank
(d) any of the above
Ans: b
44. The maximum load that a power transformer can
carry is limited by its
(a) temperature rise
(b) dielectric strength of oil
(c) voltage ratio
(d) copper loss
Ans: c
45. The voltage transformation ratio of a
transformer is equal to the ratio of
(a) primary turns to secondary turns
(b) secondary current to primary current
(c) secondary induced e.m.f. to primary induced
e.m.f.
(d) secondary terminal voltage to primary applied
voltage
Ans: c
46. Minimum voltage regulation occurs when the power
factor of the load is
(a) unity
(b) lagging
(c) leading
(d) zero
Ans: c
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