1. The function of a transformer is to
A) Convert DC to AC
B) Step up or step down AC voltage
C) Convert electrical energy to mechanical energy
D) Increase frequency
Answer: B
2. The principle of operation of a transformer is based on
A) Ohm’s law
B) Ampere’s law
C) Faraday’s law of electromagnetic induction
D) Coulomb’s law
Answer: C
3. In an ideal transformer, the primary and secondary windings are linked by
A) Air
B) Mutual flux
C) Leakage flux
D) Reluctance
Answer: B
4. The efficiency of a transformer is maximum when
A) Iron loss = Copper loss
B) Iron loss > Copper loss
C) Iron loss < Copper loss
D) Load is full
Answer: A
5. The core of a transformer is laminated to
A) Reduce copper loss
B) Reduce hysteresis loss
C) Reduce eddy current loss
D) Increase mechanical strength
Answer: C
6. The open-circuit test on a transformer is used to determine
A) Copper losses
B) Efficiency
C) Core losses
D) Voltage regulation
Answer: C
7. The short-circuit test on a transformer measures
A) Iron losses
B) Copper losses
C) Total losses
D) Voltage ratio
Answer: B
8. In a step-up transformer, the number of turns in the secondary winding is
A) Equal to primary
B) Less than primary
C) More than primary
D) Independent of primary
Answer: C
9. The no-load current of a transformer is
A) 2 to 5% of full-load current
B) 10% of full-load current
C) 20% of full-load current
D) 50% of full-load current
Answer: A
10. Transformer efficiency is usually maximum at
A) No-load
B) Full-load
C) Half-load
D) 75% load
Answer: C
11. The leakage flux in a transformer depends upon
A) Load current
B) Supply voltage
C) Frequency
D) Core material
Answer: A
12. Auto-transformer has
A) Two separate windings
B) A common winding
C) Three windings
D) No winding
Answer: B
13. In a transformer, the voltage per turn is
A) Same for both windings
B) Different for each winding
C) Proportional to turns ratio
D) None of these
Answer: A
14. The magnetizing current in a transformer lags the voltage by
A) 0°
B) 45°
C) 90°
D) 180°
Answer: C
15. The regulation of a transformer will be zero at
A) Unity power factor
B) Zero power factor lagging
C) Zero power factor leading
D) None
Answer: C
16. The main cause of humming in a transformer is
A) Load fluctuations
B) Loose laminations
C) Magnetostriction effect
D) Vibration of windings
Answer: C
17. If the supply frequency increases, the core loss will
A) Increase
B) Decrease
C) Remain same
D) Be zero
Answer: A
18. In a transformer, the power factor of the primary is
A) Same as that of the secondary
B) Always unity
C) Always zero
D) Variable
Answer: A
19. Efficiency of large power transformers is usually in the range of
A) 50–60%
B) 60–70%
C) 95–99%
D) 85–90%
Answer: C
20. Which of the following is not a type of transformer loss?
A) Hysteresis loss
B) Eddy current loss
C) Copper loss
D) Dielectric loss
Answer: D
21. In a 1-phase transformer, the primary winding has 500 turns and secondary has 100 turns. If primary is connected to 1000 V, secondary voltage is
A) 50 V
B) 100 V
C) 200 V
D) 400 V
Answer: C
22. The emf per turn in both windings of a transformer is
A) Different
B) Same
C) Zero
D) Proportional to turns ratio
Answer: B
23. The purpose of breather in a transformer is to
A) Prevent moisture in oil
B) Filter air
C) Prevent overheating
D) Increase voltage
Answer: A
24. Silica gel in the breather turns
A) Pink when dry
B) Blue when moist
C) Pink when moist
D) White when moist
Answer: C
25. Tap changing in transformers is done to control
A) Power factor
B) Frequency
C) Voltage
D) Current
Answer: C
26. In oil-immersed transformers, the oil serves as
A) Coolant only
B) Insulator only
C) Both coolant and insulator
D) Neither
Answer: C
27. The all-day efficiency of a distribution transformer is
A) Equal to ordinary efficiency
B) Less than ordinary efficiency
C) More than ordinary efficiency
D) Depends on load
Answer: B
28. The primary reason for using CRGO steel in transformer cores is
A) Cost reduction
B) To reduce eddy current and hysteresis loss
C) To increase mechanical strength
D) For high resistance
Answer: B
29. Which test is performed to find voltage ratio?
A) OC test
B) SC test
C) Load test
D) Polarity test
Answer: D
30. The polarity of transformer is important when
A) Operating in single unit
B) Connecting in parallel
C) For short circuit test
D) Measuring efficiency
Answer: B
31. Which transformer connection is best suited for small 3-phase loads?
A) Star–Star
B) Delta–Delta
C) Delta–Star
D) Open-delta
Answer: D
32. The vector group of a transformer defines
A) Voltage rating
B) Losses
C) Phase displacement between windings
D) Frequency
Answer: C
33. In a transformer, voltage regulation is negative when load is
A) Lagging
B) Leading
C) Resistive
D) Inductive
Answer: B
34. The function of conservator in a transformer is
A) Cooling
B) Oil expansion accommodation
C) Prevent leakage
D) Measure temperature
Answer: B
35. The transformer insulation resistance is measured by
A) Ammeter
B) Voltmeter
C) Megger
D) Ohmmeter
Answer: C
36. If transformer is operated at lower frequency than rated, the core loss will
A) Increase
B) Decrease
C) Remain same
D) Be zero
Answer: B
37. The no-load current consists of
A) Copper loss component only
B) Magnetizing and core loss components
C) Load current
D) Leakage flux
Answer: B
38. A transformer cannot work on DC because
A) No mutual induction
B) High eddy current loss
C) Core gets saturated
D) All of the above
Answer: D
39. The main disadvantage of auto-transformer is
A) Low efficiency
B) High cost
C) No isolation between windings
D) Heavy weight
Answer: C
40. Transformer oil should have
A) Low flash point
B) High dielectric strength
C) High viscosity
D) High water content
Answer: B
41. The voltage regulation of an ideal transformer is
A) 0%
B) 50%
C) 100%
D) Infinity
Answer: A
42. A current transformer should never be operated with
A) Short-circuited secondary
B) Open-circuited secondary
C) Balanced load
D) Rated burden
Answer: B
43. The core type transformer has
A) Windings surrounding core
B) Core surrounding windings
C) Separate cores
D) Air core
Answer: A
44. The shell type transformer has
A) One magnetic circuit
B) Two magnetic circuits
C) Three magnetic circuits
D) No magnetic circuit
Answer: B
45. In parallel operation, transformers must have same
A) Voltage ratio and polarity
B) Size
C) Core
D) Weight
Answer: A
46. A transformer steps down voltage from 220 V to 110 V. The turns ratio is
A) 1:2
B) 2:1
C) 1:1
D) 3:1
Answer: B
47. The constant losses in a transformer are
A) Copper losses
B) Eddy current losses
C) Hysteresis losses
D) Core losses (B+C)
Answer: D
48. The load current in a transformer depends upon
A) Supply voltage
B) Load connected
C) Core material
D) Frequency
Answer: B
49. The phase shift between primary and secondary in delta-star connection is
A) 0°
B) 30° lag
C) 30° lead
D) 180°
Answer: B
50. The voltage per turn in transformer is given by
A)
B)
C)
D)
Answer: A
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