Three-Phase Transformer Construction: 50 Objective Questions with Explanations

Q1. A three-phase transformer is mainly used to

1. Reduce current
2. Transfer large amount of power
3. Change frequency
4. Store energy

✅ Correct Answer: b) Transfer large amount of power

• Three-phase transformers handle high power levels.
• Used in generation, transmission, and distribution systems.
• More economical than single-phase transformer banks.

Q2. A three-phase transformer is required in power systems to

1. Increase frequency
2. Reduce losses only
3. Step-up and step-down voltage
4. Improve power factor

✅ Correct Answer: c) Step-up and step-down voltage

• Voltage is stepped up for transmission to reduce losses.
• Voltage is stepped down for safe distribution and utilization.

Q3. Three-phase transformers can be constructed using

1. Only one core
2. Three single-phase transformers
3. Air-core transformers
4. Autotransformers only

✅ Correct Answer: b) Three single-phase transformers

• Three single-phase units can be connected for three-phase operation.
• However, a single three-phase transformer is more economical.

Q4. Compared to three single-phase transformers, a three-phase transformer is

1. Heavier
2. More costly
3. More compact
4. Less efficient

✅ Correct Answer: c) More compact

• Shared core reduces material usage.
• Requires less space and weight.

Q5. Three-phase transformers are classified mainly as

1. Step-up and step-down
2. Power and distribution
3. Core type and shell type
4. Auto and instrument

✅ Correct Answer: c) Core type and shell type

• Classification is based on core and winding arrangement.
• Core-type and shell-type are most common.

Q6. In a core-type three-phase transformer, the number of limbs is

1. Two
2. Three
3. Four
4. Six

✅ Correct Answer: b) Three

• Each limb carries one phase winding.
• All limbs lie in the same plane.

Q7. In a balanced three-phase transformer, the sum of phase fluxes is

1. Maximum
2. Zero
3. Infinite
4. Constant

✅ Correct Answer: b) Zero

• Fluxes are 120° apart.
• Vector sum of balanced three-phase fluxes equals zero.

Q8. The limb which carries zero resultant flux can be

1. Strengthened
2. Insulated
3. Removed
4. Laminated further

✅ Correct Answer: c) Removed

• Zero flux means no magnetic requirement.
• Removing limb saves iron and cost.

Q9. In core-type transformer, each limb carries

1. Only HV winding
2. Only LV winding
3. Both HV and LV windings
4. No winding

✅ Correct Answer: c) Both HV and LV windings

• Placing both windings on same limb improves coupling.
• Reduces leakage flux.

Q10. LV winding is placed nearer to the core because

1. Cooling is better
2. Mechanical strength increases
3. Insulation requirement is less
4. Flux density increases

✅ Correct Answer: c) Insulation requirement is less

• LV operates at lower voltage.
• Needs less insulation thickness.

Q11. In a core-type three-phase transformer, magnetic paths are

1. Perfectly symmetrical
2. Completely isolated
3. Unequal for different limbs
4. Air-cored

✅ Correct Answer: c) Unequal for different limbs

• Outer limbs have longer magnetic path.
• Middle limb has shorter path.
• This causes imbalance in magnetizing current.

Q12. The imbalance in magnetizing current is more prominent in

1. Shell-type transformer
2. Core-type transformer
3. Auto-transformer
4. Instrument transformer

✅ Correct Answer: b) Core-type transformer

• Magnetic paths are unequal.
• Causes different reluctance in limbs.
• Leads to magnetizing current imbalance.

Q13. Shell-type three-phase transformer is constructed by

1. Removing outer limbs
2. Stacking three shell-type units
3. Using toroidal core
4. Using air-gap core

✅ Correct Answer: b) Stacking three shell-type units

• Three single-phase shell cores are stacked.
• Forms a three-phase shell-type transformer.

Q14. In shell-type transformer, winding direction of central limb is

1. Same as others
2. Opposite to others
3. Random
4. Alternating

✅ Correct Answer: b) Opposite to others

• Ensures balanced flux distribution.
• Cancels unwanted flux components.

Q15. Balanced three-phase supply produces flux that is

1. Unbalanced
2. Zero
3. Balanced
4. Distorted

✅ Correct Answer: c) Balanced

• Balanced voltages produce balanced fluxes.
• Fluxes are equal in magnitude and 120° apart.

Q16. The combined flux magnitude in shell-type transformer is

1. Double of individual flux
2. Half of individual flux
3. Equal to individual flux
4. Zero

✅ Correct Answer: c) Equal to individual flux

• Flux components add vectorially.
• Resultant flux equals magnitude of one phase flux.

Q17. Cross-sectional area of combined yoke in shell-type transformer is

1. Half of limb
2. Double of limb
3. Same as limb
4. Zero

✅ Correct Answer: c) Same as limb

• Flux density must remain constant.
• Hence yoke and limb areas are equal.

Q18. Magnetic imbalance has least effect in

1. Core-type transformer
2. Shell-type transformer
3. Auto-transformer
4. Single-phase transformer

✅ Correct Answer: b) Shell-type transformer

• Magnetic paths are symmetrical.
• Flux distribution is uniform.

Q19. Shell-type transformer offers better performance due to

1. Higher voltage
2. Balanced magnetic circuit
3. Lower resistance
4. Higher frequency

✅ Correct Answer: b) Balanced magnetic circuit

• Equal magnetic paths reduce imbalance.
• Improves regulation and efficiency.

Q20. Windings of shell-type transformer can be connected in

1. Only star
2. Only delta
3. Star or delta
4. Zig-zag only

✅ Correct Answer: c) Star or delta

• Connection depends on application requirement.
• Both star and delta are possible.

Q21. In a core-type three-phase transformer, the total flux in the merged limb becomes zero because

1. Fluxes cancel each other
2. Voltage is low
3. Frequency is high
4. Core is laminated

✅ Correct Answer: a) Fluxes cancel each other

• Three-phase fluxes are equal in magnitude.
• They are displaced by 120° electrically.
• Vector sum of three balanced fluxes is zero.

Q22. Since the merged limb carries zero flux, it

1. Must carry winding
2. Must be insulated
3. Can be removed
4. Produces leakage flux

✅ Correct Answer: c) Can be removed

• No magnetic flux flows through merged limb.
• Hence it is magnetically unnecessary.
• Removing it saves core material.

Q23. The core of a three-phase transformer generally consists of

1. Two limbs
2. Three limbs in same plane
3. Four limbs
4. Circular core

✅ Correct Answer: b) Three limbs in same plane

• Each limb carries one phase.
• Limbs lie in same magnetic plane for compact design.

Q24. Each limb of a core-type three-phase transformer carries

1. Only HV winding
2. Only LV winding
3. Both HV and LV windings
4. No winding

✅ Correct Answer: c) Both HV and LV windings

• Both windings are placed concentrically.
• Ensures better coupling and reduced leakage.

Q25. Low-voltage winding is placed nearer to the core mainly to

1. Improve cooling
2. Reduce insulation requirement
3. Increase flux
4. Reduce losses

✅ Correct Answer: b) Reduce insulation requirement

• LV winding operates at lower voltage.
• Requires less insulation from core.
• Hence placed closest to the core.

Q26. The unsymmetrical construction of core-type transformer causes

1. Voltage drop
2. Magnetizing current imbalance
3. Frequency variation
4. Core saturation

✅ Correct Answer: b) Magnetizing current imbalance

• Magnetic path lengths are unequal.
• Reluctance differs for each limb.
• Leads to unequal magnetizing currents.

Q27. Which three-phase transformer has symmetrical magnetic paths?

1. Core-type
2. Shell-type
3. Auto-transformer
4. Instrument transformer

✅ Correct Answer: b) Shell-type

• All magnetic paths are equal in length.
• Flux distribution remains balanced.

Q28. Shell-type three-phase transformer is built by

1. Using toroidal core
2. Stacking three shell-type transformers
3. Removing outer limbs
4. Using air-gap core

✅ Correct Answer: b) Stacking three shell-type transformers

• Three single-phase shell units are combined.
• Forms a single three-phase structure.

Q29. In shell-type transformer, the winding direction of central limb is kept

1. Same as others
2. Opposite to others
3. Random
4. Clockwise only

✅ Correct Answer: b) Opposite to others

• Opposite direction ensures correct flux addition.
• Maintains balanced magnetic circuit.

Q30. Balanced three-phase supply in shell-type transformer produces flux that is

1. Unbalanced
2. Balanced
3. Zero
4. Distorted

✅ Correct Answer: b) Balanced

• Balanced supply creates balanced phase currents.
• Fluxes remain equal and sinusoidal.

Q31. The magnitude of combined flux in shell-type transformer is

1. Zero
2. Equal to individual flux
3. Double of individual flux
4. Half of individual flux

✅ Correct Answer: b) Equal to individual flux

• Flux components add vectorially.
• Resultant flux equals magnitude of one phase flux.

Q32. Cross-sectional area of combined yoke in shell-type transformer is

1. Half of limb area
2. Same as limb area
3. Double of limb area
4. Zero

✅ Correct Answer: b) Same as limb area

• Flux density must remain constant.
• Hence yoke and limb areas are equal.

Q33. Effect of magnetic path imbalance is

1. High in shell-type
2. Negligible in shell-type
3. Zero in core-type
4. Same in all transformers

✅ Correct Answer: b) Negligible in shell-type

• Magnetic paths are symmetrical.
• Flux imbalance is minimal.

Q34. Shell-type transformer offers better performance mainly because of

1. High voltage
2. Balanced magnetic circuit
3. Lower resistance
4. Higher frequency

✅ Correct Answer: b) Balanced magnetic circuit

• Uniform flux distribution reduces losses.
• Improves regulation and efficiency.

Q35. Windings of shell-type transformer may be connected in

1. Only star
2. Only delta
3. Star or delta
4. Zig-zag only

✅ Correct Answer: c) Star or delta

• Depends on voltage and current requirement.
• Both connections are feasible.

Q36. Three-phase transformer uses less copper than

1. Auto-transformer
2. Single-phase transformer
3. Three single-phase transformers
4. Instrument transformer

✅ Correct Answer: c) Three single-phase transformers

• Shared core reduces copper requirement.
• Improves material utilization.

Q37. Three-phase transformer is lighter because

1. Core is absent
2. Less iron is used
3. Voltage is low
4. Frequency is low

✅ Correct Answer: b) Less iron is used

• Magnetic circuit is shared.
• Overall iron requirement is reduced.

Q38. Three-phase transformer is cheaper mainly because

1. Lower efficiency
2. Less material usage
3. Higher losses
4. Simple cooling

✅ Correct Answer: b) Less material usage

• Reduced copper and iron usage.
• Leads to lower manufacturing cost.

Q39. Core-type transformer construction is not convenient mainly due to

1. High losses
2. Unsymmetrical magnetic circuit
3. Low voltage
4. High current

✅ Correct Answer: b) Unsymmetrical magnetic circuit

• Unequal flux paths exist.
• Causes magnetizing current imbalance.

Q40. Shell-type transformer gives better regulation because of

1. Higher voltage
2. Balanced flux distribution
3. Lower current
4. Higher frequency

✅ Correct Answer: b) Balanced flux distribution

• Flux remains uniform in all limbs.
• Reduces voltage drop.

Q41. Three-phase transformers are mainly used in

1. Electronic circuits
2. Power system networks
3. Communication systems
4. Control circuits

✅ Correct Answer: b) Power system networks

• Large power transmission requires three-phase supply.
• Three-phase transformers are efficient for high power.

Q42. Transmission losses are reduced mainly by

1. Reducing voltage
2. Increasing voltage
3. Increasing current
4. Reducing frequency

✅ Correct Answer: b) Increasing voltage

• Higher voltage means lower current.
• Copper loss (I²R) reduces.

Q43. Laminations in transformer core are used to reduce

1. Copper loss
2. Eddy current loss
3. Mechanical loss
4. Leakage flux

✅ Correct Answer: b) Eddy current loss

• Laminations break eddy current paths.
• Increases electrical resistance of core.

Q44. Magnetizing current imbalance is maximum in

1. Shell-type transformer
2. Core-type transformer
3. Auto-transformer
4. Instrument transformer

✅ Correct Answer: b) Core-type transformer

• Magnetic paths are unequal.
• Causes imbalance in flux and current.

Q45. Which transformer occupies less floor space?

1. Three single-phase transformers
2. Three-phase transformer
3. Auto-transformer
4. Instrument transformer

✅ Correct Answer: b) Three-phase transformer

• Compact construction.
• Shared magnetic circuit.

Q46. Three-phase transformer occupies

1. More space
2. Less space
3. Same space
4. Infinite space

✅ Correct Answer: b) Less space

• Compared to three single-phase units.
• Installation area is reduced.

Q47. Three-phase transformers improve system

1. Voltage instability
2. Reliability
3. Harmonics
4. Noise

✅ Correct Answer: b) Reliability

• Fewer units reduce failure points.
• Stable power supply.

Q48. In core-type transformer, outer limbs have

1. Same magnetic path
2. Shorter magnetic path
3. Longer magnetic path
4. No flux

✅ Correct Answer: c) Longer magnetic path

• Flux must return through outer limbs.
• Path length is greater than middle limb.

Q49. Shell-type transformer is preferred where

1. Magnetic balance is required
2. Voltage is low
3. Frequency is DC
4. Current is zero

✅ Correct Answer: a) Magnetic balance is required

• Symmetrical magnetic paths.
• Uniform flux distribution.

Q50. Three-phase transformer is preferred over a bank of single-phase transformers because it is

1. Less efficient
2. Heavier
3. More economical
4. More complex

✅ Correct Answer: c) More economical

• Uses less copper and iron.
• Requires less space and cost.

📘 Three-Phase Transformer Construction – One-Liner Revision Notes (1–100)

🔹 Core Concepts (1–20)

🔹 1. Three-phase transformer transfers large power efficiently.
🔹 2. It is used in generation, transmission, and distribution systems.
🔹 3. Three-phase transformer can be built using three single-phase units.
🔹 4. A single three-phase transformer is more economical.
🔹 5. Three-phase transformer occupies less space.
🔹 6. It is lighter than three single-phase transformers.
🔹 7. It uses less copper.
🔹 8. It uses less iron.
🔹 9. It has better efficiency.
🔹 10. It has lower installation cost.
🔹 11. Three-phase transformer construction is compact.
🔹 12. It improves system reliability.
🔹 13. It is preferred in substations.
🔹 14. Three-phase supply produces balanced flux.
🔹 15. Balanced flux improves performance.
🔹 16. Transformer core is laminated.
🔹 17. Laminations reduce eddy current loss.
🔹 18. Core is made of silicon steel.
🔹 19. Transformer works on mutual induction.
🔹 20. Transformer does not work on DC supply.

🔹 Core-Type Transformer (21–40)

⚙️ 21. Core-type transformer has three limbs in one plane.
⚙️ 22. Each limb carries one phase flux.
⚙️ 23. HV and LV windings are placed on same limb.
⚙️ 24. LV winding is placed nearer to the core.
⚙️ 25. HV winding is placed over LV winding.
⚙️ 26. Core-type transformer is unsymmetrical.
⚙️ 27. Outer limbs have longer magnetic path.
⚙️ 28. Middle limb has shorter magnetic path.
⚙️ 29. Unequal magnetic paths cause imbalance.
⚙️ 30. Magnetizing current imbalance occurs.
⚙️ 31. Fluxes are 120° apart in three-phase supply.
⚙️ 32. Vector sum of balanced fluxes is zero.
⚙️ 33. Merged limb carries zero flux.
⚙️ 34. Zero-flux limb can be removed.
⚙️ 35. Removal saves core material.
⚙️ 36. Core-type construction is simple.
⚙️ 37. Core-type transformer is widely used.
⚙️ 38. Magnetic imbalance affects performance.
⚙️ 39. Core-type has slightly lower regulation.
⚙️ 40. Core-type transformer is economical for moderate ratings.

🔹 Shell-Type Transformer (41–60)

🛡️ 41. Shell-type transformer has symmetrical magnetic paths.
🛡️ 42. It is constructed by stacking three shell units.
🛡️ 43. Central limb winding direction is opposite.
🛡️ 44. Opposite winding ensures flux balance.
🛡️ 45. Balanced flux improves efficiency.
🛡️ 46. Combined flux equals individual phase flux.
🛡️ 47. Yoke cross-section equals limb cross-section.
🛡️ 48. Magnetic imbalance is negligible.
🛡️ 49. Shell-type has better voltage regulation.
🛡️ 50. Shell-type has better short-circuit strength.
🛡️ 51. Shell-type transformer is mechanically stronger.
🛡️ 52. It provides better cooling arrangement.
🛡️ 53. Performance is superior to core-type.
🛡️ 54. Losses are comparatively lower.
🛡️ 55. Shell-type transformer is heavier than core-type.
🛡️ 56. Construction is complex.
🛡️ 57. Used where magnetic balance is critical.
🛡️ 58. Shell-type transformer gives uniform flux distribution.
🛡️ 59. Balanced flux reduces vibration and noise.
🛡️ 60. Shell-type transformer is preferred for high ratings.

🔹 Comparison & Applications (61–80)

📊 61. Three-phase transformer is preferred over single-phase bank.
📊 62. It reduces floor space requirement.
📊 63. It simplifies installation work.
📊 64. It reduces maintenance cost.
📊 65. Three-phase transformer improves power transmission efficiency.
📊 66. High voltage reduces transmission losses.
📊 67. Transformer enables economical power transfer.
📊 68. Shell-type gives better magnetic balance than core-type.
📊 69. Core-type is cheaper for lower ratings.
📊 70. Shell-type is preferred for high power.
📊 71. Distribution transformers are mostly three-phase.
📊 72. Power transformers are always three-phase.
📊 73. Three-phase transformers support grid stability.
📊 74. Balanced load improves transformer life.
📊 75. Proper insulation is essential for safety.
📊 76. Transformer rating is in kVA.
📊 77. Transformer efficiency depends on losses.
📊 78. Copper loss depends on load current.
📊 79. Core loss is independent of load.
📊 80. Proper design improves transformer lifespan.

🔹 Exam-Oriented Facts (81–100)

🎯 81. Three-phase transformer is essential in power systems.
🎯 82. Construction affects magnetic balance.
🎯 83. Magnetic imbalance increases losses.
🎯 84. Shell-type minimizes imbalance.
🎯 85. Core laminations reduce heating.
🎯 86. Laminations are insulated from each other.
🎯 87. Eddy current loss increases with thickness.
🎯 88. Silicon steel reduces hysteresis loss.
🎯 89. Transformer efficiency is highest at full load.
🎯 90. Balanced flux reduces magnetizing current.
🎯 91. Good design improves regulation.
🎯 92. Transformer construction ensures safety.
🎯 93. Core-type has unequal magnetic paths.
🎯 94. Shell-type has equal magnetic paths.
🎯 95. Transformer is static device.
🎯 96. Transformer has no moving parts.
🎯 97. Proper cooling avoids overheating.
🎯 98. Three-phase transformer is backbone of power system.
🎯 99. Construction directly impacts performance.
🎯 100. Three-phase transformer ensures efficient power transfer.