Q1. An auto-transformer differs from a two-winding transformer because it has:
A. No iron core
B. Only one winding common to primary and secondary
C. Two separate windings magnetically coupled
D. No electrical connection between input and output
Correct Option: B
Explanation:
An auto-transformer uses a single continuous winding where part of the winding is common to both primary and secondary. Hence, power transfer occurs partly by induction and partly by direct conduction, unlike a two-winding transformer which has complete electrical isolation.
Q2. Main disadvantage of an auto-transformer compared to a two-winding transformer is:
A. Higher copper loss
B. Larger size
C. No electrical isolation between primary and secondary
D. Lower efficiency
Correct Option: C
Explanation:
The major disadvantage of an auto-transformer is the absence of electrical isolation. Any high-voltage fault on the primary side can directly appear on the secondary side, making it unsafe for sensitive loads.
Q3. For an auto-transformer, the ratio of copper saving compared to a two-winding transformer is:
A. V₂ / V₁
B. (V₁ − V₂) / V₁
C. V₁ / V₂
D. (V₁ + V₂) / V₁
Correct Option: A
Explanation:
In an auto-transformer, only a portion of the winding corresponding to the voltage difference requires separate copper. Hence, the fraction of copper saved compared to a two-winding transformer is V₂ / V₁.
Q4. An auto-transformer is not preferred when:
A. Voltage ratio is close to unity
B. Large power transfer is required
C. Electrical isolation is required
D. High efficiency is desired
Correct Option: C
Explanation:
Auto-transformers should not be used where electrical isolation is mandatory, such as medical equipment or domestic supplies, because primary and secondary are electrically connected.
Q5. A 200 V / 100 V, 10 kVA two-winding transformer is converted into an auto-transformer. The new kVA rating becomes approximately:
A. 10 kVA
B. 15 kVA
C. 20 kVA
D. 30 kVA
Correct Option: C
Explanation:
Auto-transformer kVA rating = kVAtwo × V₁ / (V₁ − V₂) = 10 × 200 / 100 = 20 kVA.
Q6. kVA rating of an auto-transformer is related to two-winding transformer rating by:
A. Same rating
B. Reduced rating
C. Increased by factor V₁ / (V₁ − V₂)
D. Reduced by factor V₂ / V₁
Correct Option: C
Explanation:
Due to conductive power transfer through the common winding, the effective kVA rating of an auto-transformer increases by the factor V₁ / (V₁ − V₂).
Q7. In conversion of a two-winding transformer into an auto-transformer, copper loss:
A. Increases
B. Remains same
C. Decreases
D. Becomes zero
Correct Option: C
Explanation:
Auto-transformers use less copper and hence have lower resistance. Since copper loss ∝ I²R, copper loss decreases.
Q8. Which of the following applications commonly uses auto-transformer?
A. Distribution transformer
B. Isolation transformer
C. Induction motor starter
D. Welding transformer
Correct Option: C
Explanation:
Auto-transformers are widely used as induction motor starters because they provide reduced starting current at lower cost.
Q9. For an auto-transformer, power transferred conductively is:
A. Entire output power
B. Zero
C. (V₂ / V₁) × total power
D. (V₁ − V₂) / V₁ × total power
Correct Option: C
Explanation:
The fraction of power transferred conductively through the common winding is equal to V₂ / V₁ of the total power.
Q10. For an auto-transformer, power transferred inductively is:
A. Equal to total power
B. Zero
C. (V₁ − V₂) / V₁ × total power
D. V₂ / V₁ × total power
Correct Option: C
Explanation:
The inductively transferred power corresponds to the voltage difference portion and equals (V₁ − V₂) / V₁ of total power.
Q11. Essential condition for parallel operation of single-phase transformers is:
A. Same efficiency
B. Same polarity
C. Same cooling method
D. Same kVA rating
Correct Option: B
Explanation:
Same polarity is essential. Wrong polarity causes secondary voltages to oppose each other, resulting in heavy circulating current.
Q12. If two transformers connected in parallel have unequal voltage ratios, it results in:
A. No load sharing
B. Circulating current
C. Reduced efficiency only
D. Over-voltage
Correct Option: B
Explanation:
Unequal voltage ratios cause unequal induced EMFs, leading to circulating current even at no-load.
Q13. For satisfactory parallel operation, transformers must have:
A. Equal % impedance
B. Equal kVA rating only
C. Equal efficiency
D. Equal losses
Correct Option: A
Explanation:
Equal percentage impedance ensures proper load sharing and prevents overloading of one transformer.
Q14. If transformers in parallel have same voltage ratio but unequal impedance, load sharing depends on:
A. Voltage ratio
B. Copper loss
C. Inverse of impedance
D. Power factor
Correct Option: C
Explanation:
Load current divides inversely proportional to impedance. Lower impedance transformer carries more load.
Q15. If transformer A has lower impedance than transformer B, then:
A. A carries less load
B. Both share equally
C. A carries more load
D. B carries more load
Correct Option: C
Explanation:
Lower impedance offers less opposition to current, so transformer A carries more load.
Q16. Circulating current in parallel transformers flows due to:
A. Load current
B. Unequal frequency
C. Unequal induced EMF
D. Core loss
Correct Option: C
Explanation:
Difference in induced EMFs causes circulating current even when load is zero.
Q17. To minimize circulating current, transformers should have:
A. Same kVA rating
B. Same polarity and voltage ratio
C. Same losses
D. Same efficiency
Correct Option: B
Explanation:
Same polarity and equal voltage ratio ensure equal induced EMFs, minimizing circulating current.
Q18. If two transformers with equal impedance but different X/R ratios operate in parallel, then:
A. kVA sharing is unequal
B. Power factor sharing is unequal
C. Voltage ratio changes
D. Circulating current becomes zero
Correct Option: B
Explanation:
Equal impedance ensures equal kVA sharing, but different X/R ratios cause unequal power factor sharing.
Q19. For equal load sharing in parallel transformers, the condition is:
A. Z₁ = Z₂
B. %Z₁ = %Z₂
C. R₁ = R₂
D. X₁ = X₂
Correct Option: B
Explanation:
Equal percentage impedance is required because it normalizes impedance for transformers of different ratings.
Q20. Parallel operation of transformers is done mainly to:
A. Increase voltage
B. Improve efficiency at all loads
C. Increase reliability and capacity
D. Reduce losses to zero
Correct Option: C
Explanation:
Parallel operation increases load capacity, improves reliability, and allows maintenance without total shutdown.