Q1. Which of the following losses in a transformer remains constant from no-load to full-load?
A. Copper loss
B. Iron (core) loss
C. Stray loss
D. Load loss
Correct Option: B
Explanation:
Iron loss consists of hysteresis and eddy current losses. These depend on applied voltage and frequency, not on load current. Since voltage is constant, iron loss remains constant from no-load to full-load.
Q2. At no-load condition, the efficiency of a transformer is:
A. Maximum
B. Zero
C. Equal to copper loss
D. Equal to iron loss
Correct Option: B
Explanation:
At no-load, output power is zero while iron loss exists. Since efficiency = Output/Input, efficiency becomes zero.
Q3. Transformer efficiency is maximum when:
A. Copper loss = Iron loss
B. Copper loss is minimum
C. Iron loss is minimum
D. Output power is maximum
Correct Option: A
Explanation:
Maximum efficiency occurs when variable loss (copper loss ∝ I²) equals constant loss (iron loss).
Q4. Which loss depends on square of load current?
A. Hysteresis loss
B. Eddy current loss
C. Copper loss
D. Dielectric loss
Correct Option: C
Explanation:
Copper loss is given by I²R and hence varies as the square of load current.
Q5. The efficiency of a transformer does not depend on:
A. Load current
B. Power factor
C. Iron loss
D. Supply frequency
Correct Option: D
Explanation:
Efficiency depends on output power and losses. Frequency affects iron loss indirectly but does not directly appear in the efficiency expression.
Q6. A transformer has iron loss of 800 W and full-load copper loss of 800 W. Maximum efficiency occurs at:
A. 25% load
B. 50% load
C. 75% load
D. Full load
Correct Option: D
Explanation:
Maximum efficiency occurs when copper loss equals iron loss. Since both are equal at full-load, maximum efficiency occurs at full-load.
Q7. At half-load, copper loss will be:
A. Equal to full-load copper loss
B. One-fourth of full-load copper loss
C. Half of full-load copper loss
D. Double of full-load copper loss
Correct Option: B
Explanation:
Copper loss ∝ I². At half-load, current = 0.5I, so copper loss = (0.5)² = 0.25 times full-load loss.
Q8. All-day efficiency of a transformer mainly depends on:
A. Copper loss
B. Load current
C. Power factor
D. Duration of load
Correct Option: D
Explanation:
All-day efficiency is based on energy over 24 hours and hence depends mainly on load duration and load cycle.
Q9. Which transformer has higher all-day efficiency?
A. Distribution transformer
B. Power transformer
C. Auto-transformer
D. Isolation transformer
Correct Option: A
Explanation:
Distribution transformers operate continuously at light load, so they are designed for high all-day efficiency.
Q10. If power factor decreases while keeping load constant, transformer efficiency will:
A. Increase
B. Decrease
C. Remain unchanged
D. Become zero
Correct Option: B
Explanation:
Output power = VIcosφ. If power factor decreases, output decreases while losses remain nearly constant, so efficiency decreases.
Q11. A transformer delivers same kVA at 0.8 lag and 0.8 lead p.f. Which statement is correct?
A. Efficiency higher at lagging p.f.
B. Efficiency higher at leading p.f.
C. Efficiency is same
D. Depends on iron loss only
Correct Option: C
Explanation:
For same kVA and same magnitude of power factor, output kW is same. Hence efficiency remains the same.
Q12. Copper loss is measured during which test?
A. Open circuit test
B. Short circuit test
C. Sumpner test
D. Polarity test
Correct Option: B
Explanation:
In short-circuit test, applied voltage is low and rated current flows. Iron loss is negligible, so measured power represents copper loss.
Q13. Iron loss is measured during:
A. Short-circuit test
B. Load test
C. Open-circuit test
D. Back-to-back test
Correct Option: C
Explanation:
Open-circuit test is conducted at rated voltage and no-load. Copper loss is negligible, so wattmeter reading equals iron loss.
Q14. A transformer operates at maximum efficiency at 60% load. Full-load copper loss = 1 kW. Iron loss is:
A. 360 W
B. 600 W
C. 1 kW
D. 2.78 kW
Correct Option: A
Explanation:
At maximum efficiency, iron loss = x² × full-load copper loss = (0.6)² × 1000 = 360 W.
Q15. Which loss is present even when transformer secondary is open?
A. Copper loss
B. Iron loss
C. Load loss
D. Stray copper loss
Correct Option: B
Explanation:
Even at no-load, alternating flux exists in the core causing hysteresis and eddy current losses.
Q16. Efficiency of transformer is highest when power factor is:
A. Zero
B. Leading
C. Lagging
D. Unity
Correct Option: D
Explanation:
For same losses, output power is maximum at unity power factor. Hence efficiency is maximum at unity p.f.
Q17. At light loads, transformer efficiency is low because:
A. Copper loss dominates
B. Iron loss dominates
C. Stray loss dominates
D. Dielectric loss dominates
Correct Option: B
Explanation:
At light load, copper loss is very small while iron loss remains constant and dominates total loss.
Q18. For maximum efficiency condition, output power is proportional to:
A. Iron loss
B. Copper loss
C. Iron loss / Copper loss
D. Iron loss + Copper loss
Correct Option: B
Explanation:
At maximum efficiency, copper loss equals iron loss. Hence output power at that point is directly proportional to copper loss.
Q19. Which efficiency is most important for distribution transformers?
A. Commercial efficiency
B. Maximum efficiency
C. All-day efficiency
D. Thermal efficiency
Correct Option: C
Explanation:
Distribution transformers supply varying load throughout the day, so all-day efficiency is the most important criterion.
Q20. A transformer supplies constant voltage to variable load. Iron loss remains constant because it depends on:
A. Load current
B. Load power
C. Applied voltage
D. Power factor
Correct Option: C
Explanation:
Iron loss depends on applied voltage and frequency. Since voltage remains constant, iron loss remains constant.