Q1. Why is a transformer rated in kVA instead of kW?
A. Core loss depends on load
B. Copper loss depends on power factor
C. Losses depend on voltage and current, not power factor
D. Power factor is always unity
Correct Option: C
Explanation:
Transformer heating depends on losses. Core loss depends on voltage and frequency, while copper loss depends on current. Both losses are independent of load power factor. Since power factor affects kW but not losses, transformer rating is based on V×I (kVA).
Q2. Transformer rating in kVA is independent of:
A. Copper loss
B. Core loss
C. Load power factor
D. Current
Correct Option: C
Explanation:
kVA rating depends only on voltage and current. Load power factor affects output power in kW but does not affect transformer losses or heating.
Q3. A transformer supplies full-load current at 0.8 lag and 0.8 lead p.f. Which statement is correct?
A. Losses are different
B. Output power is same
C. Efficiency is same
D. Voltage regulation is same
Correct Option: C
Explanation:
At the same full-load current, copper loss remains the same and core loss is constant. Hence total losses are same, resulting in same efficiency. Voltage regulation differs for lagging and leading power factor.
Q4. Voltage regulation of a transformer at lagging power factor is approximately:
A. I₂R₀₂ sinÏ• + I₂X₀₂ cosÏ•
B. I₂R₀₂ cosÏ• + I₂X₀₂ sinÏ•
C. I₂R₀₂ cosÏ• − I₂X₀₂ sinÏ•
D. I₂R₀₂ − I₂X₀₂
Correct Option: B
Explanation:
At lagging power factor, both resistive and reactive voltage drops add. Hence voltage drop is I₂R₀₂cosÏ• + I₂X₀₂sinÏ•.
Q5. Voltage regulation of a transformer becomes negative when load p.f. is:
A. Lagging
B. Unity
C. Leading
D. Zero
Correct Option: C
Explanation:
At leading power factor, reactive voltage drop becomes negative, causing terminal voltage to rise with load. Hence regulation becomes negative.
Q6. Percentage regulation of a transformer is defined as:
A. (E₂ − V₂)/V₂ ×100
B. (V₂ − E₂)/E₂ ×100
C. (E₂ − V₂)/E₂ ×100
D. (I₂R)/V₂ ×100
Correct Option: C
Explanation:
Percentage regulation is the ratio of difference between no-load and full-load secondary voltage to no-load voltage, multiplied by 100.
Q7. At unity power factor, transformer voltage regulation depends mainly on:
A. Resistance only
B. Reactance only
C. Both equally
D. Core loss
Correct Option: A
Explanation:
At unity power factor, sinϕ = 0 and cosϕ = 1. Reactive drop becomes zero, so only resistive drop affects regulation.
Q8. Percentage resistance of transformer is equal to:
A. (Copper loss / Rated kVA) ×100
B. (Iron loss / Rated kVA) ×100
C. Output/Input ×100
D. Voltage drop/Rated voltage ×100
Correct Option: A
Explanation:
Percentage resistance represents the resistive component of transformer impedance and is obtained from copper loss during short-circuit test.
Q9. Which test gives percentage resistance and reactance of transformer?
A. OC test
B. SC test
C. Sumpner test
D. Load test
Correct Option: B
Explanation:
Short-circuit test gives equivalent resistance, reactance and percentage impedance because core loss is negligible and copper loss is measurable.
Q10. If %R = 3 and %X = 4, then % impedance is:
A. 5
B. 7
C. 1
D. 25
Correct Option: A
Explanation:
Percentage impedance = √(%R² + %X²) = √(9 + 16) = 5.
Q11. Percentage impedance of a transformer is defined as:
A. Ratio of rated voltage to rated current
B. Voltage required to circulate full-load current under short circuit
C. Voltage drop at full load
D. Copper loss percentage
Correct Option: B
Explanation:
Percentage impedance is the percentage of rated voltage required to circulate full-load current when secondary is short-circuited.
Q12. In short-circuit test, applied voltage is approximately:
A. Rated voltage
B. Zero
C. 5–10% of rated voltage
D. 50% of rated voltage
Correct Option: C
Explanation:
Only a small fraction (5–10%) of rated voltage is required to circulate full-load current during SC test.
Q13. In Sumpner (back-to-back) test, two transformers must have:
A. Same rating and polarity
B. Same voltage only
C. Same current rating
D. Same efficiency
Correct Option: A
Explanation:
For safe operation in Sumpner test, transformers must be identical in rating and polarity to avoid circulating currents.
Q14. Main advantage of Sumpner test is:
A. Simple setup
B. Requires less power from supply
C. Measures no-load loss only
D. Cannot find efficiency
Correct Option: B
Explanation:
Sumpner test allows full-load testing while drawing only the losses from the supply, making it economical for large transformers.
Q15. Total power drawn from supply in Sumpner test is equal to:
A. Full-load power
B. Core loss only
C. Copper loss only
D. Core loss + Copper loss
Correct Option: D
Explanation:
In Sumpner test, no output power is delivered. Supply provides only iron loss and full-load copper loss.
Q16. Which loss is measured by wattmeter connected in secondary loop during Sumpner test?
A. Core loss
B. Iron loss
C. Copper loss
D. Mechanical loss
Correct Option: C
Explanation:
The wattmeter in the secondary loop measures full-load copper loss of both transformers.
Q17. Why Sumpner test is also called heat-run test?
A. Transformers are overloaded
B. Temperature rise can be studied
C. Core is heated
D. Short circuit occurs
Correct Option: B
Explanation:
Since full-load losses are produced, the temperature rise of the transformer can be accurately studied. Hence it is called heat-run test.
Q18. At leading power factor, voltage regulation of transformer is:
A. Positive
B. Zero
C. Negative
D. Infinite
Correct Option: C
Explanation:
At leading power factor, terminal voltage increases with load due to negative reactive drop, resulting in negative regulation.
Q19. Which component mainly affects voltage regulation at lagging power factor?
A. Resistance
B. Leakage reactance
C. Core loss
D. Hysteresis loss
Correct Option: B
Explanation:
At lagging power factor, reactive voltage drop dominates. Hence leakage reactance mainly affects voltage regulation.
Q20. If %R = 4 and %X = 3, regulation at unity p.f. (approx.) is:
A. 5%
B. 4%
C. 3%
D. 7%
Correct Option: B
Explanation:
At unity power factor, voltage regulation ≈ %R. Hence regulation is approximately 4%.