MCQ on Transformer — 20 Multiple Choice Questions with Answers
Test your knowledge of transformers with these 20 carefully selected multiple choice questions. Covering core losses, efficiency conditions, voltage regulation, cooling, and auto-transformers — this MCQ set is ideal for competitive exam preparation (GATE, ESE, SSC JE) and university viva.
Questions 1–5: Transformer Basics & Losses
Q.1 Which of the following losses is NOT present in a transformer?
- a. Copper loss
- b. Friction loss
- c. Core loss
- d. Stray loss
Q.2 What is the condition for maximum efficiency of a transformer at a given load?
- a. Core loss should be zero
- b. Copper loss should be zero
- c. Total losses should be zero
- d. Copper loss equals core loss
Q.3 Transformer core is laminated to reduce:
- a. Eddy current loss
- b. Hysteresis loss
- c. Copper loss
- d. All of the above
Q.4 A transformer has zero voltage regulation at:
- a. Unity power factor
- b. Leading power factor
- c. Lagging power factor
- d. Zero power factor
Q.5 If copper loss at half load is 300 W, what is the copper loss at full load?
- a. 1200 W
- b. 300 W
- c. 900 W
- d. 600 W
Questions 6–10: Core Properties & Hysteresis
Q.6 The desirable properties of a transformer core are:
- a. Low permeability and low hysteresis loss
- b. High permeability and high hysteresis loss
- c. High permeability and low hysteresis loss
- d. Low permeability and high hysteresis loss
Q.7 The original colour of silica gel in a breather is ______ and after absorbing moisture it turns ______:
- a. Yellow, Blue
- b. Pink, Blue
- c. Blue, Yellow
- d. Blue, Pink
Q.8 A transformer has hysteresis loss of 30 W at 240 V, 60 Hz. Find the hysteresis loss at 200 V, 50 Hz:
W_h2 = 30 × (50/60) = 25 W
- a. 20 W
- b. 25 W
- c. 30 W
- d. 35 W
Q.9 The Steinmetz exponent for hysteresis loss typically lies between:
- a. 1.5 – 2.5
- b. 1.0 – 2.0
- c. 0.5 – 1.5
- d. 0 – 1.5
Q.10 The saving in conductor material when using an auto-transformer instead of a two-winding transformer is:
- a. W/K
- b. W/(K−1)
- c. W/(1−K)
- d. K × W
Questions 11–15: Efficiency & Regulation
Q.11 The all-day efficiency of a transformer depends on:
- a. Copper loss only
- b. Iron loss only
- c. Both copper and iron loss over 24 hours
- d. Load power factor
Q.12 Which test determines the iron loss of a transformer?
- a. Short circuit test
- b. Open circuit test
- c. Sumpner's test
- d. Polarity test
Q.13 Voltage regulation of a transformer is defined as:
- a. Change in secondary voltage from no-load to full-load
- b. Change in primary voltage
- c. Ratio of copper loss to iron loss
- d. Ratio of output to input
Q.14 At what load does a transformer achieve maximum efficiency if iron loss (P_i) = 500 W and full-load copper loss (P_cu) = 800 W?
- a. Full load
- b. 79% of full load
- c. 50% of full load
- d. 62.5% of full load
Q.15 The short circuit test of a transformer is performed to determine:
- a. Iron loss and magnetising current
- b. Copper loss and equivalent impedance
- c. Voltage regulation only
- d. Turns ratio
Questions 16–20: Auto-Transformer & Applications
Q.16 An auto-transformer is NOT suitable when:
- a. Transformation ratio is close to unity
- b. Electrical isolation between primary and secondary is required
- c. Variable voltage output is needed
- d. High efficiency is desired
Q.17 The EMF equation of a transformer is:
- a. E = 4.44 f N Φ_max
- b. E = 2.22 f N Φ_max
- c. E = 4.44 f N² Φ_max
- d. E = 4.44 f N B_max
Q.18 Which cooling method is used for large power transformers (above 30 MVA)?
- a. ONAN (Oil Natural Air Natural)
- b. ONAF (Oil Natural Air Forced)
- c. OFAF (Oil Forced Air Forced)
- d. ODAF (Oil Directed Air Forced)
Q.19 The purpose of Buchholz relay in a transformer is:
- a. To measure oil temperature
- b. To detect internal faults by gas accumulation
- c. To regulate voltage
- d. To measure load current
Q.20 In a transformer, the flux is:
- a. Proportional to supply voltage and inversely proportional to frequency
- b. Proportional to both voltage and frequency
- c. Independent of voltage
- d. Proportional to load current
Answer Key
Detailed Explanations
Q.1: A transformer is a static device with no rotating parts. Therefore, friction and windage losses (mechanical losses) are absent. Copper loss, core loss (hysteresis + eddy current), and stray losses are all present.
Q.2: Maximum efficiency occurs when variable losses (copper loss) equal constant losses (iron/core loss). This is derived by differentiating the efficiency expression with respect to load current and setting it to zero.
Q.3: Laminating the core increases the resistance to eddy currents by breaking the core into thin insulated sheets. This reduces eddy current loss (P_e ∝ t², where t = lamination thickness). Hysteresis loss is unaffected by lamination.
Q.5: Copper loss is proportional to the square of load current. At half load, I = I_FL/2, so P_cu(half) = P_cu(full)/4. Therefore P_cu(full) = 4 × 300 = 1200 W.
Q.8: When V/f ratio is constant (240/60 = 200/50 = 4), B_max remains constant. Hysteresis loss W_h = η × B_max^1.6 × f × Volume. Since B_max is same, W_h ∝ f. So W_h2 = 30 × (50/60) = 25 W.
Q.14: The fraction of full load at which maximum efficiency occurs is x = √(P_iron/P_cu_FL) = √(500/800) ≈ 0.79 or 79% of full load.
Q.18: For very large transformers (above 30 MVA), ODAF cooling is used where oil is directed through specific cooling ducts in windings with forced air cooling on radiators. This provides the most efficient heat removal.
Q.20: From the EMF equation E = 4.44fNΦ_max, we get Φ_max = E/(4.44fN). Since E ≈ V (supply voltage), flux is proportional to V/f.
Frequently Asked Questions
Q: Why is a transformer rated in kVA instead of kW?
Transformer losses depend on current (copper loss = I²R) and voltage (iron loss depends on V/f), not on the power factor of the load. Since kVA = V × I (independent of PF), the rating is expressed in kVA. Read more: Why Transformer Rating is in kVA.
Q: What is the difference between core loss and copper loss?
Core loss (iron loss) consists of hysteresis and eddy current losses — it remains constant regardless of load since it depends on supply voltage and frequency. Copper loss (I²R loss) varies with the square of load current. At no-load, copper loss is negligible while core loss is at its rated value.
Q: How does lamination thickness affect eddy current loss?
Eddy current loss is proportional to the square of lamination thickness (P_e ∝ t²). Thinner laminations mean higher resistance to circulating currents, reducing losses. Standard CRGO steel laminations are 0.27–0.35 mm thick for power transformers.
Q: What is all-day efficiency and why is it important?
All-day efficiency = (Total energy output in 24 hrs) / (Total energy input in 24 hrs). Distribution transformers operate at varying loads throughout the day, so all-day efficiency (which accounts for iron loss running 24/7) is more meaningful than maximum efficiency at a single load point.
Q: Why does a transformer have zero voltage regulation at leading power factor?
At leading power factor, the capacitive component of load current causes a voltage rise across the leakage reactance that compensates for the resistive drop. At a specific leading PF angle, the secondary terminal voltage at full load equals the no-load voltage, giving zero regulation.