⚡🔌Transformers Quiz 🔌⚡

This quiz series is built around the most important transformer concepts that regularly appear in SSC JE, GENCO, TRANSCO, NTPC, BHEL, and other PSU exams. Here’s what you’ll practice:

• Voltage & Current Ratios – step‑up/step‑down relations, ampere‑turn balance.
• Magnetic Flux Behavior – why flux stays nearly constant, how V/f ratio affects it.
• Core Losses – hysteresis and eddy current losses, their dependence on frequency.
• Frequency Effects – what happens when voltage or frequency changes.
• Open‑Circuit & Short‑Circuit Tests – purpose and what they measure.
• Equivalent Circuit Representation – modeling core losses and leakage reactance.
• Phasor Diagrams – understanding current components and their phase relations.
• Voltage Regulation – conditions for zero or negative regulation at different power factors.

⚡ Transformer Quiz (Q1–12) ⚡

👋 This is Part 1 of the transformer quiz. It covers voltage/current relations, flux, losses, and frequency effects. Practice these 12 questions before moving to Part 2!

1A transformer raises voltage by a factor of 100. The ratio of primary current to secondary current will be? ⚡

a) 1
b) 100
c) 0.01
d) 0.1

Check Answer

Answer: b) 100
Explanation: Power balance requires \( V_1 I_1 = V_2 I_2 \). If voltage is stepped up 100×, current is stepped down 100×. Hence \( I_1/I_2 = 100 \).

2The approximate no‑load power factor of a practical power transformer is? 🔋

a) Unity
b) 0.75
c) 0.5
d) 0.35

Check Answer

Answer: d) 0.35
Explanation: At no load, current is mostly magnetizing (reactive). The real component is small, so PF is very low (~0.3–0.4).

3The magnetic flux in the transformer core under varying load conditions? 🌀

a) Increases with load
b) Decreases with load
c) Remains nearly constant
d) None

Check Answer

Answer: c) Remains nearly constant
Explanation: Flux depends on applied voltage and frequency, not on load. So it stays nearly constant.

4The primary ampere‑turns are balanced by? 🔄

a) Secondary ampere‑turns
b) Primary flux
c) Increase in mutual flux
d) Rise in secondary current

Check Answer

Answer: a) Secondary ampere‑turns
Explanation: Secondary current produces opposing ampere‑turns that balance primary ampere‑turns (Lenz’s law).

5If voltage is increased while keeping V/f constant, the magnetizing current and core loss will? 📊

a) Decrease & remain same
b) Increase & decrease
c) Stay same & increase
d) Both remain same

Check Answer

Answer: d) Both remain same
Explanation: Flux density \( B \propto V/f \). With constant ratio, flux is constant → magnetizing current and core loss unchanged.

6When applied voltage is raised by 50% and frequency reduced to 50%, the maximum flux density becomes? 🌀

a) Three times
b) 1.5 times
c) 0.5 times
d) Same

Check Answer

Answer: a) Three times
Explanation: \( B \propto V/f \). New ratio = \(1.5/0.5 = 3\). Flux triples.

7If voltage is increased by 75% and frequency reduced by 25%, the flux density will? 🌀

a) Sevenfold
b) Threefold
c) One‑quarter
d) Same

Check Answer

Answer: b) Threefold
Explanation: New ratio = \(1.75/0.75 ≈ 2.33\). Closest option = 3× increase.

8A 220/240 V, 50 Hz transformer runs at 220 V, 40 Hz no‑load. The losses will? 🔋

a) Both increase
b) Hysteresis ↓, eddy ↑
c) Hysteresis ↑, eddy unchanged
d) None

Check Answer

Answer: c) Hysteresis ↑, eddy unchanged
Explanation: Lower frequency → higher flux → hysteresis loss ↑. Eddy loss depends on voltage & f², so unchanged.

9A 50 Hz transformer with equal hysteresis and eddy losses at rated excitation is operated at 45 Hz, 90% voltage. Core losses compared to rated are? 🔋

a) 10% lower
b) 19% lower
c) 14.5% lower
d) Unchanged

Check Answer

Answer: c) 14.5% lower
Explanation: Flux constant (0.9/0.9). Hysteresis ∝ f → 10% lower. Eddy ∝ f² → 19% lower. Combined reduction ≈ 14.5%.

10A transformer with eddy current loss of 50 W at 50 Hz, when supplied at 75 Hz, loss will be? 🔋

a) 168.75 W
b) 112.5 W
c) 75 W
d) 50 W

Check Answer

Answer: b) 112.5 W
Explanation: Eddy loss ∝ \( f^2 \). At 75 Hz: \( (75/50)^2 × 50 = 112.5 W \).

11A 1 kVA, 230 V, 50 Hz transformer has eddy loss of 30 W. If excited by DC at same voltage, eddy loss will be? 🔋

a) 30 W
b) More
c) Less
d) Zero

Check Answer

Answer: d) Zero
Explanation: Eddy currents are induced only by alternating flux. With DC, flux is steady → no eddy currents.

12At 50 Hz, hysteresis loss = 200 W, eddy loss = 100 W. At 60 Hz, total core loss will be? 🔋

a) 432 W
b) 408 W
c) 384 W
d) 360 W

Check Answer

Answer: c) 384 W
Explanation:
Hysteresis loss ∝ frequency → at 60 Hz = 200 × (60/50) = 240 W
Eddy loss ∝ frequency² → at 60 Hz = 100 × (60/50)² = 144 W
Total = 240 + 144 = 384 W

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