Why this topic is important for ECET?
Transformers are one of the most frequently tested concepts in ECET (EEE). The equivalent circuit helps us analyze performance, efficiency, and losses, while regulation determines how much the secondary voltage changes under load conditions. Understanding these makes it easy to solve numerical problems in Power Systems, AC Machines, and Utilization, which account for 20–25% of marks.
📘 Concept Notes
1. Equivalent Circuit of a Transformer
The real transformer is not ideal – it has winding resistance and leakage reactance. To simplify analysis, we draw its equivalent circuit:
- Primary side elements: Resistance
and Leakage Reactance 
- Secondary side elements: Resistance
and Leakage Reactance 
- Core losses: Represented by resistance
- Magnetizing reactance: Represented by
Usually, all elements are referred to one side (primary or secondary) using the turns ratio 
.
Final simplified equivalent circuit:
- Series resistance:
- Series reactance:
- Core loss branch:
2. Voltage Regulation of Transformer
Definition: It is the percentage change in secondary voltage from no-load to full-load, keeping primary voltage constant.
- If load is lagging (inductive) → Regulation is positive.
- If load is leading (capacitive) → Regulation may become negative (improved voltage).
Approximate formula (per unit):
Where:
= secondary current
= load power factor- “+” for lagging, “–” for leading
3. Example
A 1ϕ transformer has 
, 
, 
, 
, power factor 0.8 lag.
⚙️ Formulas
- Turns ratio:
- Referred resistance:
- Referred reactance:
- Voltage Regulation (exact):
- Approx Regulation:
🔟 10 MCQs
Q1. The equivalent circuit of a transformer includes:
a) Only leakage reactance
b) Only resistance
c) Resistance, reactance, core loss, magnetizing reactance
d) Only magnetizing reactance
Q2. Voltage regulation of a transformer depends on:
a) Winding resistance
b) Leakage reactance
c) Load power factor
d) All of the above
Q3. A transformer has 
, 
. Find regulation.
a) 4.76%
b) 5%
c) 10%
d) 2%
Q4. In lagging PF loads, regulation is:
a) Negative
b) Positive
c) Zero
d) None
Q5. A 1ϕ transformer has 
. Find %VR.
a) 8%
b) 12%
c) 10%
d) 20%
Q6. Maximum regulation occurs at which power factor?
a) Unity PF
b) Lagging PF
c) Leading PF
d) Zero PF
Q7. For a capacitive load, regulation is:
a) Positive
b) Zero
c) Negative
d) Infinite
Q8. The shunt branch in equivalent circuit represents:
a) Leakage reactance
b) Winding resistance
c) Core loss and magnetization
d) Copper loss
Q9. A transformer has 
. Regulation = ?
a) 0%
b) 100%
c) 50%
d) None
Q10. Which test is used to determine equivalent circuit parameters of a transformer?
a) OC and SC test
b) Load test
c) Hopkinson’s test
d) No-load test only
✅ Answer Key
| Q.No | Answer |
|---|---|
| 1 | c |
| 2 | d |
| 3 | a |
| 4 | b |
| 5 | c |
| 6 | b |
| 7 | c |
| 8 | c |
| 9 | a |
| 10 | a |
🧠 Explanations
- Q1: Equivalent circuit has all elements → (c).
- Q2: VR depends on resistance, reactance, and PF → (d).
- Q3:
→ (a). - Q4: Lagging loads cause positive VR → (b).
- Q5:
→ (c). - Q6: Regulation maximum at lagging PF → (b).
- Q7: Leading PF can cause negative VR → (c).
- Q8: Shunt branch → core loss (Rc) & magnetizing reactance (Xm) → (c).
- Q9: Same no-load and full-load voltage → VR = 0 → (a).
- Q10: OC & SC test gives equivalent circuit → (a).
🎯 Motivation / Why Practice Matters
For ECET 2026, transformer questions appear in both objective theory and numerical form. Mastering equivalent circuits & regulation ensures you can solve OC/SC test problems, efficiency calculations, and power system applications in seconds. Speed + accuracy here will save crucial exam time and boost your rank.
📲 CTA
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