Day 7 ECET 2026 Night – Transformers: Equivalent Circuit & Regulation

Concept Notes

1. Equivalent Circuit of a Transformer

The real transformer has losses and leakage, so we represent it with an equivalent circuit that closely matches practical performance.

• Primary Side Parameters:
  • = Primary winding resistance
  • = Primary leakage reactance
• Secondary Side Parameters:
  • = Secondary winding resistance
  • = Secondary leakage reactance
• Magnetizing Branch (Core losses):
  • = Core-loss resistance (hysteresis + eddy current losses)
  • = Magnetizing reactance (accounts for magnetizing current)

👉 To simplify, secondary parameters are referred to primary side (or vice versa) using turns ratio .

• Referred secondary resistance:
• Referred secondary reactance:

Final Equivalent Circuit:
Series part:
Shunt branch:

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2. Voltage Regulation of a Transformer

Voltage regulation is the change in secondary voltage from no-load to full-load, expressed as a percentage of full-load voltage.

Formula:

In terms of equivalent circuit parameters:

• 
• 
• = Power factor (lagging or leading load)
• Lagging load:
• Leading load:

👉 Good transformer design: Voltage regulation should be low (near 0%), ideally negative under leading PF.

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Example:
A 100 kVA transformer has , load PF = 0.8 lagging, current = 500 A.

(too high just for illustration; in reality <10%).

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⚙️ Formulas

• Turns ratio:
• Referred resistance:
• Referred reactance:
• Equivalent resistance:
• Equivalent reactance:
• Voltage regulation:

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🔟 10 MCQs

Q1. The equivalent circuit of a transformer is used to:
a) Reduce losses
b) Represent practical performance
c) Increase efficiency
d) None

Q2. The shunt branch in transformer equivalent circuit represents:
a) Winding resistance
b) Leakage reactance
c) Core losses & magnetizing current
d) Load

Q3. If turns ratio and , then referred resistance = ?
a) 0.4 Ω
b) 2 Ω
c) 10 Ω
d) 25 Ω

Q4. The series part of equivalent circuit represents:
a) Core losses
b) Leakage & winding drops
c) Magnetizing current
d) Open-circuit test

Q5. Voltage regulation of an ideal transformer is:
a) Zero
b) Unity
c) Infinite
d) Negative

Q6. A transformer has lagging. Find %VR.
a) 1.5%
b) 2%
c) 3%
d) 5%

Q7. Which load condition can give negative voltage regulation?
a) Unity PF
b) Lagging PF
c) Leading PF
d) No-load

Q8. If , find .
a) 1Ω, 2Ω
b) 2Ω, 4Ω
c) 3Ω, 2Ω
d) 2Ω, 1Ω

Q9. A transformer with better design should have:
a) High leakage reactance
b) Low regulation
c) High copper loss
d) None

Q10. Thevenin’s equivalent of transformer secondary is useful in calculating:
a) Efficiency
b) Regulation
c) Short circuit current
d) All

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✅ Answer Key

Q.No	Answer
1	b
2	c
3	d
4	b
5	a
6	c
7	c
8	b
9	b
10	d

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🧠 Explanations

• Q1: Equivalent circuit models actual transformer → (b).
• Q2: represent core loss & magnetizing current → (c).
• Q3:

→ (c).

Q7: Leading PF can make VR negative → (c).

Q8: → (b).

Q9: Good transformer = low VR → (b).

Q10: Equivalent circuit helps find all parameters → (d).

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🎯 Motivation / Why Practice Matters

Transformers are the backbone of power systems. ECET 2026 will definitely test equivalent circuits and regulation because they connect theory to practical performance (like voltage drop in distribution). Practicing these problems ensures you can handle numerical + conceptual mix questions quickly.

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