📘 Auto Transformer – Theory, Copper Saving & Numericals
Auto transformer uses a single winding for both primary and secondary. It saves copper and improves efficiency. Very important for GATE.
🔹 1. Construction
Single continuous winding with tapping. Portion common to both primary and secondary.🔹 2. Working Principle
Power transfer occurs in two ways:- Partly by induction
- Partly by direct conduction
🔹 3. Voltage Ratio
V₁ / V₂ = N₁ / N₂
Same as normal transformer.🔹 4. Copper Saving
Copper used in two-winding transformer: Proportional to:(N₁ I₁ + N₂ I₂)
Copper used in auto transformer: Proportional to:(N₁ − N₂) I₁
Percentage copper saving:Copper saving = (1 − 1/a) × 100%
Where: a = Voltage ratio🔹 5. Example 1
Step-down transformer: 2200 V / 200 V Find copper saving. Turns ratio:a = 2200 / 200 = 11
Copper saving:= (1 − 1/11) × 100 = (1 − 0.09) × 100 = 91%
Huge copper saving.🔹 6. Advantages
- Less copper
- Smaller size
- Higher efficiency
- Better voltage regulation
🔹 7. Disadvantages
- No electrical isolation
- Not suitable for high voltage isolation needs
🔹 8. Applications
- Voltage control
- Starting induction motors
- Interconnecting systems with small voltage difference
🎯 GATE Important Points
- Copper saving formula frequently asked
- Best when voltage ratio close to 1
- No isolation between primary & secondary
- Higher efficiency than two-winding transformer
Auto Transformer = Economical Transformer
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