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Autotransformer kVA Advantage

A two-winding 200V/100V, 1.0 kVA transformer is reconnected as an auto-transformer of 100V/300V. What will be the rating of auto-transformer?
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Answer & Explanation
Correct AnswerA. 1.5 kVA

Quick Explanation

The original two-winding transformer is 200/100 V, 1 kVA. Reconnected additively for 100/300 V service, only part of the output power is transformed electromagnetically and the remainder is transferred conductively, increasing the usable kVA rating. Applying the autotransformer rating relation gives the value in option A.

Formula / Key Relation

Sauto = Stw × Vtotal/Vseries (for the stated connection)
Autotransformer kVA Advantage explanatory diagram

Detailed Explanation

Correct Answer

A — 1.5 kVA

Concept and Reasoning

The original two-winding transformer is 200/100 V, 1 kVA. Reconnected additively for 100/300 V service, only part of the output power is transformed electromagnetically and the remainder is transferred conductively, increasing the usable kVA rating. Applying the autotransformer rating relation gives the value in option A.

Calculation / Key Relation

Sauto = Stw × Vtotal/Vseries (for the stated connection)

Exam Takeaway

Autotransformer kVA exceeds the original two-winding kVA because part of the power is conducted directly.

Common Mistake

Identify which winding is the series portion before applying the kVA multiplication factor.

Quick Trick

Autotransformer kVA exceeds the original two-winding kVA because part of the power is conducted directly.

Common Mistake

Identify which winding is the series portion before applying the kVA multiplication factor.

Exam Tip

Autotransformer kVA exceeds the original two-winding kVA because part of the power is conducted directly.

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