Main advantage of auto transformer over two windings transformer is that

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Examoogle · Accepted Answer

it uses only one winding

Explanation: An autotransformer is a transformer that uses a single continuous winding shared by both the primary and secondary sides. This common winding provides both electrical conduction and magnetic induction, allowing for a more compact and efficient design compared to the two-winding construction found in conventional transformers.

Working Principle: In an autotransformer, the secondary voltage is obtained by tapping a portion of the single winding. Because the common portion of the winding carries the difference of the primary and secondary currents, the required cross-sectional area of the conductor is reduced, leading to significant savings in copper material and lower leakage reactance.

Key Formulas: $k = \frac{V_2}{V_1}$ — Transformation ratio; $W_{auto} = W_{2w}(1 - k)$ — Weight comparison

Key Points: Uses a single winding with a common tap point.; Provides physical electrical connection between primary and secondary circuits.; Smaller size and higher efficiency due to reduced copper and iron usage.; No electrical isolation between high and low voltage circuits.

Advantages: Lower cost and lighter weight; Higher efficiency due to lower losses; Lower leakage reactance; Improved voltage regulation

Disadvantages: No galvanic isolation; Higher short-circuit current stress; Hazard of high voltage appearing on secondary if common winding fails

Applications: Variable voltage supply (Variac); Starting induction motors; Boosting voltage in transmission lines

Additional Info: The primary advantage is economic: the autotransformer requires significantly less copper and iron for the same kVA rating.. The core losses are generally reduced because of the reduced size, but the defining structural feature is the single shared winding.

Answer: B is correct — An autotransformer uses a single winding as both the primary and secondary, which inherently reduces the amount of copper and core material required.

Exam Tip: Always remember that autotransformers provide no galvanic isolation, which makes them unsuitable for applications where safety or isolation from the supply is a primary design requirement.

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