The statements which support the points that auto transformers are advantageous
I. Weight of conduct

Question

The statements which support the points that auto transformers are advantageous
I. Weight of conductor reduces
II. Ohmic losses reduces
III. Leakage reactance reduces
IV. Lower short-circuit current

Examoogle · Accepted Answer

I, II, III

Explanation: An autotransformer is more efficient than a two-winding transformer because it uses a single continuous winding to serve as both primary and secondary, leading to reduced material usage and lower losses. This design results in lower weight, reduced ohmic losses (due to less copper), and improved voltage regulation due to lower leakage reactance.

Working Principle: In an autotransformer, power is transferred through both magnetic induction and electrical conduction. Because the portion of the winding that is common to both primary and secondary carries only the difference between the currents, the total conductor cross-section required is smaller. This physical reduction directly decreases total resistance, leakage flux paths, and weight.

Key Formulas: $W_{auto} = W_{tw} 	imes (1 - \frac{V_2}{V_1})$ — weight of copper relative to two-winding transformer; $I_{common} = |I_2 - I_1|$ — current in the common winding portion

Key Points: Autotransformers are more compact due to the common winding section.; The reduction in conductor material lowers the total resistance, thereby reducing ohmic ($I^2R$) losses.; Leakage reactance is lower because the windings are physically close and share a single magnetic core circuit.; Autotransformers are NOT used when DC isolation is required between primary and secondary.

Advantages: High efficiency; Smaller size and weight; Lower cost of manufacturing; Better voltage regulation

Disadvantages: No electrical isolation between input and output; If the common winding breaks, the full input voltage may appear at the output; Low impedance results in higher short-circuit currents compared to two-winding transformers

Applications: Voltage regulators in power systems; Starting induction motors; Interconnecting systems operating at different voltage levels

Additional Info: Statement IV is incorrect because autotransformers actually have higher short-circuit currents due to their lower leakage reactance.. Because impedance is lower, the fault current under a short-circuit condition is significantly higher than that of an equivalent two-winding transformer.

Answer: A is correct — Statements I, II, and III are valid advantages of an autotransformer, while IV is a disadvantage due to lower impedance causing higher fault currents.

Exam Tip: Always remember that 'lower leakage reactance' results in 'higher short-circuit current', which is often a disadvantage, not an advantage in power systems.

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