Which of the following is correct for a step down single-phase transformer?

Question

Examoogle · Accepted Answer

The correct answer is Option C: Input volt ampere = output volt ampere. An ideal transformer is a lossless device where the power transferred from the primary side to the secondary side remains constant. Therefore, the input volt-ampere (VA) is equal to the output volt-ampere (VA), assuming 100\% efficiency.

Step-by-Step Solution & Concepts:
An ideal transformer is a lossless device where the power transferred from the primary side to the secondary side remains constant. Therefore, the input volt-ampere (VA) is equal to the output volt-ampere (VA), assuming 100\% efficiency.

Working Principle: The transformer operates on Faraday's law of electromagnetic induction. When an alternating voltage is applied to the primary, it produces a magnetic flux that links the secondary winding, inducing a secondary voltage. In an ideal case, the product of voltage and current at the primary side must balance the product of voltage and current at the secondary side to satisfy the law of conservation of energy.

Key Formulas: $V_1 I_1 = V_2 I_2$; $\frac{V_1}{V_2} = \frac{I_2}{I_1} = K$; $S_{	ext{in}} = S_{	ext{out}}$

Key Points: A step-down transformer reduces voltage but increases current proportionately.; The product of voltage and current (Volt-Ampere) represents the apparent power.; For an ideal transformer, losses like copper loss ($I^2R$) and core loss are assumed to be zero.; In practical transformers, input VA is slightly greater than output VA due to internal losses.

Advantages: Efficient voltage level transformation; Galvanic isolation between primary and secondary circuits

Disadvantages: Practical transformers have winding resistance leading to copper losses; Hysteresis and eddy current losses in the magnetic core

Applications: Power distribution systems; Voltage level adaptation for electronic devices

Additional Info: The input VA is defined as $V_1 I_1$ and output VA as $V_2 I_2$.. Option A, B, and D are incorrect as they violate the principle of power conservation in an ideal transformer model.

Answer: C is correct — In an ideal single-phase transformer, the input volt-ampere is equal to the output volt-ampere.

Exam Tip: Remember that while Power (Watts) stays nearly constant in ideal cases, practical transformer efficiency $\eta$ is defined as $\frac{	ext{Output Power}}{	ext{Input Power}}$. Always check if the question implies an 'ideal' or 'practical' scenario.

Examoogle · Answer

Option A: Input volt ampere ≠ output volt ampere

Examoogle · Answer

Option B: Input volt ampere > output volt ampere

Examoogle · Answer

Option D: Input volt ampere

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