If a transformer is fed from a 220V and dc supply rather than a 1-phase ac supply, then the transfor

Question

If a transformer is fed from a 220V and dc supply rather than a 1-phase ac supply, then the transformer will

Examoogle · Accepted Answer

burn its windings

Explanation: If a transformer is connected to a DC supply, it fails to function as a transformer because there is no change in magnetic flux. Since a DC voltage presents only the winding resistance to the circuit (ignoring the near-zero inductive reactance), a massive, excessive current flows, leading to the burning of the windings.

Working Principle: In AC, the impedance $Z = \sqrt{R^2 + X_L^2}$ limits the current, where $X_L = 2\pi fL$. With DC, the frequency $f=0$, making $X_L=0$. The current is then limited only by the extremely low DC resistance ($I = \frac{V}{R}$), leading to massive Joule heating ($I^2R$) which incinerates the transformer insulation and windings.

Key Formulas: $I_{DC} = \frac{V}{R}$ — Current under DC supply limited only by resistance; $X_L = 2\pi f L$ — Inductive reactance which becomes zero at $f=0$

Key Points: Transformers require time-varying flux to induce EMF according to Faraday's Law.; DC produces a constant magnetic field, inducing zero voltage in the secondary.; The primary winding acts as a short circuit due to the lack of inductive reactance.; The resulting high current causes rapid thermal destruction.

Additional Info: Transformer windings have very low DC resistance because they are designed for high efficiency in AC operation.. Option C is technically true as it won't operate as a transformer, but Option A is the catastrophic physical outcome, making it the most appropriate choice in engineering exams.

Answer: A is correct — The transformer will burn its windings due to the high current resulting from the lack of inductive reactance under DC conditions.

Exam Tip: Always remember that transformers are AC devices; a DC supply effectively treats the primary as a low-value resistor connected directly across the source.

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