Which of the following does not change in a transformer?

Question

Examoogle · Accepted Answer

Frequency

Explanation: A transformer is a static electromagnetic device that transfers electrical energy between two circuits through electromagnetic induction. Throughout this energy conversion process, the supply frequency remains constant because the input and output voltages are linked to the same magnetic flux variation rate, which is governed by the source frequency.

Working Principle: The transformer operates on Faraday's law of electromagnetic induction. When an AC voltage $V_1$ is applied to the primary winding, it produces a time-varying magnetic flux $\Phi$ in the core. Since the same core links both windings, the induced EMFs in the primary and secondary coils share the same temporal variation, forcing the output frequency to match the input frequency ($f_1 = f_2$).

Key Formulas: $E = 4.44 f N \Phi_m$ — EMF equation showing dependency on frequency ($f$); $f_1 = f_2$ — Fundamental condition of steady-state transformer operation

Key Points: Transformers are constant-frequency devices.; Voltage and current are scaled inversely based on the turns ratio $a = \frac{N_1}{N_2}$.; Power remains theoretically constant (ignoring losses), hence $V_1 I_1 \approx V_2 I_2$.

Advantages: Efficient voltage level adjustment for power transmission; Provides galvanic isolation between circuits

Disadvantages: Large size for low-frequency applications; Fixed frequency operation only (cannot be used for DC)

Applications: Power grid voltage step-up/step-down; Impedance matching in electronics

Additional Info: Changing the frequency would fundamentally alter the magnetizing current ($I_m = \frac{V}{2\pi f L_m}$) and flux density, potentially leading to core saturation.. Option A and B are incorrect because transformers are specifically designed to change voltage and current levels via electromagnetic induction.

Answer: C is correct — Frequency is the only electrical parameter that remains constant across the input and output windings of an ideal transformer.

Exam Tip: Always remember that transformers act as impedance transformers: $Z_2 = Z_1 \left(\frac{N_2}{N_1}\right)^2$. This is a frequent derivation required in competitive exams.

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