The transformer ratings are usually expressed in terms of

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kVA

Explanation: Transformer ratings are expressed in kVA (kilo-Volt-Ampere) because the total power handled by the transformer depends on both voltage and current, independent of the power factor of the connected load. Since the transformer losses (copper losses and iron losses) are dependent on voltage and current (and not phase angle), the kVA rating provides a universal limit for the device.

Working Principle: The internal power losses of a transformer consist of iron losses (dependent on voltage and frequency) and copper losses (dependent on current). Because the manufacturer cannot know the power factor of the load the transformer will eventually serve, they define the capacity based on the product of rated voltage and rated current, i.e., $S = V 	imes I$. This ensures that regardless of whether the load is resistive, capacitive, or inductive, the device stays within its thermal design limits.

Key Formulas: $S = V_{rms} 	imes I_{rms}$ — The apparent power (kVA) representing the vector sum of active and reactive power.; $P = S 	imes \cos\phi$ — The relationship between Apparent Power (S) and Active Power (P).

Key Points: Transformer iron losses are a function of voltage.; Transformer copper losses are a function of current ($I^2R$).; Total losses depend on both $V$ and $I$, hence the rating is in kVA.; The power factor ($cos\phi$) is determined by the load, not the transformer itself.

Advantages: Uniform rating for all load types; Safeguards against overheating due to overcurrent

Disadvantages: Does not explicitly specify the active power (kW) capacity without knowing the power factor

Applications: Power distribution systems; Impedance matching in signal circuits

Additional Info: kW (Kilowatts) represents active power, which changes based on the load's power factor.. If a transformer were rated in kW, it would be incorrect for varying loads as the current would exceed safety limits at low power factors.

Answer: D is correct — Transformer ratings are expressed in kVA because their thermal limits are determined by the product of voltage and current, irrespective of the load power factor.

Exam Tip: Always remember: If you are asked why machines (transformers/alternators) use kVA but motors use kW/HP, remember that motors drive mechanical loads (fixed work requirement), whereas transformers are static devices whose primary constraints are thermal (V and I).

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