Efficiency of a power transformer is of the order of

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Examoogle · Accepted Answer

98%

Explanation: Power transformers are highly efficient electrical devices, typically operating at efficiencies between 97% and 99.5%. This high efficiency is achieved because transformers are static devices (no moving parts) and are designed to minimize both core (iron) and winding (copper) losses.

Working Principle: The efficiency is defined by the ratio of output power to input power. Since transformers do not have friction or windage losses found in rotating machinery, the primary losses are limited to Hysteresis and Eddy Current losses in the core, and $I^2R$ (ohmic) losses in the windings. Designers use high-permeability magnetic materials and optimal conductor sizing to keep these losses at a minimum under rated load conditions.

Key Formulas: $\eta = \frac{P_{out}}{P_{out} + P_{iron} + P_{copper}} 	imes 100$; $P_{loss} = P_{iron} + P_{copper} \approx I^2R + P_{h} + P_{e}$

Key Points: Transformers are static devices, which eliminates mechanical losses.; Efficiency is highest when variable copper losses equal constant iron losses.; Power transformers are designed for near-full load operation.; Losses are minimized using silicon steel laminations and high-conductivity copper windings.

Advantages: Extremely low energy wastage during voltage conversion.; Negligible mechanical wear and tear.; High reliability for long-term power grid operation.

Disadvantages: Efficiency drops significantly at very light loads.; Requires cooling systems for high-capacity transformers to dissipate residual heat.

Applications: Transmission and distribution networks.; Industrial power supply systems.; Substations.

Additional Info: In large utility transformers, the efficiency can even exceed 99%.. 100% efficiency is physically impossible due to the Second Law of Thermodynamics (heat dissipation).. Options like 50% or 25% are representative of extremely poorly designed systems, not functional power transformers.

Answer: B is correct — Power transformers exhibit very high efficiency, typically in the range of 98%, because they lack moving parts and are engineered to minimize electromagnetic losses.

Exam Tip: Always remember that transformer efficiency is maximum when the variable copper loss equals the constant iron loss ($I^2R = P_{iron}$).

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