The efficiency of a transformer at full load, 0.8 p.f. lag is 90%. Its efficiency at full load, 0.8 p.f. lead will be

• A. Less than 90%
• B. 80 %
• C. 90 %
• D. More than 90%

Answer: (C)

Efficiency= output power/input power
efficiency = (v*i*cosφ)/((v*i*cosφ)+(x² Pc)+(Pi)) x=fractional part of full load