Question

A 40 kVA transformer has a core loss of 400 W and full load copper loss of 800 W. The fraction of rated load at maximum efficiency is.​

Answer 1

The important thing is the transformer efficiency is maximum when copper losses (Pcu) are equal to no load losses(Pnl). The formula for the above equality is

x2 * Pcu = Pnl

so x = sqrt(pnl/Pcu) = Sqrt(400/800)

• No-load losses (Core losses) are fixed for a given transformer and Full load losses (copper losses) are proportion to share of load current.