Page 565 - Basic Electrical Engineering

When copper loss at a particular load equals the core loss, efficiency will be

maximum at that load. The condition for maximum efficiency of a
transformer is

Core loss = Copper loss

The value of the load current at maximum efficiency is determined as

If we want to know at what percentage of full-load, the efficiency will be
maximum, (ie if we would like to know if efficiency is maximum at 75% of

the full load or at 80% of the full load or at 100% of the full load) we can
determine as follows.

Determination of load at which efficiency will be maximum

Let us assume that x is the fraction of the full load at which the efficiency is
maximum. The core loss or the iron loss W will remain constant at all loads.
i
The copper loss will vary as the square of the load.

This means if at full load copper loss is = W cu

at half of full load copper loss