Page 564 - Basic Electrical Engineering

Efficiency,

or,

where W = iron loss or core loss
i
and W = copper loss in primary and secondary windings
cu
V = output voltage
2
I = load current
2
cos ϕ = load power factor.
2

The load at which efficiency will be maximum, that is the condition for
maximum efficiency, can be determined by differentiating the expression for
efficiency with respect to the load current, I .
2

6.14 CONDITION FOR MAXIMUM EFFICIENCY
The expression for η after dividing the numerator and denominator of the

right-hand side of the expression for η by I can is written as
2

where R″ is the equivalent resistance of the transformer windings referred to
e
the secondary side. Efficiency η will be maximum if the denominator is
minimum. We can, therefore, minimize the denominator by differentiating it

with respect to the load current, I and equating to zero as
2