The efficiency
of a transformer is defined as the ratio of output power (in watts) to
input power (in watts).
Thus, transformer
efficiency formula is given by
where,
V2 = load voltage
I2 = load
current
Cos θ2 = load power factor
PC = transformer core loss
PI = transformer copper (ohmic) loss = I22
Re2
Re2
= equivalent transformer winding resistance referred to secondary or load side
Since transformers are almost always operated at rated voltage
and rated frequency, therefore, making V2 and Pc constant
in above formula. Thus, transformer efficiency depends only on load power
factor cos θ2 and load
current I2 , as long as transformer is operated at rated voltage and
rated frequency.
If load current I2 is kept constant and power factor
is varied, then, maximum efficiency would be obtained when load power factor is
unity.
Variation of transformer efficiency with load power factor is
shown as
Maximum Transformer Efficiency
If the load power factor cos θ2 is kept constant but
load current I2 is varied ,
then , the condition for maximum efficiency is obtained as follows
we have,
i.e Core loss = Copper
loss
or, constant loss =
Variable loss , is the condition for maximum efficiency.
Load
current I2 at which maximum efficiency occurs is given by
By putting Pc =
I22Re2
in efficiency formula, we get transformer maximum efficiency as
In practical situations, both load current and load power factor
vary ( when we turn on and off domestic appliances etc ) , therefore,
transformer efficiency variation with load power factor and load current
together is shown below.
