The transformer efficiency is defined as the ratio of output power to input power. It is denoted by ἠ.
As the output power is always less than the input power due to losses in the transformer, practically the transformer efficiency is always between 0 and 1 i.e. 0% and 100% but it can never be 1 of 100%.
The efficiency of an ideal transformer is equal to 1 or 100% since the losses in the ideal transformer are zero.
transformer efficiency formula
The graph of output power versus transformer efficiency is shown in the figure. The figure shows that the efficiency increases with the increase in the output power up to a certain value and after a particular value of output power, the transformer efficiency decreases.
The value of transformer efficiency will be maximum when the copper losses will be equal to iron losses in the transformer. The value of maximum efficiency can be found by taking total losses equal to 2Pi. It also depends on load power factor and has the maximum value at a power factor of unity.
The transformer on which load is variable (like distribution transformer) is designed to give maximum efficiency at about 75% of full load.
And if it is continuously operated near the full load (like
power transformers), then it is designed to give maximum efficiency at or near the full load.

Transformer Efficiency Formula

transformer efficiency formula image
The transformer has no moving parts so the losses due to friction and windage are absent therefore its efficiency is very high. It can be at least equal to 90%. Its output and input are almost of the same value. Hence their ratio cannot be found accurately by measuring input and output power.
To overcome this problem it is better to measure the transformer losses separately and then find the transformer efficiency by the transformer efficiency formula. The iron losses and copper losses of can be determined very easily and accurately by no-load test and short-circuit test on transformer respectively.

All Day Transformer Efficiency

The efficiency discussed so far is the ordinary or commercial or power efficiency of the transformer. But for the distribution transformer, it does not give the true idea about the transformer performance because the load on distribution transformer fluctuates throughout the day. This transformer is energized for twenty-four hours, but for the major portion of the day, it delivers the very light load.
Thus iron losses take place for the whole day but copper losses take place only when the transformer is loaded.
Hence, the performance of such transformer (like distribution transformer) cannot be judged by the power efficiency. But it can be judged by the special type of transformer efficiency known as energy efficiency or all-day efficiency. The all-day efficiency is computed on the basis of energy consumed during the period of twenty-four hours.
The all day transformer efficiency is defined as the ratio of output energy (in kWh) to input energy (in kWh) for twenty-four hours.
To find all day transformer efficiency, we have to know the load cycle of the transformer.
Thanks for reading about transformer efficiency formula.

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