Question.

Distinguish between Power and all day efficiency.

All-day Efficiency

The ordinary or commercial efficiency of a transformer is given by the ratio

Output in watts / Input in watts

But there are certain types of transformers whose performance cannot be judged by this efficiency,

Transformers used for supplying lighting and general network i.e., distribution Transformers have their primaries energised all the twenty-four hours, although their secondaries supply little or no-load much of the time during the day except during the house lighting period.

It means that whereas core loss occurs throughout the day, the Cu loss occurs only when the transformers are loaded.

Hence, if is considered a good practice to design such transformers so that core losses are very low.

The Cu losses are relatively less important, because they depend on the load.

The performance of such is compared on the basis of energy consumed during a certain time period, usually a day of 24 hours.

Efficiency(all day) = Output in KWh / Input in KWh [ for 24 hours]

This efficiency is always less than the commercial efficiency of a transformer.

To find this all-day efficiency or (as it is also called) energy efficiency, we have to know the load cycle on the transformer is, how much and how long the transformer is loaded during 24 hours. Practical Calculations are facilitated by making use of a load factor.