Question

Distinguish between power efficiency and all day efficiency why is all day efficiency considered more reasonable basis for comparison than ordinary efficiency.

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

Output to watts / Input in watts

But there are certain types of transformers whose performance cannot be judged by thin efficiency.

Transformers used for supplying lighting and general net work i.e., distribution 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 arc loaded.

Low iron loss is required when units are continuously excited but supply loads only intermittently

Low Cu loss is required where the load factor is high.

Again for a transformer working on full load the greater part

for a transformer working on full load for the greater part of the day, maximum efficiency should be arranged to occur somewhere around the full-load value but for a transformer whose full-load value may be supplied for only 1 of the day and the unit is only lightly loaded for the rest of the time, it would be desirable to arrange maximum efficiency to occur at about 1/2full-load value.

This indicate that the efficiency of a transformer is better estimated on an energy rather than a power ratio and thus we have the term "all-day efficiency" which can be defined as

All-day efficiency = Output in kWh for 24 hours / Input in kWh for 24 hours