The all-day efficiency of a transformer is defined as the ratio of ______.

All-day efficiency

The all-day efficiency of a transformer is defined as the ratio of kWh output in 24 hours to the kWh input in 24 hours.

It is calculated on the basis of energy consumed during a period of 24 hours.

Need for all-day efficiency

• Some transformer efficiency cannot be judged by simple commercial efficiency as the load on certain transformers fluctuate throughout the day.
• For example, the distribution transformers are energized for 24 hours, but they deliver very light loads for the major portion of the day, and they do not supply rated or full load, and most of the time the distribution transformer has 50 to 75% load on it.
• The iron loss takes place at the core of the transformer. Thus, the iron or core loss occurs for the whole day in the distribution transformer.
• The second type of loss is known as copper loss and it takes place in the windings of the transformer and is also known as the variable loss. It occurs only when the transformers are in the loaded condition.
• Hence, the performance of such transformers cannot be judged by commercial or ordinary efficiency, but the efficiency is calculated or judged by all-day efficiency which is computed by the energy consumed for 24 hours.