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# Transformer loss, efficiency and voltage reduction

Transformers also consume a little bit of energy, which causes iron and resistance loss. The transformer's load does not affect iron loss, but resistance loss increases as the current increases.

Iron loss (Pr) is the sum of hysteresis loss (Ph) and eddy current loss (Pr).

Resistance loss (copper loss Pk) is power loss formed in the transformer's primary (R1) and secondary coil (R2) resistance due to primary and secondary current (I1 and I2).

They can be defined using the following formula:

Pk = (I12 x R1) + (I22 x R2)

Efficiency of power refers to how much power the transformer gives out and how much is uses, or the ratio of output power P2 and input power P1 .

The transformer's efficiency of power η is:

η = P2 / P1

The transformer's secondary voltage is stated in the nominal load. If the secondary load is smaller than the nominal load, the transformer's resistance loss is smaller and the secondary voltage is higher than the nominal secondary voltage. In other words, the transformer produces the highest secondary voltage in idle running.

The difference between no-load voltage and loaded secondary voltage depends on the size and structure of the transformer. It varies between 5 and 25 %. The difference between no-load and loaded voltage is larger with small transformers than larger ones. Transformers are classified using standards that give a certain fluctuation range for no-load voltage. When designing a transformer for a specific purpose, the difference between no-load voltage and nominal load voltage can be minimised.

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