I was reading about single phase transformers. I came across these two terms 'full load' and 'rated' and they left me thoroughly confused.

In some places they're treated as the same thing, in others they aren't.

Are they the same thing? I know that rated values are those at which the transformer is meant to be used. If the two terms aren't the same, what is the distinction?

Also, suppose I were to apply rated voltage at primary side, what current and voltage would I get at the load? Would both of them be rated values?

In case the terms are different, what happens at the load when I apply full load voltage at the primary side? (Is full load voltage even a thing?)

Please help


In general, rated values are the values for which the transformer is designed and can be expected to perform as expected over the expected lifetime when operated continuously.

For the primary voltage, the transformer will overheat if operated continuously with a voltage that exceeds the rated value. It can tolerate 5% or 10% above rated, but the lifetime will be reduced somewhat. At some level of voltage, the transformer will fail quickly. If the primary voltage is below rated, the secondary voltage will be below rated, but otherwise, there is no problem for the transformer. The term "full-load" does not really apply to the primary voltage.

For the secondary, the full-load voltage voltage is applied to the load when the rated or full-load current is drawn. At less than rated current, there is less internal voltage drop in the transformer, so the voltage will be a little higher.

Exceeding the rated secondary current for very long will cause the transformer to overheat. For both the primary and secondary, full-load and rated current mean the same thing.

  • \$\begingroup\$ So I take that the two terms refer to the same thing? \$\endgroup\$ – AkaiShuichi Nov 27 '19 at 23:26
  • \$\begingroup\$ They are almost the same, but there are contexts in which it makes more sense to use one than the other as I attempted to explain but apparently didn't quite succeed. \$\endgroup\$ – Charles Cowie Nov 28 '19 at 1:54

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