Answer / parth belani

The efficiency can be found by finding the losses. because,
efficiency= output/input = output/(output+losses)
Now there are mainly two types of losses in a transformer.
1) Iron losses which can be found by the "No load" test.
2) Copper losses which can be found by the "Full load"
or "Short Circuit" test.
by knowing the losses, and the output rating of the
transformer, we can find efficiency.

Answer / sumanta & arup

from efficiency curve or oc & sc test

Answer / sunil kumar

The efficiency of transformer can be predetermined by open
circuit and short circuit test.in case of open circuit test
the high voltage side is kept open and rated low voltage is
applied to the primary(for ex:25mva,230(p)/115(s),230 kept
open and115v applied to secondary winding and the
wattmeter accounts for iron losses.in case of shortciruit
test the lv winding is shorted and the hv winding is
energised because in order to limit current drawn from the
source and the wattmeter accounts for full load copper
losses.in addition to above tests sumpners test or back to
backtest is most accurate where two identical transformers
are loaded simultaneously it gives the actual temp rise in
the transformers wherwe as oc and sc tests can't.

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