Transformer Loading Efficiency and Losses
The efficiency and losses of a transformer at a given load: output kW = kVA x load x PF, total loss = the fixed no-load (core) loss plus the load^2 x full-load (copper) loss, efficiency = output / (output + loss). A 75 kVA unit with 200 W core and 1,200 W copper loss at 75% load runs 98.47% efficient, with peak efficiency near sqrt(200/1200) = 41% load; a lightly loaded oversized transformer suffers because the core loss becomes a bigger fraction of a small throughput. A design/screening aid; the manufacturer's test report governs.
Formula and source
output = kVA x load x pf; losses = (noload_w + load^2 x loadloss_w)/1000; efficiency = output/(output+losses); peak at load = sqrt(noload/loadloss).
Transformer loading efficiency and losses, standard power-transformer practice (IEEE C57 loss definitions) by name; first-principles core-plus-copper loss model.
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