Load connected with the transformer always varries i.e. it may be sometime resistive load or inductive load or capacitive load. As a result p.f. sometime unity, sometime lagging or leading and it is not equal. We know that KW=KVA*COSθ As p.f. varries so KW varries. So on the nameplate of the transformer KW rating is not appropriate.Again cu loss of a transformer depends on current and iron loss on voltage. Hence, total transformer loss deoends on volt-ampre(VA) and not on phase angle berween voltage and current i.e. it is independent of load power factor. That is why rating of transformer is in KVA and not in KW.
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