The Alternator/Transformer has to operate on electrical load, which is continuously varying. Now during the design of the electrical machine such as Alternator/Transformer , the manufacturer doesn’t have idea about the operating condition of the transformer/alternator, at what type of load is it going to operate. And with change in type of load the power factor of the system changes, and there by changes the power factor of Alternator/Transformer.
Now suppose if the manufacturer design the Alternator/Transformer in KW at unity power. But when it will get loaded the power factor will decrease. Which will result in increase in current of Alternator/Transformer here it may have chance that current may increase beyond the rated value.
Which will result in increase of heat, losses and possibility of fire in Alternator/Transformer. Which may damage the machine. Thus to provide protection to Alternator/Transformer they are rated for rated value of voltage and rated value of load current separately.
Therefore their ratings are in VA and for higher value KVA/MVA.
Here VA (volt-ampere) means voltage and current
NOTE. check our another post on IS DC TRANSFORMER POSSIBLE ?
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