why transformer rating in kva and motor rating in kw

All electrical devices that supply power are usually rated in kVA. This includes transformers, UPS's & the like. Simply put, the difference between kVA & kW is that kVA is "Apparent Power", which does not take into account the Power Factor & kW is "True Power". See the below equations:
The above equations are only for single phase. When a manufacturer makes a transformer, generator, UPS etc, they have no idea of the type of load that will be used & consequently can only rate the device according to its maximum current output that the conductors can safely carry (at untiy Power Factor) & the insulation rating of the conductors (voltage & temperature).

Example. 500 x 100 = 50 000 VA (or 50 kVA). If the Power Factor of any given load was 0. 5, the "True" power output of the transformer would be 25 kW: 500 x 100 x 0. 5 = 25 000 W (or 25 kW).

Now since Iron losses depend upon the voltage and copper losses on current so we can infer that the total loss of a transformer is dependent on 2 values that are voltage and current but not on the phase angle between voltage and current which is also known as power factor. KVA does not include the term power factor in it while KW does.

So since the total losses is dependent only on Voltage and Current values (not on power factor), the ratings of the transformers are also given in terms of VA or KVA. This question was asked by one of my visitors using the. If you have any similar queries related to electrical and electronics engineering, do comment below. I would be glad to answer your queries.

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