why generator rating in kva instead of 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). The ratings of all electrical equipments are all rated in KVA or Kilo Volt Amp. But the practical use is in KW. We can say it also that electrical equipments are rated in apparent power (KVA or VA) other then active power (KW or W). Electrical equipments rated in KVA is due to the variety of power factor rating in consumer side.

KVA is independent of power factor. The KW depends on Power factor of the consumer utility. The trend of electrical rating in terms of VA(volt amp), KVA(kilo volt amp), MVA(mega volt amp) is due to the variation in the consumer side. In consumer or user side the electrical parameters are so varieties that a manufacturer can not cope of to all these variation. So the safe side is to rate the equipment in KVA. The main criteria of the consumer side is variation in power factor you can not predict it. And KVA rating has no terms or influence of power factor as it is the direct multiplication of voltage and current.

On the other hand the kw is calculated by power factor which a manufacturer can not predict. With KVA rating we can get the rated value of voltage and current. Say a small motor rated as 2KVA, we can get that its maximum current is 5amp with 400volt. Say a high capacity power plant alternator is rated by 12MVA, 0. 80 power factor. We can get the MW value with simple calculation (12*0. 80=9. 6) that is 9MW. So we can get it that the KVA rating is not fully sufficient, we need the power factor also if it is applicable.

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