Why are transformers rated in KVA and Not Kilowatts?

I Need to know why transformers are rated in KVA and not KWatts? Can someone help me please with a detailed description?Why are transformers rated in KVA and Not Kilowatts?A volt Amp or VA is a unit of power roughly equal to a watt....but not exactly.



In AC circuits if the load is more capacitive or inductive the current can lead or lag behind the voltage curve so there is a difference between true power and apparent power. True power divided by apparent power gives the power factor. So a transformer is rated on what it has to deliver in actual voltage and current in order to deliver the true power, and this minimizes possible misunderstanding, misuse, and over-stressing a transformer by calculating its requirements by the actual power delivered to a highly reactive load.





.Why are transformers rated in KVA and Not Kilowatts?You need to know the difference between real power and apparent power.



Apparent power is the actual voltage times the actual current but this current might be used in charging a capacitor which then discharges back into the supply in the next half cycle.

So no real power is dissipated.



Motors and transformers can also get a magnetic field which also holds energy until the next half cycle delivers it back to the mains.

So no real power is dissipated.



However, all this current has to be provided by the transmission lines and transformers. So the transformers have to be rated in KVA where the VA is just the product of the voltage and the current.



Real power is dissipated (and charged for ) when the Voltage and Current happen at the same time each instant even though they are both varying. This is measured in Kilowatts and the Electric Company would like as much as possible of the Volt Amps passing through their system to be Watts.Why are transformers rated in KVA and Not Kilowatts?Both the primary and the secondary windings and the core have to be designed on the basis of the full voltage and current which the transformer will experience. Voltage goes into determining insulation thickness, core cross section and numbers of turns. Current provides the basis for the copper cross sections of windings. So basically the capability (rating) of the transformer is determined by Volts x Current i.e. VA or kVA.



The kilowatts it transforms are normally lower than that number by the power factor.