[Three Reasons] Why Transformer is Rated in KVA


Today we are going to know the Three Reasons Why Transformer is Rated in KVA, not KW or KVAR.
The transformer is the most important electrical machine which transfers electrical power. If you do not know how a Transformer works then you can read our article Working of a Simple transformer. The link is given at the end of the article.

Three Reasons Why Transformer Ratings in KVA, Why Transformer Ratings in KVA, not KW.


The three reasons why transformer ratings in KVA are given below,


Why Transformer is Rated in KVA.





(1) KVA= KV(killo Volt) * A(ampere)
KW= KV(killo Volt) * A(ampere) * Power factor

We already know that Power factor only depends on the type of Loads like,
  • Inductive Load - Lagging Power factor
  • Capacitive Load - Leading Power Factor
  • Resistive Load - Unity Power Factor
 The Transformer is not a Load, it is a device which can transfer power not consume power. If you think that a Transformer is a Load that's wrong. So as the transformer does not consume power it can only transfer power with increasing and decreasing voltage and current that is why the Transformer always rated in KVA.




(2) When the transformer is designed the manufacturer does not know which type of load will be connected in future. And the power factor depends upon the load. If an Inductive Load is connected then the current will be lag which flows through the secondary winding of the transformer as well as the primary winding of the transformer.



Three Reasons Why Transformer Ratings in KVA, Why Transformer Ratings in KVA, not KW.



We also know that pure inductive and pure capacitive load does not practically exist. Every load has some resistance even it inductive or capacitive. For example, a motor is connected to the transformer which is inductive + resistive. So the motor draws both reactive(KVAR) and active power(KW). So the power supplied by the transformer is the vector sum of reactive(KVAR) power and active power(KW) that is KVA.




(3) The last reason is power loss. Most of the people think that is the main reason "Why Transformer Ratings in KVA" but I will say that is not the main reason. The first and second reason is more important which is described above.
The Copper loss(I2R)  occurs due to the flow of the current in the transformer winding and the Iron or core loss occurs due to the voltage. These losses do not depend on the power factor so that is why the transformer rating in KVA not KW.

These are the Three Reasons Why Transformer is Rated in KVA.




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