Why rating of transformer in kVA?



Rating of transformer:

      Transformers are generally specified with Apparent power rating rather than active power rating due to following reasons: 

Reason (1) :

  • Rating of a transformer depends upon temperature rise which depends upon the losses taking place in the Transformer.
  • The temperature can be maintained within permissible limits by using proper cooling system.
  • More the effectiveness of cooling system more will be the rating of transformer.
  • For a given cooling system rating of any electrical machine is indirectly determined by the losses present in the machine
  • In a transformer
         Constant losses depends on the voltage rating
         Variable Losses depend upon current rating
         Total losses depends upon v and I ratings
∴ Transformer rating is in terms of v*I which is called as Apparent power rating

Reason (2) :
  • During manufacture stage of the transformer, the manufacturer doesn't know the power factor of a load at watch the transformer is going to be operated. Therefore the manufacturer with the proper design can fix required voltage and current ratings of a transformer without considering power factor of the load. Hence, the rating fixed by the manufacturer is always in the terms of Apparent power only.
    The KVA rating mentioned on the name plate of a transformer represents load-side KVA at FL.
           

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