8 Feb 2012

Why transformers are rated in KVA not KW

Reason why transformers are rated in KVA not KW.

While enjoying electrical engineering you must have come across transformers and might have noticed that the power ratings of the transformers are in KVA not in KW which is conventionally used for denoting power ratings in electrical devices.

Well in this post I am going to tell you why the transformers are rated in KVA but not in the conventional KW manner. Before going into this topic you need to know about the various kinds of losses in transformers. Well there are 2 kinds of losses in transformers, they are:

  1. Iron losses
  2. Copper losses
750 KVA Power Transformer


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 contact page of my blog. 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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21 comments:

  1. Induction motor is also a generalized transformer,and it also has core losses and ohmic losses which depends upon voltage and current respectively and independent of pf.Then why induction motors are not rated in KVA but in KW?
    Hoping for an early response :)

    ReplyDelete
    Replies
    1. the diff is that even though both are elec. equipments, Xformer is considered as power source & induction motor as a load .
      in the case of power source you cannot predict the type of load (R,L,C) that will be connected to it. Hence the actual usage capacity rating depends upon the power factor of the load.P= ViR
      if the load is resistive pf=1, for inductive <1.
      in the oter case i.e the load yhe manufacturer gives the rated voltage & full load current taking into acct. the pf.

      Delete
  2. Why do we connect neutral of a transformer(11kv/220v) with earth?If it would not occur perhaps we may not get shock because circuit between transformer and us not completed.

    ReplyDelete
    Replies
    1. This has been done due to Fault.. If there would occurs fault any time, there will be flow of over current. To take take save from that it has been grounded.

      Delete
    2. This ground has been done due to Fault condition. If there occurs any fault any time there will be flow of over current. To save it from fault current this ground has been done.

      Delete
  3. Why we short neutral and earth terminal at transformer(11kv/220v)?
    If it would not so perhaps we may not get shock on touching phase wire of the transformer because circuit would not be complete.

    ReplyDelete
  4. Anonymous12/01/2012

    what do u need to do to be able to connect or link two substations

    ReplyDelete
    Replies
    1. Substations are mainly used to take a connection line to our home. We need different voltage at diff. places. for example we need high voltage/power in the industrial area. To make up these kinds of voltage to supply we need to set up diff. kinds of substation at diff. place.

      Delete
    2. Very interesting blog.I like this blog.Thanks for sharing this information.electrical transformers

      Delete
  5. Anonymous1/08/2013

    Please i'm having a tough time with this question on transformers and i need your help.
    The efficiency of a 250kVA, single phase transformer is 96% when delivering full load at 0.8 p.f. and 97.2% when delivering half full load at unity p.f. Determine the efficiency at 75% of full load at 0.8 p.f. lagging.

    ReplyDelete
  6. Anonymous1/11/2013

    The main reason is the transformer does not have the P.F. it consist of apparant power & there is no change in the phase difference between the vtg & current hence it is in KVA......

    ReplyDelete
  7. why the transformer ratings are in multiples of 11,22,44 etc like that?

    ReplyDelete
    Replies
    1. its because of form factor which is 1.11

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  8. Thanks for sharing this article about transformer. For your article people can know about KVA and KW.

    ReplyDelete
  9. why battery not discharge rapid bt capcitor is rapid disch

    ReplyDelete
    Replies
    1. I am not clear about your question. What do you want to ask?

      Delete
  10. Correct me if i am wrong but KVA in
    transformers stands for kilo watt ampere.right?

    ReplyDelete
  11. Why motor ratings in kw instead of kva? Plz rply

    ReplyDelete
    Replies
    1. hi Atanu,i see you are confused about kva and kw.just keep in mind that where ever there is a generation point of electricity its not loaded with anything so its calculated in kva,but when we put on the load on to it then we calculate it on kw because of the power at the consuming end,if you are not consuming the electricity then there is no kw.

      Delete
  12. there is another reason for giving the transformer rating in KVA instead of KW is that the manufacturer donot know at what power factor that transformer is used .................................................................................................. so he rates it in KVA

    ReplyDelete
  13. there is another reason for rating T/F in KVA instead of KW because the manufacturer donot know at what powerfactor it is being used ................................so he mentions only KVA but not KW................................

    ReplyDelete

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