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How to Calculate/Find the Rating of Transformer in kVA (Single Phase and Three Phase)?

 How to Calculate/Find the Rating of Transformer in kVA (Single Phase and Three Phase)?
We know that, Transformer always rated in kVA.  Below are two simple formulas to find the rating of Single phase and Three phase Transformers.
Rating of Single Phase Transformer:
P = V x I.
Rating of a single phase transformer in kVA
kVA= (V x I) / 1000
Rating of a Three Phase Transformer:
P = √3. V x I
Rating of a Three phase transformer in kVA
kVA = (√3. V x I) /1000
But Wait, A question is raised here… Look at the General nameplate rating of a 100kVAtransformer.
Transformer name plate and rating.
Did you notice something????Anyway, I don’t care what is your answer J  but lets me try to explain.
Here is the rating of Transformer is 100kVA.
But Primary Voltages or High Voltages (H.V) is 11000 V = 11kV.
And Primary Current on High Voltage side is 5.25 Amperes.
Also Secondary voltages or Low Voltages (L.V) is 415 Volts
And Secondary Current (Current on Low voltages side) is 139.1 Amperes.
In simple words,
Transformer rating in kVA = 100 kVA
Primary Voltages = 11000 = 11kV
Primary Current = 5.25 A
Secondary Voltages = 415V
Secondary Current = 139.1 Amperes.
 Now calculate for the rating of transformer according to
P=V x I (Primary voltage x primary current)
P = 11000V x 5.25A = 57,750 VA = 57.75kVA
Or P = V x I (Secondary voltages x Secondary Current)
P= 415V x 139.1A = 57,726 VA = 57.72kVA
Once again we noticed that the rating of Transformer (on Nameplate) is 100kVA but according to calculation...it comes about 57kVA…
The difference comes due to ignorance of that we used single phase formula instead of three phase formula.
Now try with this formula 
P = √3 x V x I
P=√3 Vx I (Primary voltage x primary current)
P =√3 x 11000V x 5.25A = 1.732 x 11000V x 5.25A = 100,025 VA = 100kVA
Or P = √3 x V x I (Secondary voltages x Secondary Current)
P= √3 x 415V x 139.1A = 1.732 x 415V x 139.1A= 99,985 VA = 99.98kVA
Consider the (next) following example.
Voltage (Line to line) = 208 V.
Current (Line Current) = 139 A
Now rating of the three phase transformer
P =  √3 x V x I
P = √3 x 208 x 139A = 1.732 x 208 x 139
P = 50077 VA = 50kVA
Note: This post has been made on the request of our Page fan Anil Vijay.

27 comments:

  1. Why is transformer rating always written in KVA? We know that the unit of Power is Watt. then why don't we write transformer rating in Watts?

    ReplyDelete
    Replies
    1. Dear Mukesh Khatri@
      Read This Article
      Why Transformer rating in kVA and not in kW?
      http://www.electricaltechnology.org/2012/02/why-transformer-rating-in-kva-not-in-kw.html

      Delete
    2. The two types of transformer are core loss and ohmic losses.
      The core loss depends on transformer voltage and ohmic loss depends on transformer current.As these losses depends on transformer voltage and current and are almost unaffected by the load pf, transformer rated output is expressed in VA or in KVa.

      Delete
    3. because the power factor of the load is not known so transformer rating is given in KVA

      Delete
  2. dear in last consideration u shows that line to line volts is 208V then after calculation over value is came 50 KVA but actual value is 100 KVA have. plz Kindly explain it

    ReplyDelete
    Replies
    1. Dear Waqas Ahmed
      Here are two examples.
      The first one is for 100kVA which shown in the image.
      The Second one is for 50kVA (where line to line voltage is 208V)

      Delete
  3. Your article looks great!Thanks for you sharing. I love it.

    High Voltage Transformers

    ReplyDelete
  4. If we dont know the line current,how 2 calculate the rating?

    ReplyDelete
    Replies
    1. It is already given on every transformer's nameplate

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    2. This system arrangement is very common, both at the utilization level as 480 Y/277 V and 208 Y/120 V, and also
      on most utility distribution systems.

      Delete
  5. Anonymous20:33

    kW=kva×pf
    is it right?

    ReplyDelete
    Replies
    1. Yes it is 100% right.
      When considering the apparent power we also considered the effect of inductor and capacitor which always will be minimized by using power factor correction technique.
      Points to be remembered:-
      ->KVA is always greater then KW
      because KW only have the effect of resistor.

      -> KVA=KW if and only if the power factor is one so the load is pure resistive.

      -> Single Phase KVA
      VI/1000

      ->three Phase KVA
      VI*1.73/1000

      ->Single Phase KW
      VI*PF/1000

      ->Three Phase KW
      VI*1.73*PF/1000

      (1000 if conversion factor for Kilo (10^3) & square root of 3 = 1.132051)

      Delete
  6. sir could u explain DYn-11.....

    ReplyDelete
    Replies
    1. Wait for the upcoming posts... Thanks

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    2. http://en.m.wikipedia.org/wiki/Vector_group

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  7. Hello Sir, Can u explain why in transformer,the power factor is not considering.? And why in others.?

    ReplyDelete
    Replies
    1. Correct me if I'm wrong ' The power factor is determined by the load, not the transformer. The PF on the load side of the transformer will be the same as it is on the primary, except at very light loads where the magnetizing current could have some effect. Having said that, the impedance in the primary circuit could effect the PF of the system. In other words, for a given load (with reactance) the system PF depends on the impedance in the primary circuit.'

      Delete
  8. hi sir, how do the transformer designer calculate the turns, emf, loss, flux, area if we say we need1600KVA

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  9. hi sir, how does transformer designer calculate flux, emf, loss, turns if v just say the rating eg,1600Kva if u reply with a example calculation it will be comfortable

    ReplyDelete
  10. Anonymous18:18

    Hi Sir
    what size transformer do I need for 5A,230V Load?

    ReplyDelete
  11. dear 220kv/500kv transmission poll install in the forest.and we don't know the load and the production side.so tell the method of load and production identification?

    ReplyDelete
  12. Anonymous21:44

    hi sir, i want to design a 500VA rating of transformer.but i dont understand how to select the v/g & c/n value? and no. of turn on each side of transformer?
    pls kindely rply me

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  13. Anonymous22:40

    Sir ,why we dont consider power factor for transfrmer but for generator or motor?

    ReplyDelete
  14. Anonymous14:33

    Sir ,why we dont consider power factor for transfrmer but for generator or motor

    ReplyDelete
  15. Dear Sir,
    I want to know about how to select step down transformer rating. 415/230V and VA rating. ..?
    ..I mean how va can be calculated. .based on the which factors. .

    ReplyDelete