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Why Transformer Rating In kVA, Not in KW?

 In Simple words, 
There are two type of losses in a transformer;
1. Copper Losses
2. Iron Losses or Core Losses or  Insulation Losses
Copper losses ( I²R)depends on Current which passing through transformer winding while Iron Losses or Core Losses or  Insulation Losses depends on Voltage.
That’s why the Transformer Rating may be expressed in kVA,Not in kW.Why Transformer Rating In kVA, Not in KW?
Also read here about : 
Transformer
How to Calculate/Find the Rating of Transformer (Single Phase and Three Phase)? 
TRANSFORMER NAMEPLATE (GENERAL REQUIREMENTS).


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46 comments

  1. Great post, now im knowing why tranformer rating in KVA..

  2. Again I am little bit confuse about the unit of Transformer.<br /><br />We write, P=VxI and whatever product of VxI comes out to be, we just denote it with unit &quot;WATT&quot;. Why not &quot;VA&quot; as in Tranformers? <br /><br />then in case of Transformer due to CU loss (I) and Iron loss (V) we write its unit &quot;VA&quot;. Why not &quot;WATT&quot; as in case of P=VI?

  3. this is the question I am asking that why the unit of Apparent power (S)=VI is VA? Why not to write Watts? <br /><br />So what if it is apparent power and we are writing it as VA and in case of active power we write Watts. Why? Couldn&#39;t get the logic behind.<br /><br />I read your post it is really good but unit is still not clear to me logically.

    • Dear Mukesh Khatri@<br />See…. Watt is a unit of real power or actual power…i.e. the power which we utilize in our home [ Electrical Appliances operate on Real Power] <br />in other hand… Apparent Power [ S= VxI = VA and its formula <br />Apparent Power = √ (True power2 + Reactive Power2)] is not real power..its complex power [ … I.e we can&#39;t use Reactive power but only real power in

    • Muhammed Ibrahim

      hi mukesh,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 becuase of the power at the consuming end,if you are not consuming the electricity then there is no kw.<br />

    • Real Power=Voltage*Current*Cos(Pi).Watts<br />Reactive Power=Voltage*Current*Sin(Pi)<br />Apparent Power=Resultant of Real and reactive power so cos(pi) and Sin(pi) will become one.hence we will get apparent power Volt-Amps only

    • it is the totoal power for real and reactive power so it is to the v-i

  4. my question in simple words is how you come to know that P=VxI= VA?<br /><br />If you have the proof of denoting VxI with unit VA then what was the necessity of using unit the VA inspite of Watts as a unit for apparent power? <br /><br />What is the problem with the unit WATT that we don&#39;t mention it as a unit of apparent power when we clearly see it is product of VxI ..

    • Dear Mukesh Khatri@<br />See…. Watt is a unit of real power or actual power…i.e. the power which we utilize in our home [ Electrical Appliances operate on Real Power] <br />in other hand… Apparent Power [ S= VxI = VA and its formula <br />Apparent Power = √ (True power2 + Reactive Power2)] is not real power..its complex power [ … I.e we can&#39;t use Reactive power but only real power in

    • I know, Apparent power(s) is sum of route of Active Power(wattful) and Reactive Power(wattless).<br /><br />I am afraid how does this justify that the unit of Apparent Power will be &quot;VA&quot;? <br /><br />You mean to say it is complex power and we utilize real power not reactive power therefore Apparent power&#39;s unit should be &quot;VA&quot;.<br /><br />Does common sense accept this

    • Look…There are three kinds of Power. Active Power, Re-active Power and Apparent power. now use common sense,,,is it right to just use the same unit for these three different quantities. The same question rises here that we also know that Re-active power = P = VxI…But the unit is in VARS….Why??? Should was not it in Watts. Also this is not a technical answer [ as we have already given

    • See @Mukesh…Apparent power is the product of voltage(V) &amp; current(I). Now, the unit of voltage is volt &amp; unit of current is ampere. So the unit of apparent power is volt-ampere.<br /><br />Now to get back to watts…when the apparent power is further multiplied with power factor(cosine of the angle between voltage &amp; current in an AC circuit) the result is active power which is

    • great sir tq now 1ly iam clear about this tx a lot <br />

  5. great work by your team

  6. great explanation….hats of to u…….<br />

  7. dear sir;<br /> I need a clarification regarding..<br /> 1.why we should not connect earth to the ground?<br /> 2.how to calculate the bus bar size?

  8. Fist the understading P=VA=WATT is wrong,<br />P=VA Cos(phi) = watt for AC power<br />S=VA = Apparent Power <br />One can write P=VA only withe explanation that power factor Cos (phi) is unity.<br /><br />For DC curcuit P=VA=Watt since there is no power factor as well as no apparenat power.<br />

  9. W and VA are the same, they are both power. VA unit is just used for apparent power to discriminate from the real power unit (W), if we use the unit watt(W, for both apparent and real power, how would somebody what power you are referring to? this is to avoid confusion. The same logic on big engines, they have this ratings: BHP, KW

  10. Why motor rating is in KW? not in VA..

    • simply put: its bcoz of power factor, the mptor is at the load side hence PF is taken into consideration, when u check on the name plate you will find the following rated parameters:<br />Voltage<br />Amperage<br />Power Factor….thus<br />Voltage*Amperage*PF =Kw

  11. Why motor rating in kw nt in kva,there are also cu and iron loss

  12. please help to solve this problem:<br /><br />A 440V, 60hz, six-pole, 3-phase induction motor is taking 50kVA at 0.8 power factor and is running at a slip of 2.5 %. The stator copper losses are 0.5kW and rotational losses are 2.5kW. Calculate: <br />a) the rotor copper losses<br />b) the shaft power<br />c) the efficiency<br />d) the shaft torque

  13. whats a explanation by faruq sir…you clear the concept of watt and volt-amp

  14. If i want to install a 250 kva generator set, how can i choose the rating of the transformer t be installed? Can anybody help me please!

  15. Pls Answer this Q…. What is the current of an 237V, 328.4kW three-phase load with cosFI=0.99 ?

  16. P=VI<br />Power=Voltage*Current

  17. Quote &quot;we can&#39;t use Reactive power but only real power in watts but we pay for both real and reactive power&quot;<br />Electrical Technology kindly clarify, i read somewhere that our energy meters only charge us based on the Real power(Kw) consumed bt here you&#39;ve stated that even the reactive power is paid for….

  18. Can you please explain the effect of low/high impedance of transformer?

  19. why we need to connect neutral to earth in a Transformer??

  20. why in theTransformer, Neutral is Connected to Earth??

    • One thing, since the earth is a good conductor, it can be used as a common line in distribution system. So the metal usage for the neutral wire can be minimized by earthing the neutral wire in many places of a distribution network.<br />The other thing is safety related. The leakage current of consumer equipments and machinery can be bypassed to earth only if the neutral point of the transformer

  21. i like to know about multiplication factor

  22. The answer to the common man:<br />The power indicated in Watts (W) is the power consumed by an equipment.<br />But the power indicated in Volt-Ampere (VA) is the capacity of an equipment to deliver the power.<br />You can connect a 1000W heater to a 1KVA transformer. Here the transformer is just transferring 1000W power to heater.

  23. very nice explenation

  24. why the rating of generator and transformer are same plz explain the real logic behind it..

  25. Transformer rating Depends upon the power factor of the load but this is not specified at the time of manufacturing so rating of transformer and generator in KVA but in motor nameplate manufacturer always mentioned the pf thats why its rating is in kw.

  26. Tranformer rating depand on the cu-losses and core(iron) losses.So tranformer rating has been kVA.

  27. Great post thank u

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