Why is the transformer rated in KVA?

Editorial / Best Answer

Answered by: NagaseshaReddy.M

  • Sep 10th, 2007


kVA is the unit for apparent power. Apparent power consists of active and reactive power. Active power is the share of the apparent power which transmits energy from the source (generator) to the user. Reactive power is the share of the apparent power which represents a useless oscillation of energy from the source to the user and back again.

It occurs when on account of some »inertia« in the system there is a phase shift between voltage and current. This means that the current does not change polarity synchronous with the voltage. But the heat generated in a winding as well as the eddy current losses generated in a transformer core depend on the current only, regardless of whether it aligns with the voltage or not.

Therefore the heat is always proportional to the square of the current amplitude, irrespective of the phase angle (the shift between voltage and current). So a transformer has to be rated (and selected) by apparent power.

Showing Answers 1 - 29 of 29 Answers

NagaseshaReddy.M

  • Sep 10th, 2007
 

kVA is the unit for apparent power. Apparent power consists of active and reactive power. Active power is the share of the apparent power which transmits energy from the source (generator) to the user. Reactive power is the share of the apparent power which represents a useless oscillation of energy from the source to the user and back again.
It occurs when on account of some »inertia« in the system there is a phase shift between voltage and current. This means that the current does not change polarity synchronous with the voltage. But the heat generated in a winding as well as the eddy current losses generated in a transformer core depend on the current only, regardless of whether it aligns with the voltage or not.
Therefore the heat is always proportional to the square of the current amplitude, irrespective of the phase angle (the shift between voltage and current). So a transformer has to be rated (and selected) by apparent power.

chinmay4u

  • Sep 25th, 2007
 

Before knowing why Transformer is rated in KVA, you should know What is the correct rating for Transformer. Actually Transformer should be rated in KW.But the reason for not rating it in KW reather then KVA is ,

            KW=KVA*PowerFactor

And as the manufacturer of Transformer don't have any idea about PF of the user they rated the Transformer in kva so that according to the users PF the rating will become the KW.

prathibha.B

  • Oct 4th, 2007
 

Any device is rated depending on losses of the machine. In a transformer the losses are iron losses which depends on voltage and copper losses which depends on current so there are no losses which depend on power factor hence transformer is rated in KVA.

because,ironloss is depends on voltage and cu_loss
depends on current.so the transformer ratings in kva.since both the losses
viz copper loss(depends on current) and iron loss(depends
on voltage) are independent of power factor, that is why a
Transformers rating is not on kW, but on KVA

WE know the relation of power in three phase P=root of three VIcos*.hence we can say that the load is not constant it very accordingly.so power factor is also get vary with load it is hard to define the power of transformer so it is rated in KVA.

Transformer is rated in KVA because in transformers flux is constant so to measure power we depends upon the voltage and current so we rated in KVA.
Depending upon this  transformers are also called as FLUX CONTROLLED MACHINE

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subasree

  • Aug 31st, 2009
 

kVA is the unit for apparent power. Apparent power consists of active and reactive power. Active power is the share of the apparent power which transmits energy from the source (generator) to the user. Reactive power is the share of the apparent power which represents a useless oscillation of energy from the source to the user and back again. It occurs when on account of some inertia in the system there is a phase shift between voltage and current. This means that the current does not change polarity synchronous with the voltage. But the heat generated in a winding as well as the eddy current losses generated in a transformer core depend on the current only, regardless of whether it aligns with the voltage or not. Therefore the heat is always proportional to the square of the current amplitude, irrespective of the phase angle (the shift between voltage and current). So a transformer has to be rated (and selected) by apparent power.

surendra kumar

  • Sep 27th, 2011
 

There are 2 losses in transformer. One is copper loss which depends on current and the other is iron loss which depends on voltage. These two factors are not affected by the power factor. This is why transformers are rated in KVA and not KW. Single phase KVA = Amps x Volts/1000. Single phase KW = Amps x Volts x pf/1000. 3 phase KVA = Amps x Volts x 1.73/1000. 3 phase KW = Amps x Volts x 1.73 x pf/1000

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Ranjith

  • Oct 2nd, 2011
 

maximum load of that transformer some volts and amps = power

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rama krishna

  • Oct 29th, 2011
 

generally that which type T/F set ? depending on we require load
power = voltage * current , P = V I
in this V = voltage , I = current NOT equal V and I sum angle so define T/F rated k v a

constant flux and constant frequency

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Manoj Goyal

  • Nov 11th, 2011
 

Because in kva means reactive power in t/f reactive power occurs that's why t/f is rated in kva

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Prasanna

  • Jan 26th, 2012
 

While any medium will take the electric charge and consumption of some unit of power is called Kilo watt (KW), Example tube light..other than the medium will take the power again it will produce charge to the conductor is called KVA, ex. Transformer

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sanjay saini

  • Feb 16th, 2012
 

In transformer, there are two types of loss-
1. cu losses and
2. Core losses

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DHARMESH SHUKLA (rdec gzb)

  • Apr 25th, 2013
 

If the load in a transformer will change then the copper loss also change because the current in transformer also change.
Poh=I^2=KVA^2
this expression show that iron loss and core loss depend on the supply voltage and it will be dependent of the phase angle between voltage and current hence rating of transformer is in KVA

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dhananjay kumar

  • May 3rd, 2013
 

since core loss depend on voltage and copper loss depend on current .so,product of these two is volt ampere.
it is independent of load power factor

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Anshu

  • Jun 10th, 2013
 

Load power factor is not known.

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atul sudhakar

  • May 2nd, 2014
 

as it included both copper loss and iron loss so to calculate both of them it is rated as in KVA rating

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it is rated in volt amp because we need to take take both useful power, i.e. active power and non useful power(reactive power ) into consideration while we rate the amount of load it can take, so that we can estimate about the losses that might occur due to presence of interfering reactive power

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ravi

  • May 18th, 2014
 

rating defines the maximum limit within which a device works satisfactory,the maximum limit of transformer and many other electric machines depends on a certain temperature. over this temperature the insulation of transformer get damaged.as the temperature rise is due to losses, and the transformer losses depends on current and voltage, not on the phase difference between them.hence transformer is rated in terms of apparent power

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YUVARAJA

  • Jul 2nd, 2014
 

An efficiency of the transformer depends on core loss(related to voltage) and copper loss(which relates to current),so the transformers are rated in volt ampere

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RITU

  • Jul 4th, 2014
 

An efficiency of the transformer depends on core loss(related to voltage) and copper loss(which relates to current), and that does not depend on power factor so the transformers are rated in volt ampere

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kalyan sundar dalai

  • Jul 5th, 2014
 

transformer loss depends on voltage & current and its efficiency is depends its losses obviously its rating in kva.

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ashutosh

  • Sep 5th, 2014
 

Because of core losses and iron losses as core losses are measured in amps and cu losses in voltage ....this is definatly reason for the rating

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venkat.y

  • Nov 7th, 2014
 

Iron losses of transformer are depend on supply voltage and copper losses on load current. Therefore total losses are depends on V and I, not on power factor.

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TRUSHAR UPADHYAY

  • Nov 15th, 2014
 

Here the copper loss depends on current and iron loss depends on voltage. Therefore the unit comes out V and A. Therefore it is independent from power factor and transformer rating measured in kVA.

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DEVANG MISTRI

  • Nov 18th, 2014
 

in a.c system the load bearing capacity of a transformer varies in accordance with the power factor and thus kw is not a stable rating that is why they are rated in kva.

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laxman

  • Dec 17th, 2014
 

kva/ kv in transformer means how much maximum apparent or total power a transformer supply. At the time of transformer manufacture type of load doesn't know. so that why its rating in kva

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