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Why is the transformer rated in KVA?

Asked by: Interview Candidate | Asked on: Sep 6th, 2007

Editorial / Best Answer

Answered by: NagaseshaReddy.M

Answered On : 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 - 25 of 25 Answers
NagaseshaReddy.M

Answered On : 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.

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chinmay4u

Answered On : 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*PowerFactorAnd as the manufacturer of Transformer don't have anyideaaboutPFoftheuserthey rated the Transformer in kva so that according to the users PF the rating will become the KW.

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prathibha.B

Answered On : 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.

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amarnath.nalla

Answered On : Dec 22nd, 2007

View all answers by amarnath.nalla

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

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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.

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Because transformer's iorn loss is depend on voltage and core loss is depend in current i.e volt amper. their is nothing to do with power.

  
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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

Answered On : Aug 31st, 2009

View all answers by subasree

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.

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Transformer rated in VA bcoz its powerlosses depends only on voltage & current not on power factor. Copper loss depends on current & iron loss depends on voltage.

  
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surendra kumar

Answered On : 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

Answered On : Oct 2nd, 2011

Maximum load of that transformer some volts and amps = power

  
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rama krishna

Answered On : 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

Answered On : 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

Answered On : 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

Answered On : 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)

Answered On : 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

Answered On : 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

Answered On : Jun 10th, 2013

Load power factor is not known.

  
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atul sudhakar

Answered On : 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

Answered On : 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

Answered On : 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

Answered On : 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

Answered On : 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

Answered On : 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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