Answer / minal

Transformer are always rated in Kva because there are 2
losses in transformer.
one is copper loss which depends on current & the other is
iron loss which depends on voltage & not affected bypower
factor.
Thus transformer rating in kva & not in kw.

Answer / md. jafar iqbal

The total losses of a Transformer= Core loss+Copper loss.
Core loss depends on voltage and copper loss depends on
current that means total losses depends on current & volage
but not depends on their phase angle.There is no relation
between the total loss of a transformer and the change of
power factor. This is why rating of a transformer is
written KVA not KW.

Answer / integratorman

Transformer losses are related to the efficiency of the
transformer. The Power factor of a transformer depends on
the load attached to it. if that load is resistive, power
factor is usually considered 1, in which case KVA = kW. if
the load is inductive or capacitive, then power factor
should be taken into consideration when sizing the
transformer.
Hence the transformer is designed for voltage and current
output(VA) - how you apply the transformer determines how
many watts there is in the circuit.

Answer / amolkarne

x'mers,UPS,DG etc are rated in KVA beacause at the time of
manufacturing manufacturer is unknown about the type of
load (inductive,capacitive or resistive)that perticular m\c
is going to suypply.so there is no question of pf to be
considerd to give it's rating in KW...........
KVA = KV X Amps -(1)
KW = KV X Amps X P.F. -(2)

it gets more clear by looking towards above formulae

Answer / blake

The KVA rating of a transformer takes into account the
power factor of the supply system. i.e. If you have a 100kW
load on your system with a supply power factor of 0.8 then
your transformer will need to be at least 125kVA. p.f =
KW/KVA. So to answer the question a KVA rating allows for
the fact that differnt installations can have different
power factors.

Answer / ramesh kumar k

All answers explains two different reasons for this..
Both are correct...Mr. Integratorman's answer gives good
explanation for Mr. Blake's answer.. This reason is not
explained in many text books.. These two people are also
right... along with others..

Answer / nibel

transformers are rated in kva coz we cant judge the type of
load(either inductive or capacitive) used by us. so we cant
find the power factor on the load side. finally no one use
kw only kva, (Kw=kva*pf)

Answer / devi

Transformer rating written in always kva because depends
upon the voltage and current rating.

Answer / rudra

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 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.

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, Therefore the heat is always proportional to the square of the current amplitude, irrespective of the phase angle. So a transformer has to be rated by apparent power.

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