Answer / gans

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.

Answer / hazari sharma

primary and secondary voltage are rated on accont of rated current flowing both winding that,why cu losses which depands on current and iron loss which depands on voltage and easily calculate that'swhy t\f rating done in kva.

Answer / sumesh raut

As we know that the cu loss of depends upon current and iron
loss depends upon voltage so total loss of transformer
depends upon voltage and current i.e(VI)or (KVA) and doesnot
depends upon angle between them i.e(cos(phi)).So the rating
of transformer is in (KVA).

Answer / rockyxyz

the only answer to this question is that the power factor
of load is not known while designing (it is leading or
lagging) so transformers are rated in kVA. The required
size of X-mer depends on the power factor of load connected
to it.

Answer / prabas

transformer rating only in KVA because copper losses depends
upon current(I^2R) and iron losses depends upon voltage.
iron losses are 2 types 1)eddy current losses 2)hysterises
losses so total losses depends upon I,V not on phase angle
between i and v.so t/f rating always in KVA not in KW.

Answer / manikandan

Copper loss depands on current and Iron loss depands upon
the voltage.hence the total loss depands upon only VA. not
a phase angle between voltage and current . ie it is
independant of load power factor .Thats why rating of
transformer given in KVA and not in KW. It may be further
point out that temp. rise of the transformer is directly
proportional to apparent power which flow thought it.

Answer / dipak sonkusare

Transformer is a static machine. As there are two types of
losses - Cu loss and Core loss. Cu loss depends on current
where as core loss depends on voltage and power factor does
not matters. Also the connecting load is unknown where it
will supply the power i.e., it may be inductive /
capacitive / resistive load. Hence the transformer rating
indicated in KVA instead of KW.

Answer / vamsidhar

copper loss depends on current(amp) and iron loss depend on
voltage(volt) ,hence transformer rating is in kva

Answer / ss

rating of any machine depends on temparature rise of the
machine ,temparature rise depends on losses in the machine
,in t/f losses r independent of power factor they depends on
voltage and current,but in the case of other machine losses
depends on pf

Answer / rajesh

we know
output=Input-Losess. 1

and the losess in transformer are two types ohmic, core
losess and these are depends on the Voltage and secondary
current , independent of P.F.

so the rating of losess is in VI not VICOSPIE.From equation
1 losess in KVA so rating of T.F also in KVA.

Eg.if any machine connected at noload, the output put is
zero, because of no current from below equation.
VICOSPIE.

But in T.F there will be some power cosumption inform of
loss.i.e. core loss.

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