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why transformer ratings in kva?
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Answer Posted / pradyumna chandra panda
Why the motors are rated in KW and transformers are rated in
KVA?
Ans . Motor is a consuming device it consume the power
(wt) so it is rated in KW. But the
Transformer is a static device which transfers
the power (Apparent power) from the
Generating point to the user. Apparent power is
the product of voltage and current
I.e. Increase voltage with decrease current and
decrease voltage with increase
current. Power factor of the load does not
affect apparent power it affect the Active
Power.
Active power is the real power which the transformer
gives. Transformer should not be rated on the base of
Active power (KW) which is variable because the power factor
of the load is not knows before which to be connected across
the transformer.
Reactive power Apparent power (k)
KVA
VI sin Q k
Kw
Q q
Active power VI cos Q
Active power is equal to apparent power at resistivity load
where power factor is unit at cos Q is equal to zero degree.
Active power is decrease power where cos Q (not fi) is
increase
Therefore in simple we can say due to unknown power factor
transformer and generator are rated in KVA.
Hint – we know cos FI is the value of cos Q i.e. apparent
power/ active power
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