why transformer rating is always in KVA
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Answer Posted / aditi
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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