In power generator sets, when they are coverted from 50hz
to 60hz, the kva rating after conversion increases. For
example, the manufacturer of a perkins 200kva @ 50Hz
generator set has mentioned it to become 220kva after
converstion to 60hz. What is the relation between the
frequency & the KVA rating. Any formula please.

Answer Posted / kj

The main reason for the increase in the kva is that the
diesel engine produces more HP at 1800 rpm than at 1500 rpm.
This allows a higher kw output. The alternator can also
handle the higher current and therefore the kva is
increased. Note that the rated power factor stays the same.
This is because the amount of cooling air is increased at
the higher speed.
The increase in the EMF per turn allows a higher voltage to
be generated but this is reduced by the AVR reducing the
excitation. This has no effect on the kva capacity of the
generator set.

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