The alternators are designed
to supply a specific voltage to the various loads. This voltage is
called its rated terminal voltage denoted as VL. The power
drawn by the load depends on its power factor. Hence instead of
specifying rating of an alternator in watts, it is specified in terms of
the maximum apparent power which it can supply to the load. In three
phase circuits, the apparent power is √3VL IL,
measured in VA (volt amperes). This is generally expressed in kilo volt
amperes and is called kVA rating of an alternator where IL is
the rated full load current which alternator can supply. So for a given
rated voltage and kVA rating of an alternator, its full load rated
current can be decided.
Consider 60 kVA, 11 kV three phase alternator.
In this case kVA rating = 60
........ 10-3 to express the product in kilo volt amperes
... 60 = √3 x 11 x 103 x IL x 10-3
... IL = 3.15 A
This is the rated full load current of an alternator. But load current
is same as the armature current. So from kVA rating, it is possible to
determine full load armature current of an alternator which is important
in predicating the full load regulation of an alternator for various
power factor conditions. Similarly if load condition is different than
the full load, the corresponding armature current can be determined from
its full load value.
Note : Ia at half load = 1/2 x Ia at full load. It reduces in the same proportion in which load condition reduces.
Hence regulation at any p.f. and at any load condition can be determined.
