Generator is producer of Voltage and Current, we can
connect load as such draining full load current or half
load, even noload,but voltage remain moreover same.So it
willl rated in terms of product of V*I, secondly it gives a
electrical output for any Load(may Capacitor,Inductor &
resistor),so it is consider as Procuct of Voltage & Current
Whereas motor is machines coverts electrical Energy to
Mechanical energy, so it is rated in KW
Generator and transformer are rated in KVA becoz at that
time we dont know the consumer power factor as we know that
the (KW= KVA X Cos Phi) as we know the power factor we have
ratings in KW and on the other hand motor has defined power
factor and the rating has been mentioned in KW for example
if we have transformer of rating 50KVA this rating of
transformer is its apparent power that is KVA if we
multiply the same withh the power factor 0.85 we will get
the real power that is KW
Generator is a source of power. It includes active as well
as reactive power. It deliver only active power while
working on unity power factor. It deliver reactive power
only while connected to a purely reactive load. It can
deliver both of these powers while connected to a composite
load. This signifies that the current limit is the main
factor here. You can draw a purely rective current of rated
value or purly active current of rate value. combination of
output depends upon the load but the maximum limit is
When you are drawing both of the currencts active and
reactive, in such condition how much the alternator can
deliver? it can be descibed by KVA only not by KW. That is
why alternators are rated in KVA.
Motors are rated in KW only because for motors the load is
mechanical and thus only active power which is converted to
mech. is significant.The power factor of the motor is
independent of the load and at whatever powerfactor it
works it is due to its desing.
Generator rating is given in kVA,as generator is the souce
and delivers power to load. Load may be resistive,inductive
or capacitive.A 10 MVA generator will deliver 10 MW load if
power factor is unity,however if the power factor of the
load is 0.8 then generator can deliver active power up to
8MW only.It is up to user who efficienly generator power
may be utilised. Therfore, manufacturer always specify the
generator rating in KVA or MVA.
On the other hand, motor converts electrical energy into
mechanical energy.Therfore,its rating is given in KW as
power factor of the motor remain constant.
The rating of generator is described in terms of KVA by its
manufacturer because the generator will produce a constant
voltage on which it is designed and the current will depend
on the nature of load,for same load it delivers more
current for poor power factor and less current for unity
power factor, since the manufaturer does not know the
nature of load he desribed its rating in terms of voltage
While in case of motor, motor have fixed power factor.
We all know that the factor "Power factor" comes only when
the voltage & current waves are travelling at a time
difference. This will happen due to the
inductive /capacitive nature of the load(Depends how the
load receives the power from the sourcei.e whether voltage
first (inductive)or current first(capacitive).
But at the time of producing the power, voltage and current
waveforms are starting from the source at the same time
only. Hence the question of power factor does not arise...
Any device is rated depending on losses of the machine. In
a transformer the losses are iron losses which depend on
voltage and copper losses which depend on current so there
are no losses which depend on power factor hence
transformer is rated in KVA.the same thing applies for
generators and trasformers are rated i kva.kw=kvaxcos
(phi).the manufacture didn't no custmor load(in kw).one
more answer iron losses depends on supply voltage and cu
losses on current.so taht kvxi=kva
bcz the output in kw. is dependent on p.f. and kva o/p.
independent of p.f.for example asingle phase alternator
drawing 50 amp atunity p.f. at 230v has an o/p. of
(50*230)=11.5kw at unity p.f: (50*230*0.6)=9.6kw.at
0.8 p.f.and (50*230*0.6)=9.6kwat 0.6 p.f. but the kva
rating of alternator is (230*50)=11.5kva.
We wish to test 60 Hz systems from a 50 Hz supply. To
acheive frequency conversion, we use a VFD. The VFD powers
40 kVA transformer, which has motor and non-motor loads on
it. Will the transformer work?