Answer / karthik

Watt is the product of voltage, current and power factor.
VA is the product of voltage, current and power factor.

Motor is an inductive load and operates at a power factor
less than unity. So the current drawn by it is more than the
apparent at constant power.
For a 3ph motor, P=√3*V*I*cosø where V,√3 are constants.
At constant P, I*cosø is also a constant. if pf cosø
varies, I too varies. i.e. if pf is poor (cosø reduces), I
increases. Hence rating needs to be specified in KW.

Whereas for a generator, it isn't a load. It supplies power
to a load. So power factor doesn't come into picture. Hence
its rating is specified in KVA.

Answer / shahulsnfa

See, KVA is apparent power which means the power
available. and KW is real power which means how much power
utilised from available power.

motors are utility machines which consumes the power
from the source for its working. and the generator are
producers and not consumers. the motor consumes the power
in terms of voltage and current.

when phase angle difference btwn current and voltage
is zero, then KW=KVA. but the phase angle difference
between current and voltage will have some value regarding
load. so the real power is determined by the load and not
by the source.

that is why macines are rated in KW and generators are
in KVA. and the transformers are transforming power rather
than consuming.. so they also rated in KVA.

Answer / navmit bhimani

In transformer there are two losses,copper loss depend on
current and iron loss depend on voltege and nothing
relation with power factor.so and so....

Answer / vinamra

generator and the transformer is not a load,means its power
fector is not decided,its pf is depend upon the loads which
will connect to the gen. or trans.,therefore it rating is
in kva.but in case of motor its power fector is predecided
so its rating is in kw

Answer / pillay

We buy/fix a motor to do a specific job.Job done is measured in watts.So a machine say a pump will use up to 3 KW to run as designed than we fix a motor of 3KW or slightly bigger to run it.Now the motor is 3KW or bigger say 30 KW and the voltage is AC or DC operated is up to you.

generators are rated in KVA because we dont know at what pf will it be operated. Say you have fitted a pf correction board to maintain at 0.95pf, you dont have a guarantee it wont breakdown or take some time to adjust. What is important the generator voltage is constant and the generator dont trip or burn.So if the pf can change anytime the current also can change anytime this may make your generator to trip on overcurrent,overvoltage(leading pf)or even destroy your genset.By rating your generator by KVA with a constant designed voltage you can know the maximum current the genset can give irrespective of pf.It is a safer and sure way of protecting your generator.you can even load yor genset at 0.2 pf so long you dont exceed the rated amps or KVA.

Answer / expert

when kva is calculated pf comes into play, and not in KW.
as we know that the tranf and DG pf is dependent on the
load,but the motor draws an constant power with specified
pf..........if not clear plz reply

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