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# why do we rate transformers in KVA not in KW ?

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#### munzir

##### Full Member level 6
why do we rate transformers in KVA not in KW ?

because of :

1) we don't know the pF of the load
2) its custom / customary
3) due to the VA losses in the transformer

which one ?

HI,,

the reason is the Power of the load and its powerfactor.\\not all about the copper and core losses because we know that transformer is one of the most efficient electrical devices we have.

why do we rate transformers in KVA not in KW ?

because of :

1) we don't know the pF of the load
2) its custom / customary
3) due to the VA losses in the transformer

which one ?
Explaination:
Maximum current carrying capacity of the transformer is determined by the thickness of the conductor and Maximum voltage supplied to the winding depends on the insulation strength of the conductor

Manufacturers of Transformers and alternators does not know at what power factor the consumer uses the machine and the nature of the load (capacitive, resistive and inductive) connected to the machine. Hence they rate the transformer depends on the maximum current carry and the voltage to be applied. This rating of the transformer corresponds to apparent power of the machine (VA). Also iron losses in transformer depends on the the voltage applied and copper losses depends on the current flowing through the winding. These losses are independent of the power factor at which the transformer operates.
On the other hand induction motors are rated with real power (Watts) as the machine operating at defined power factor at full load is pre determined.

https://electricalquestionsguide.blogspot.com/2011/11/why-transformer-rating-kva-not-kw.html

you see transformers are rated in KVA or VA because it supplies both active(P) and reactive power(Q) to load . in case of induction machine it consumes reactive energy not supplies , and synchronous machine supply both active and reactive power so it's rated in KVA(Kilo volt Amp) .
KVA is the unit of Apparent power which is vector addition of active and reactive power (P+jQ) .

active power is utilized as work and reactive energy used to excite field coil in transformer and machine which flows in line and cause addition to line drop . where ever you connect inductive load there comes reactive power .

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