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# Transformer rating

Why do we rate transformer in KVA not in KW?

View all questions by Sajjad Hussain Turi

sravz

Answered On : Jun 4th, 2008

since the losses in transformers are mostly due to copperlosses and corelosses  which in turn  depends on  current  and voltage ( ie.,  copper loses is nothing but i^2R losses depends on current and core losses v*i depends on the voltage) hence the transformer ratings are taken in  KVA but not in KW.

1 User has rated as useful.

ashwin.s.zola

Answered On : Jul 12th, 2008

1.cgs- centimeter, gram and second sysytem
mks- metre, kilogram and seconds systems
which means that all quantities are represented in d respective dimensional units acc 2 d sysytem applied

2. i did nt understand ur 2nd q, be a bit more specific

3. thr is no difference betwn earthing and grounding, jus difference terms used in d u.s and england repectively

Transformer rating is always in KVA rather than in KW. it is due to the fact tha the core loss inside the conductor (which are mainly hysteresis loss and eddy current loss) depends upon the voltage and the iron loss depends upon the current core loss=v^2/R and the iron loss = i^2R. Tansformer is a static device so there is no rotational losses, so the vi input is aproxly equal to vi output hence one can write: vicosk1=vicosk2 hence cosk1= cosk2. The transformer is the device which have highest efficiency among electric machinery; because it does not have mechanical and ratational losses.

anupam_srivastav

Answered On : Oct 23rd, 2009

Though wattage is the product of v*i*p.f,
and while rating transformer we consider
iron loss and cu loss which depend on voltage and current respectively, hence no use of rating
is in wattage but rating is simply in KVA which is only a product of i
and v.

omswaroop30

Answered On : Oct 24th, 2009

Efficiency of transformer depends on core loss and copper loss. Core loss depends mainly on voltage and copper loss depends on current passing through the winding. So basically total loss i.e. efficiency depends on voltage and current that is why we rate transformer in KVA (voltage*current) not in KW.

friend

Answered On : May 13th, 2012

bcox at the time of designing the transformer we do not know the power factor of the load which s going to be connected accross it.... i mean we do not know at what power factor it will operate... since KVA=V*I... KW=V*I*COS@... so its rated in KVA

fayyaz ahmed laghari

Answered On : May 29th, 2012

Bcoz losses r in form of current and voltage

cvijayk

Answered On : Jun 29th, 2012

Heating of the transformer is based on both KW and KVAR, in other words it is dependent on KVA. Hence transformer rating is given in KVA instead of KW.

meenakshi

Answered On : Jul 1st, 2012

Becoz copper loss depends on current & core loss on voltage.hence total losses depends on volt-ampere not on phase angle between V & I ,i.e. it is independent from load power factor.

yogi

Answered On : Oct 7th, 2012

transformer rating is dependent on heat generated when in operation and heat is proportional to square of current, KVA rating gives maximum allowable current at which transformer can work but KW rating doesnt give the same.

pinaki patra

Answered On : Oct 9th, 2012

For a transformer power factor is dependent on load, which is variable. Hence fixed quantity VA is used in place of watts.

s harikumar

Answered On : Nov 16th, 2012

Transformer rating in KVA instead of KW since its iron loss depends on voltage and copper loss depends on load current only. It is not dependent on the power factor of the load.