necnotevenclose
Senior Member
Why are transformers rated in kVA and not just KW?
Transformers KVA & KW
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necnotevenclose said:
KVA is a product of volts X amps without the powerfactor involved. Knowing what the p.f. is will give the actual wattage used.I suppose if they added a p.f. in the name plate rating ,it would'nt sound as good. If a building has a .8 p.f. with a 75 kva xfrmr it would use a maximum 60kw.
Rick
robbietan
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transformer manufacturers does not know what power factor the transformer load will be. if they put in a pf of 0.8, a customer might ask them what about my facility with a pf of 0.9?
necnotevenclose said:
Transformer losses are determined by the voltage--core losses--and by the current--copper losses. With a zero PF, one could max out the transformer while delivering zero power. Rating in real power--KW--would be meaningless in this case and in any other case where the PF is less than one.
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- Lockport, IL
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The KVA rating of a transformer, or a generator for that matter, is based on its ability to reject heat to the surrounding area. Generation of heat is the result of current flowing in the windings. The parameter ?KW? is related to a portion of the current, the portion associated with real power. The parameter ?KVA? is related to all of the current, the portion associated with real power and the portion associated with reactive power. So to accurately address the ability to reject heat, you have to account for all the heat. Thus, you describe the rating in terms of KVA, and not KW.
kingpb
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- SE USA as far as you can go
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charlie b said:
The rating of the transformer, according to IEEE C57.12.80, states that it shall be based on the volt-ampere rating (KVA).
transformer capability have more then just the copper losses (winding) to deal with. The Hysteresis losses, Eddy current Losses, and leackage flux also play an important role.
As well, with a generator or motor you also have more then rotor and stator copper losses (I^2R) you also have core losses, mechancial losses, and stray losses.
Motors are rated on hp, which is directly related to kW, but generators are rated on KW, or MW with a known power factor.
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