Why transformer rating in kVA?

+83 votes
asked Aug 21, 2015 in Science by alessia (440 points)
Why transformer rating in kVA?

3 Answers

+16 votes
answered Aug 31, 2015 by LuigiHarrill (370 points)

Transformer ratings are known to be measured in kVA and not in kW. A transformer's rating or the rating of any electrical machine shows its ability to carry mechanical load without showing any signs of overheating. Rises in temperature, which are known to be major threats to insulation, comes up as a result of internal loss within the machine.

Two known types of losses exist in a transformer;

  • Insulation losses or core losses or Iron losses
  • Copper losses

Copper losses (I2R) are uneven losses that are mostly dependent on the amount of current that passes through the windings of the transformer while Insulation losses or Iron losses or Core he voltage losses are mostly dependent on the voltage.

This shows that the transfer is designed for Iron losses-rated voltage and copper losses-rated current. It is not possible for us to make a prediction on the power factor during the process of designing the machine, since power factor is dependent on the load which is not stable.

When manufacturers produces UPS, transformer, etc., they are never able to predict the exact type of load that would be used and consequently rate the device based on its maximum output of current that can be safely carried by the conductors (at unified Power Factor) as well as the conductors' insulation rating (Voltage & temperature). That is why the transformer rating can be in kVA, and not in kW.

+2 votes
answered Sep 16, 2015 by MathewMohr75 (220 points)
KVA*Power Factor=kW

The output of a transformer is simply an electrical quantity. In cases where the manufacturers do not know the pf rating the transformer will be used – the power factor is a variable that is dependent on the load connected to source of power.

So manufacturers make use of KVA rating for Transformers alone, and never on generators.

In a similar way, the output in motors is mechanical energy so pf is not variable, which is why KW rating is used in cases involving motors.

Similarly for motors output is mechanical energy so power factor is not variable so in case of motors KW rating is used.
+1 vote
answered Sep 19, 2015 by FredrickJass (230 points)
Why transformer rating in kVA? One very brief and accurately precise answer would be that no decision was taken during manufacturing as regards the type of load the transformer would carry (capacitive or Inductive) so he can only consider voltage and capacity and the multiplication of these two shows kVA or mVA.

One other answer might be linked to the losses that occur in the transformer which does not depend on power factor, so no attention is paid to this. But this cannot be the right answer, though you will still find a number of dumb professors screaming this as the answer.
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