about transforner

Transformer rating is given in kVA, so that the transformer full load current ( current the transformer is able to supply) may be calculated without knowing the power factor of the load, the transformer is supplying.

As the power factor differs from different types of loads (resistive and capacitive loads has a power factor very close to 1, inductive loads such as motors as a poor power factor and also changes according to the load on the motor)

What this means is that transformer will be able to supply a "larger" kW load, when a resistive load is supplied compared to a inductive load with a poor power factor. Because of the poor power factor on the inductive load the current drawn from the transformer will be the same as the load with a good power factor.

Lets use a 100kVA transformer to try and illustrate

kW = kVA * pf = 100 * 0.9 (good pf) = 90kW (load supplied)

kW = kVA * pf = 100 * 0.5 (bad pf) = 50kW (load supplied)

In both the above cases (although the kW is different), the current drawn from the transformer will be the same. Using the formulae P = √3 * Vl * Il * cosΘ the current drawn will be 144 amps if assuming the voltage to be 400v (Vl).

For this reason the rating of transformer is given in kVA and not kW (same for generator)