Why are electrical transformers rated in kVA (kilo Volt Ampere) rather than kW (kilo watts)?

Answer 1

There are 2 reasons.

1) The magnetic saturation depends on the current, not the power.

2) The resistive heat dissipation (I^2 R loss) depends on the current, not the power.

For these two operational limits, it does not matter whether the load is resistive (kVA = kW) or reactive (kVA = kW / (power factor) ).

Answer 2

Goes back to the concept of total power being composed of two parts, the real and imaginary power. For most applications, we only think about the amount of real power we need to get a job done. And, if the power factor is close to one (reactive loads primarily with very few motor loads), this is true. However, the imaginary power is still flowing and has to be generated and accounted for. If the power factor is above 95%, the effect is small enough that you could ignore it in most applications. However, tell that to a transformer outside a heavy industrial area where the power factor, if not corrected, could be 60%. In that case, the transformer may only be 100% loaded at, say 1000 kW but the total load would be 166% based on kVA. On a summer day, most transformers like that would overheat and fail. So, again, the transformer would fail from the total power in kva, not just the real.

Answer 3

Because if an inductive or capacitive load changes the phase relationship between voltage and current, the KVA will exceed the actual power, so a transformer can't handle as much power out of phase as in phase.

Answer 4

Like the first person said, there is inductance associated .... this means that the current is complex valued.

So kVA is the complex power ... kW is the real value, which makes sense because that's what actually does work.

Answer 5

Must be a big transformer your talking about...need to know the current and the voltage when determining the perticular transformer for a perticular purpose. Individually.