Why the rating of a transformer is done in KvA and not in KW?

Question

Why the rating of a transformer is done in KvA and not in KW

Answer 1

Watts have no complex component. KVA is used to measure an apparent power on AC signals with a particular inductance/capacitance, since the power delivery is non-constant. At 0Hz, AC and DC have the same value.

AC is a wave. DC is a straight line. So... AC doesn't have a constant power associated with it. DC does.

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

Transformers are designed and cataloged by KVA ratings, just as horsepower ratings designate the power capacity of an electric motor, a transformer’s KVA rating indicates its maximum power output capacity.

What does KVA mean? K= Abbreviation of the Greek word kilo, meaning ‘times 1000 V= Volts A= Amperes or Amp.

Calculating KVA for single phase:
KVA = 1 x E/1000
(1= Amps, E = Volts)

Answer 4

Just want to add in a bit more:
S(kVA)=P(kW)+jQ(kVAr) where S is total power, P is active power (Power uses by a resistive load E.g:light, heater, etc) & Q is reactive power (Power causes by an inductive load E.g: Induction motor). For a load, we only wants to know how much work it will do, so information of kW is enough. However, for a transformer as it has to supply power to the whole system, it will be affected by both active & reactive power. So you will have to concern about the total kVA of your system to choose the correct transformer. If you only concern about the active power the transformer can handle and not about the reactive power, the total power of your system sometimes will be larger than the total power of the transformer and will damage it.

Answer 5

all the AC machines are rated in KVA. because its operation is depend on power factor also.If rating is expressed KW it will not give a clear idea of power output.because effective utlization of energy will depend upon power factor and power factor will vary with load nature.so by KW=KVA* power factor

Answer 6

The transformer power rating is based on apparent power (i.e. VA, kVA, or MVA) instead of real power because the transformer is insensitive to the power factor. It only cares about the magnitude of current put through, and magnitude of voltage applied to, its windings, not their relative phase angles. (Obviously, there are some fine print notes regarding waveforms, harmonics, fluxes, etc., but they are not germane to a fundamental understanding of why transformers are rated in volts, amps, and VA.)

Answer 7

Because when making a design, you need to consider both powers : R= reactive (KVAr) and P = real (KW), the composite of both of them S = ( R^2+ P^2)^0.5. will give you the total rating of a transformer.

Answer 8

its because the nature of the load may vary,in turn varying the power factor(ie cos @).....inductive load has cos@=0
capacitive load also has cos@=0
resistive load has cos @=1
power = v * i * cos @
hence if the rating of a transformer is expressed in terms of power,this rating will keep varying....in order 2 standardize the rating it is only expressed in terms of v and i(ie kva)

Answer 9

volts times amps = watts.
amps are generally what a wires capacity is measured by
since they are the driving force. And what will burn through a wire.