Why is an Alternator or Generator Rated in kVA, Not in kW?
As we already know that why is a transformer rated in kVA instead of kW, same is the case and reason as well here i.e. the power √3 VL IL Cos Φ delivered by the alternator and generator for the same value of current, depends upon P.F. (Power Factor = Cos ϕ) of the load. In other words, generators and alternators don’t deliver Watts but both of active and reactive powers which depends on the power factor of the connected load to it.
Keep in mind that the conductors of an alternators are calculated for a definite current and the insulation at the magnetic system are designed for a definite voltage independent of P.F. (Cos Φ) of the load. For this reason apparent power measured in kVA is regarded as the rated power of the alternator.
In very short, a generator generates both active and reactive power which is rated in volt-amperes (VA) instead of watts (W).
The main factors manufacturers consider while designing electrical devices and appliances which provide electric power like transformer, UPS, alternators and generators etc, are load and power factor. As they don’t know exactly what is the power factor and which kind of load (resistive, inductive, capacitive or a mixture of all of them) will be connected to the device and appliances.
So they simply design and rate the electrical device according to its maximum current output that the conductors can safely carry while they consider unity power factor (In case of pure resistive load).
If we connect inductive or capacitive load (When power factor is not at least unity), The output would differ as there are losses due to low power factor.
For this reason, KVA is an apparent power which does not take into account the PF (Power factor) instead of KW (Real Power).
Where:
KW = KVA × Cos ϕ
And
kVA = KW ÷ Cos ϕ
For example,
In case of unity Power factor (1) i.e. pure resistive load. A 100kVA generator or alternator would provide exactly 100kW according to the following formula.
P = V x I x Cos Φ
kW = V x I x Cos Φ
kW = 100 x 1 = 100 kW
If we put the value of Cos Φ as 1, The real power (kW) would be equal to the apparent power (kVA).
In case of inductive or capacitive load, suppose the power factor is 0.80.
Now the real power would be
kW = KVA X Cos Φ
kW = 100 X 0.80
kW= 80 kW.
Now you know why alternators, generators, transformers and UPS etc are rated in kVA instead of kW.
FAQ
Why generator rating in kVA instead of kW?
Ans. Alternators/generators are rated in kVA (kilovolt-ampere) because they are designed to handle both real power (kW) and reactive power (kVAR). The kVA rating represents the total apparent power that the alternator/generator can deliver, while the kW rating represents the real power or the actual work done
Why is the motor in kW not kVA?
Since the power factor is unknown both generator and transformer are rated in kVA. But in the case of a motor, it has a fixed power factor. That is why the power factor is included in the nameplate data. Hence, Electric Motors are rated in kW and not in kVA
Why are transformers not rated in kW but in kVA?
The losses that occur in transformers are independent of power factor, which is why they are rated in kVA. The apparent power unit is kVA. Reactive and actual power are combined in it. Transformers are made without taking into account the associated load
Why are inverters rated in kVA not kW?
Because the AC output may have the voltage and current with a phase difference. Only at zero phase the KVA is equals real KW, otherwise VA is only the apparent power
Why fuse in kW and alternator in kVA?
Because KVA gives you the maximum out of phase current which will always be higher than the current defined by KW
What is the difference between kW and kVA in generators?
kW is the unit of real power and kVA is a unit of apparent power (or real power plus re-active power). The power factor, unless it is defined and known, is therefore an approximate value (typically 0.8), and the kVA value will always be higher than the value for kW
Why are alternators rated in kVA?
Generators are rated in kVA instead of kW because they measure different aspects of the power output. kW is the real power that a generator can supply based on its engine’s horsepower. kVA is the apparent power that a generator can supply if it was 100% efficient
Why is UPS rated in kVA?
KVA is the instantaneous power that the UPS can provide whereas Ah is the battery capacity i.e. stored energy in the form of charge. KVA over time will be dependent on the Ah of the battery used in the UPS
Can kVA be less than kW?
kW denotes the unit of real power and kVA denotes the unit of apparent power. The power factor, which is defined and known, is, therefore, an approximate value (typically 0.8), and in the kW to kVA calculations, kVA value will always be more than the kW value
