Everyone who’s dealing with a generating set has for sure heard about kW and kVA and found it easier to take the kW as a unit of measurement.
Knowing the difference between watts (kW) and Va (kVA) is useful for more detailed determination of the scope of the generator set.Knowing the difference between Watt (kW) and Va (kWA) is useful to determine in greater detail the specific application of the generating set.
Last year during class I used a classical example but obvious to the audience to explain to my student the difference between kVA and kW: Comparing the electric power of a generator to a glass of beer.
Pouring some beer in a glass, two separate parts are formed: the beer itself and the foamy head. We can consider the glass as the maximum power, the foam as the reactive power and the beer as the real power.
“Apparently” the glass is full but what will quench our thirst is only the part of the beer not the foam. However, to drink the beer you should also drink the foam which, even though it tastes a bit bland, helps to fill out the glass first than the belly.
Just like the foam in the beer, the reactive power is to be considered even when it has no need and it is connected to the power factor which is 0,8 for the alternator of a generator.
The power factor is the ratio between the real power vector module flowing to the electrical load and the apparent power in the circuit.
Therefore, to determine the real power of a generator we just need to multiply the kVA value by 0,8 and vice versa divide by 0,8 to switch from kW to kVA:
kW = kVA * 0,8
kVA = kW / 0,8
Example: To which nominal kW does a 20kVA generator correspond? You should multiply 20 x 0,8 to obtain the real power in kW which is 16kW.
The term “apparent” also gives the idea of something that looks like but it isn’t that.
If the reactive power is almost considered as waste why then the apparent power is taking into consideration when talking about diesel generators?
This is a common question that I have always heard because, in fact, it seems not to make sense taking into consideration the apparent power when looking for the real one.
The answer is simple: The apparent power, being connected to the current value present in the circuit, is useful. It can be considered as the maximum value of real power that we can obtain by reversing the discrepancy between voltage and current.
Remember the beer example? Well, imagine you are reducing the foam quantity in the glass and refill it with other beer!
The real and apparent power values get close until they become equal when the power factor tends to be 1. This happens when the generators powers a resistive load.
For the single phase generating sets with 230V voltage, there is no discrepancy therefore, considering a power factor 1, the real power will be the only useful power to be considered.
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