Why is the power of transformers indicated in kVA, and not in kW

We all know perfectly well that power is measured in watts (W), but much more often we have to deal with kW (1000 W). And most of us believe that it is in these units that the characteristics of all electrical equipment are indicated.

But, let's say, if you take a voltage stabilizer or look at the TP (transformer substation) near your house, you can see that the power will be indicated in kVA - kilo Volt-Amperes.

And now let's look at what kVA is and for what reason the power of the transformers is recorded in these units.

Simple explanation

Let's not go deep into boring definitions and formulas, but let's analyze the issue in a simple way. And first of all, let's figure out what kind of power our electrical appliances consume.

So, first, understand that not every electrical appliance that operates on an AC mains spends all the absorbed power on doing useful work - warming up, light, sound of music speakers and etc.

The load can be divided into four main types and all of them can be connected directly to the transformer.

Resistive load

A striking representative of this type of load is the most ordinary electric kettle or iron, in which the heating element is heated when an electric current passes through it.

In fact, ten is nothing more than resistance, and here it is absolutely unimportant how the current flows through it. Everything is simple here: the more current flows, therefore the heating is stronger, which means that absolutely all the power is spent on this process.

So the power, which is spent on a resistive load, is called active. It is precisely this load that is measured in kW.

Inductive load

An example of an inductive load is the most common electric motor. When the current passes through the electric motor, not all the energy is spent on rotation.

A certain part is spent on creating an electromagnetic field, and is also dissipated in the conductor. This power component is referred to as reactive power.

It is not expended directly to perform the work, but it is needed for the equipment to function fully.

Capacitive load

This is a special case of the reactive power component. As you know, a capacitor works according to the principle: accumulated charge - given charge. This means that inevitably part of the power is spent on the accumulation and transfer of charge and does not directly participate in useful work.

So now it is extremely difficult to find an electrical appliance at home in which at least a couple of capacitors will not stand.

Mixed load

Well, everything is extremely simple here. A mixed load contains all of the above components. And 99 out of 100 electrical appliances are just like that.

So the total power just consists of the reactive and active components, and it is the total load that is measured in kVA.

Transformer manufacturers cannot determine in advance what type of load will be connected and where exactly their product will be used. Therefore, the technical parameters indicate the total power for the mixed type of load.

It's important to remember. Many manufacturers indicate the power of the device in kW, but also indicate the power factor K. So, in order to find out the full power of the device, you need to remember a simple formula:

Let's look at a simple example for a better understanding. Let's say you purchase a drill and its power, according to technical data, is 3 kW. But the power factor is 0.8.

So, knowing this data, you can calculate the full power of the drill:

S = 3 / 0.8 = 3.75 kVA

It is for this value that the drill will load our transformer with you.

Conclusion

Now I think it is clear why it is kVA and not kW that is indicated on transformers, since this parameter takes into account all types of loads, and not just active ones.

That is why when you want to connect, for example, any load to your transformer, then take into account the full load, and not only the active one.

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