Lesson 6 - Electricity

Power factor correction plays a big role in reducing overall electricity charges.

Power factor is the ratio of true power to apparent power. True power is the actual power drawn by a piece of equipment, and is measured in kW. Apparent power is the power applied, and is measured in kVA. The reason for the difference is reactive power, which is measured in kVAr.

A way to look at this is to take a glass of beer. The beer consists of liquid and foam. The apparent power is the volume of the glass. The true power is the liquid, and the reactive power is the foam. We would like to have more liquid, as that is what quenches our thirst. This is why we want a higher power factor. Apologies to the brewers trying to ensure good foam.

This example is a bit simplified, as it does not simply add up, but we do not want to get into the complicated electrical maths. To understand this a bit more, we have to look at different types of electrical loads. Loads such as heating elements and light bulbs are resistive loads, as the load depends on the resistance of the equipment. All the power has to go through the equipment, and the voltage and current drawn are perfectly in phase, and thus the power factor is one. Thus, all the power applied is turned into useful energy. However, if we have an inductive load such as the load from an inductive motor, the power applied ‘induces’ rotation. Here, the current lags the voltage because the electromagnetic field created causes the current to lag behind the voltage applied. This is what happens in most industrial applications.

Another form of load is a capacitive load. A capacitor can be seen as a ‘storer’ of electricity. In this case, the current leads the voltage. These three examples are shown in the figure below.

Figure: Unity, lagging, and leading power factors

Power is charged based on the supply to equipment and not on the work done, just like beer is charged for a glass regardless of how much foam is in it. We would therefore want to get the power factor as close to one as possible. This brings us to power factor correction. This is done using devices that add capacitive load to bring the voltage and current back into phase, or as close as possible to it. Now that we understand the key concepts around electricity, let us have a quick look at how it is distributed.