FAQ: What is Power Factor and Power Factor Correction?
The power factor of a power supply is a ratio of the real power consumed by a load (expressed in Watts) to apparent power (expressed in VA). It is expressed either as a number between 0 and 1 or as a percentage between 0 and 100%.
Real power is the actual power drawn by the supply, whereas apparent power is the product of the input current and the input voltage. Since voltage and current may be out of phase in non-linear loads such as switch mode power supplies, apparent power can be much greater than real power.
In order to maintain a high power factor, many power supplies (including LED drivers) must employ some form of Power Factor Correction (PFC) to ensure that the input current waveform matches the input voltage waveform as closely as possible in both waveform shape and phase.
A power supply with low power factor will draw more current for a given power consumption. This will result in greater power losses in all transmission lines and a large number of low power factor loads may even require a modification of the utilities infrastructure. There are several standards now in effect requiring certain minimum levels of power factor in power supplies, including LED drivers.
Low power factors also negatively impact the environment as a result of wasted energy in transmission lines as well as generators and transformers consuming more fossil fuels and generating more pollution and higher costs. It is clear to see how a high power factor is beneficial in all electrical products. It becomes more pertinent in LED drivers when you consider that approximately 20 percent of the world’s electricity is consumed by lighting.
Power Factor Correction is a tricky thing to explain, but our favourite analogy on the subject is ‘The barge being towed along the canal’. This involves a barge and a horse. The horse is on the towpath towing the barge along the canal by a rope connecting the two. As the horse is not directly in front of the barge, as well as being pulled forward the barge is also being pulled towards the edge of the canal. This affect is counteracted by the barge steering to keep a steady line in the centre of the canal. The extra work being done by the horse (which is not being used to pull the barge forward) represents the Power Factor of the barge. If the rope was longer, the horse would not have to work so hard because it would be nearer to the centre-line of the barge. In this case, the additional length of the rope represents Power Factor Correction. So, by reducing the angle (improving the Power Factor Correction) less energy is wasted in performing the same task.
Other than the harmonics issues surrounding Power Factor (which we will discuss in another FAQ), energy usage is a core principle in Power Factor Correction. This simple graphic can show the advantages of utilising high Power Factor Correction power supplies and drivers: