If you have ever been confused by terms like power factor, cosine phi, or harmonics in the context of an LED lighting installation, then this article is for you. Our specialist Adriaan Van Nuffel explains it simply.

The role of Cosine Phi

An electrical installation is powered by alternating current (AC). In this case, the voltage (in volts) constantly changes direction, resulting in a sinusoidal profile. When the installation is loaded, for example, when we turn on the lights, a current (in amperes) will flow, which, in the simplest case, depending on the type of light source or electrical device, will also take on a sinusoidal shape.

Sometimes there is a shift between the moment the current changes direction and the moment the voltage changes direction. This phase shift is quantified by the 'cosine phi'. In the ideal case, there is no phase shift, and then the cosine phi = 1. The phase shift is caused by inductive or capacitive elements in the installation.

Cos ɸ : The relationship between voltage and current

If the cosine phi is not equal to 1 but, for example, only 0.8, the installation will require more current from the grid to deliver the same useful power. This is not good because higher current will lead to more losses due to heating of the electrical cables.

Harmonics in the electrical world

Today, many electrical devices, including LED fixture drivers, are built with electronic circuits that do not draw continuous current from the grid but rather short pulses. This leads to a current profile that is no longer sinusoidal but has an irregular shape.

This irregular shape can be mathematically transformed into a sum of multiple sinusoidal profiles, each with a frequency that is a multiple of 50 Hertz. These are the harmonics: the first harmonic has a frequency of 50 Hz, the third harmonic has a frequency of 150 Hz, the fifth has a frequency of 250 Hz, and so on.

Only the first harmonic delivers useful power! (This is due to a mathematical principle that relates to the interaction of different frequencies in sine and cosine waves). This is called the active, working, or real power. But the other harmonics will require additional current from the grid and therefore lead to additional losses. This is called the apparent, reactive, or reactive power.

Moreover, higher harmonics can affect the operation of other devices connected to the grid.

All of this is measured by the Total Harmonic Distortion (THD), which is the sum of all the currents of the higher harmonics divided by the current of the first harmonic.

THD: a measure of current distortion

A perfectly sinusoidal current will have no harmonics, and thus THD = 0. As the current demanded by the higher harmonics increases, the THD also increases. 

The meaning of power factor

The power factor (PF) takes into account both the effects of harmonics and cosine phi. Thus, these two negative effects are evaluated in one value:

• The shift of the current relative to the voltage (cosine phi)
• The distortion of the current compared to a perfect sine wave (THD)

Power Factor: A Balance Between Cos Phi & THD

In the ideal case, the power factor is equal to 1. If the cosine phi is too low or the THD is too high, the value of the power factor will decrease.

ETAP Lighting and Power Factor

• According to the EN61000-3-2 standard, for electrical devices with a power greater than 25 watts, a minimum power factor (PF) of 0.9 is required. ETAP luminaires with a power above 25W always have a power factor PF > 0.9.
   
• The problem usually arises with electrical devices with a power lower than 25 watts. These can have a very poor power factor. This can be the case, for example, with phone chargers, computers, television screens, but also low-power LED lamps (residential market) or inferior LED tubes.
   
• There will be no problems if only a few devices have a low power factor. However, if fluorescent lamps are replaced by LED tubes with a poor power factor throughout an entire installation, this will demand too much current from the grid. This can cause blown fuses, additional electricity costs, and so on.