There is one more load type that needs to be considered; the non-linear load. So far, with linear loads, the voltage and current have been sine waves of a single fundamental frequency. Non-linear loads cause the input waveforms to be distorted such that they are no longer pure sine waves.
Fourier analysis explains that the distorted waveform can be broken down into a series of sine waves that have specific magnitudes and phase shifts, and whose frequencies are integer multiples of the input frequency (i.e., the line frequency). For example, a square wave is the sum of all odd harmonics, where the magnitude of each is equal to the inverse of the harmonic number and there is no phase shift. Only the fundamental frequency contributes to real power, or work. Undistorted (linear) loads can have a power factor from 0 to 1 since they are only composed of the fundamental frequency. Non-linear loads, because of the presence of harmonics, will always have a power factor of less than 1.
A very common non-linear load is the switching power supply. In its simplest form, the power converter uses an input circuit that comprises a half-wave or full-wave rectifier followed by a storage capacitor to provide an unregulated DC voltage. This topology only draws current from the input when the mains supply peaks, and each pulse contains enough energy to sustain the load until the next peak as shown below.
Voltage and current waveforms in a rectifier circuit
The resulting output-current waveform is, compared to the sinusoidal input, heavily distorted and contains a number of strong harmonics. As stated previously, because the rectifier presents a non-linear load to the AC source, it can never achieve a unity power factor.
The problem for utilities is that the current drawn by equipment with a low power factor is far higher than the current required, which adds cost to their generating and distribution capacity.
To reduce the environmental impact of electric power and limit interference with other loads on the same supply network, governments around the world have drawn up and enacted legislation to limit the harmonic distortion of power supplies. For example, electrical equipment with an input power requirement of 75W or more supplied in Europe and Japan must comply with the IEC61000-3-2 standard. The standard specifies the maximum amplitude of line-frequency harmonics up to and including the 39th harmonic.
Although the US does not have the same level of legislation as the European Union, the Energy Star program that is operated by the US Department of Energy, as well as schemes such as 80 PLUS for computer and datacenter power systems, are placing an increased emphasis on maintaining a high power factor, calling for a power factor of ≥0.9 at 100 percent of rated output in the system's power supply.
Elsewhere, similar legislation is being enacted, with China, for example, bringing in regulations based roughly on Europe’s IEC61000-3-2.
The solution to poor power factor and excess harmonics is to use power factor correction (PFC), which shapes the input current of the power supply to maximize the real power level from the mains and minimize harmonic distortion. Ideally, the electrical appliance should present a linear load, such as a simple resistor, rather than the reactive load of an uncorrected power supply. A corrected waveform minimizes losses as well as interference with other devices being powered from the same source.
Standard triangle vector diagram showing the effects of power factor correction