Fighting the waves

The benefits of active Power Factor Correction (PFC)


Since the introduction of the standard 61000-3-2 for reducing current harmonics, active Power Factor Correction (PFC) has become ever more widespread as an active filtering measure for devices operated on single-phase grids. Active PFC tracks the input current of the sinusoidal input voltage, so that the current harmonics in the input current are significantly reduced. This generally requires a transistor, a diode, an inductance and a controller.

The power factor value (PF) indicates how far the current waveform of the input current resembles the ideal sinus form and how large the phase angle is between current and voltage. An ideal case would be: PF = 1 (ideal sinus form for the current and phase angle between input voltage and input current φ = 0°). Modern technology allows values of PF = 0.99 to be achieved. These kinds of high values are not necessarily required to comply with the limit values of the standard.

“As soon as multiple fans are used in parallel operation, use of active PFC becomes almost unavoidable.”

Günter Haas, Group Leader Electronics Development, ebm-papst Mulfingen

A passive solution can also be used in individual devices as an alternative to active PFC. This consists of an inductance arranged either in the supply line to the device or in front of the intermediate circuit capacitor.

Minimising harmonics also significantly reduces both the reactive power and the effective current. If such devices are installed, this means that smaller cable cross-sections can be used for the supply line.

If only a single fan is used as an individual device, the aforementioned standard can also be complied with using a passive PFC solution. As soon as multiple fans are used in parallel operation, use of active PFC becomes almost unavoidable.

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The image shows the harmonics in a parallel operation of two sets of three fans, one with passive and one with active PFC. According to the standard, the set of fans with passive PFC in this example is not permitted.

Comparison: active vs. passive

The table shows a comparison between a device with active PFC and one with passive PFC. Both devices were measured at the same operating point (identical air performance). The THD value (total harmonic distortion) indicates the ratio of the effective value of the sum of all current harmonics to that of the fundamental oscillation. The THC value (total harmonic current) indicates the overall effective value of the current harmonics. From the perspective of the standard, only harmonics with the ordinal numbers 2 to 40 are relevant.

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