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.

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.

techtalk_Vergleich_aktiv PFC

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