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Motor efficiency depends on PFC (Part 2)

Posted: 29 Oct 2009     Print Version  Bookmark and Share

Keywords:motor efficiency  PFC motor  power factor correction 

Active power factor correction (PFC) can achieve very high power factors—0.98 and above—with reasonably sized components, although the energy efficiency may be slightly lower than with passive techniques (94 per cent compared with 96 per cent, for example) due to the addition of switching components.

In one of the simplest architectures, an inductor, a MOSFET or IGBT switch, and a diode are added between the rectifier bridge and the bulk capacitor, in a boost switch-mode power supply configuration. Figure 1 shows an improved rectifier with active PFC, which draws current from the AC mains exactly in-phase with the mains voltage for a high power factor.

This is how the AC-to-DC boost converter works: The intermediate DC bus voltage is chosen to be higher than the peak voltage of the rectifier bridge, so the switch-mode controller will be working in boost mode. The controller driving the switch (Q) will adjust the duty factor of the switch control signal so that the desired current and voltage targets are maintained. The switching frequency is chosen to be much higher than the AC mains frequency (60Hz compared with 20kHz, for example). The small current ripple at the switching frequency and its harmonics can be filtered using a passive filter on the AC mains input, similar to passive PFC, but much easier because of the lower amplitude and higher frequency of the ripple current.

View the PDF document for more information.

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