Home » FAQ » Can Variable Frequency Drive Improve Power Factor?

Can Variable Frequency Drive Improve Power Factor?

Variable frequency drive can change power factor. Internal filter capacitor of VFD reduces the reactive power loss and increases active power of power grid, thus improving the power factor. Appropriate reactors can be connected in series at the input side and output side of VFD to absorb harmonics and increase the impedance of power supply or load, so as to achieve the goal of suppressing harmonic and reducing electromagnetic radiation in the transmission process. By suppressing harmonic current, the power factor could be improved form previous (0.5-0.6) to (0.75-0.85). VFD has a good effect in improving the power factor.

As shown in figure above, when charging the capacitor, there will be current only when the peak voltage of input side exceeds the capacitor voltage, so the input side current of VFD is shown in figure (c) above. From a perspective of frequency analysis, it contains strong harmonic components, so the power factor decreases, about 0.7-0.75. Firstly, we will talk about VFD improve power factor. Due to the existence of DC bus, the influence of power factor output by variable frequency drive on its input side is greatly reduced, while its input side uses uncontrollable diode rectifier, so the power factor will not be very low. Therefore, the saying that VFD can improve power factor is correct.

Secondly, the function of reactor. If a reactor is added (input side), harmonics can be suppressed, the existence of harmonics will affect the decline of power factor. So adding the reactor to variable frequency drive can improve power factor.

The generation of harmonics and influence of harmonics on power factor are various, they are related with specific system structure, technical route and installation. To analyze the generation and elimination of harmonics, it shall be related with field situation. Of course, it’s certain that no input reactor is allowed in large-scale systems.

If the reactor (DC or AC) value on the rectifier side of VFD is too small, the input current harmonics will be very large, which will lead to the reduction of power factor. If the reactor value is set too large, the input current waveform would be improved, but voltage waveform will be lagged to form a larger angle. Similarly, it will also lead to the reduction of power factor. As a result, some VFDs add power factor correctors on the input side, the cost is rather high.

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