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Variable Frequency Drive THD(i)

The Total Harmonic Current Distortion (THDi) of a variable frequency drive (VFD) depends on the pulse assys. Standard for the VFD is 6 pulse, with a THDi at or about 40%, +/- 5%. With 12 and 18 pulse, this is of course reduced. These levels are also induced by source impedance, that being the line supply and distortions within it. VFD also employs a DC bus line reactor to help with harmonic distortions. This allows distances between the motor and drive of 1000 plus feet without aid. The distances between the motor and drive can go up to 500 feet unaided. The VFD with no DC link reactor has a THD(i) of 70%, =/- 5%. With any of these, significant reductions in harmonic generation is accomplished with the addition of an output RFI filter, which then enhances distance capability between the motor and drive to 1,000 feet unaided, and meets the stringent European RFI requirements. All aspects and all refinements are of course, cost driven. Many variable frequency drives on the market induce a significant electronically induced whine in the motor when running. This is caused by spiking in the output wave form, IE distortions. This should give an idea of comparable distortion levels compare the VFDs on the market.

Other solutions can be passive filters that reduce distortions to 5%, in accordance with specs from IEEE and euro standards. I believe it's IEEE 519.

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