
Harmonic Filters for VFDs

Industry standard three‐phase variable speed drives draw harmonic currents from three phase voltage sources. If harmonic distortion is left unaddressed, it can further distort the utility grid or cause damage to nearby equipment. The IEEE-519 standard sets a current and voltage distortion limit of 5% for electrical systems. To mitigate the amount of harmonic distortion produced by Variable Speed Drives (VFDs), active and passive filters are used.
Active harmonic filters are effective at reducing the harmonic currents in power systems to safe levels while allowing the VFD to draw the harmonic currents it needs to operate. The active filters rely on active power conditioning electronic devices to compensate for harmonic currents. They are used for more critical and complicated systems because they have a better capability to act in response to drastic disturbances. However, they are more expensive to install than passive filters.
Passive harmonic filters are commonly used on the input side of motor drive systems. They are made up of reactive components like inductors, capacitors and resistors. They are tuned to provide a low impedance path for specific currents with undesirable harmonic frequencies.