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Active filter `gives AFE performance at a lower cost`

25 July, 2008

Danfoss has developed an active filter (AF) technology for mitigating drive harmonics which, it claims, gives a similar performance to active front-end (AFE) technologies, but at a much lower cost. It has achieved this, in part, by re-using many of the components and structures from its range of high-power drives.

Like AFE systems, the AF technology is claimed to reduce total harmonic distortion to less than 10%, but while an AFE can cost about three times as much as the drive it is mitigating, the AF is around 1.8 times the cost of the drive. As well as compensating for harmonics, the filters also adjust power factor levels.

Another distinguishing feature of the AF technology is that it is fitted in parallel to the drive installation. This makes it easy to retrofit to an existing installation and also means that it can compensate for several drives and other loads at the same time. Large plants can be compensated centrally at the medium-voltage level via an auto-transformer.

When compensating groups of drives (which do not need to be made by Danfoss), the power factor of non-controlled motors is adjusted to unity, and harmonics are cancelled simultaneously. The active filter adjusts automatically to the load, and is not affected by the stability of the power supply.

Danfoss plans to start marketing the AF technology later this year. It will be offering combined packages of drives and filters which, it says, will be smaller than passive filters as well as providing a lower THiD.

Gregers Geilager, Danfoss’ manager for high-power products, explains that the active filter works in a similar way to noise-cancelling headphones which produce an anti-phase signal that counteracts the noise signal. The filters are sized for 40% of the harmonic level of a drive, up to a maximum filter rating of 500A, which is suitable for a 630kW drive.

Danfoss Rockford plant

The active filter shares up to 95% of its components with Danfoss’ high-power drive range and will be produced in the same US plant in Rockford, Illinois (shown above). As well as cutting costs, this has other advantages, according to Geilager. For example, it should improve reliability because it is using components of a known quality. It also uses Danfoss’ back-channel cooling technology, which makes it less vulnerable to environmental factors. Also lead times should be short – the specialised transformers needed for active front-end systems can take 20 weeks to deliver.

One limitation of active filters compared to active front-end systems, is that they cannot be used to regenerate power.




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