In previous articles we have defined the Active Harmonic Filter (AHF) as the best low harmonic choice for variable speed drives. There are often questions whether the drive has to be fitted with a line filter as well.

In theory a well designed Active Filter can compensate a drive without a choke but this may not be the best solution. The Active Filter would then need to be bigger and the drives rectifier would be stressed by the compensation power from the filter, reducing the drives life span.

A very common solution is to install a 2% or 5% choke on the drive. Many high quality drives are fitted with such chokes as standard. The choke reduce the harmonics from 85-100% to about 35-40% which therefore is a very cost effective solution. The remaining THD is significantly lower meaning the size of the final and more expensive filtering is much smaller.

The choke will also dampen the compensation power affecting the drive. Without the choke the drives rectifier will degrade over time.

So the answer is yes, a Low Harmonic Drive system using Active Harmonic Filters will benefit from using a choke as well. The overall system will have lower cost and higher availability.