Earlier on the blog, we have defined the Active Harmonic Filter (AHF) as the best low harmonic choice for variable speed drives. A common question often put is 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 optimal solution. This would create the need for a bigger Active Filter, and the drives rectifier would be stressed by the compensation power from the filter. This in turn would reduce the drive’s life span.

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

The choke will also dampen the compensation power affecting the drive. Leaving out the choke, the drive’s 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 to complement. It gives the overall system a lower cost and higher availability.