Active Filters:

This “Did You Know” article will discuss active filters.

So first what’s the difference between passive and active?  
Passive filters are just pieces of electrical hardware that are tuned to remove a single harmonic frequency or possibly 2 frequencies.

Active filters have a “brain” and electronics that actively adjust to remove many harmonic frequencies.

Passive filters are “in series” with the VFD, meaning they must be sized exactly as the VFD.

Active filters are “in parallel” with the VFD(s), meaning their size is based on total harmonics to be removed.

In our case, passive filters are working with 60Hz. (Send a passive filter overseas and that will change to 50Hz and your State-side filter is a worthless chunk of scrap iron!)

Active Filters though are much “smarter” and can adjust as power usage fluctuates and harmonic frequencies move around.  As you would guess, these filters are much more costly than a passive filter.  A passive filter is meant for one VFD load, where an active filter could possibly filter multiple VFD loads.

Another way to look at the Active Filter is a set of Bose™ noise-cancelling headsets.   If you’ve ever worn these headsets, you notice that it’s much quieter and really knocks out the noise.  But how does it remove a sound headed straight for your ear?  It actually sends a negative sound to your ear.  The result?  A sound that gets cancelled out.

A TCI active filter does exactly the same thing.  It sees harmonics and injects negative harmonics back to the grid.

The result?  The harmonic gets removed. 

These are actually easier to size, you simply look at the harmonic content and size to remove that much “noise”.  Add more harmonics later to the facility, just add another Passive filter module.  Remove loads in the plant and the filter just reduces its workload.

So there you go.  We have covered Line Reactors, Passive Filters, and Active Filters, all meant for the input of a VFD to reduce harmonics.  Now it comes down to how much reduction does your customer need and how much he or she is willing to spend to get it.

Special thanks to National Power Quality and
Consulting for providing these Did You Know articles.

For electrical consulting needs, NPQC can be contacted at