Passive Filters:

This “Did You Know” article will deal with VFD input filters.

In simple terms, the power company supplies a sinusoidal waveform, nice and clean.  Traditionally consumers have used motors operating without a VFD.  This causes no problems with the waveform, but for the consumer, it limits the motor to only one speed.

Introduce the VFD, and now the consumer has a method to change the motor speed.  However, the VFD doesn’t consume power perfectly as seen by the utility. VFDs produce “non-linear” power and/or “harmonics”.  Add a few of these VFDs to your facility and you may not notice much changes to the power source.

Convert your entire facility to all VFDs and you can create a harmonics mess, or “poor power quality”.  So, no big deal, not a consumer problem, right?

Well, yes and no.  The utility will take note of the poor usage of their power source and may complain with a slap on the wrist or may go with a penalty charge. (As a reminder, a VFD without any filtering can have harmonics as high as 70%.)

So what are some of the tools we have to fix these issues?
In this article we will only consider the Harmonic Guard series.  For many years, the HG7 series has been around from TCI, reducing harmonics down to the 7% range.

TCI recently introduced its HGP filter.  This one reduces the harmonics down to the 5% level. 

One thing that should get you excited is that the price difference between the HG7 and HGP is almost negligible.   And the hardest standard to meet is the “5%” rating.  So, you now have a product to get down to this tough standard.

Ok, that’s been a lot of words, so let’s summarize it.  If you are selling a VFD and the customer is concerned with harmonics, then add a HG7 or HGP filter to it.

So today we covered what is termed in the industry as “passive filters.” In another “Did You Know” article we will cover, “active filters”.

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

For electrical consulting needs, NPQC can be contacted at