Why is improper EMI grounding so common?

Despite their demonstrated benefits, variable-frequency drives (VFDs) have a tendency to generate electrical noise (EMI) and be susceptible to interference. In a recent article, Frank Bartos explains how drive design and installation techniques can minimize EMI. Here, he discusses the importance of proper grounding and bonding.

By Renee Robbins December 18, 2009

Proper grounding and bonding is a must to mitigate electromagnetic interference (EMI) in variable-frequency drives (VFDs) and electrical systems. Unfortunately, improper EMI grounding is common. Mike Olson, power & control sales manager-HVAC applications at ABB Inc ., suggests two reasons for this situation:

1. VFDs are the most EMI-prone devices that installers handle, and their routine grounding methods used with other devices are not always adequate for drives;

2. Many buildings in the U.S. don’t have a good, solid common ground, placing one floor-or one side of a building-at a slightly different potential than the main ground connection in the basement. "All building grounds should be at the same potential, which is often not the case," Olson says.

Grounding effectiveness for electromagnetic compatibility (EMC) is enhanced by an added bonding component, says Olson. Electrical bonding connects metallic, non-current-carrying components at a location, placing them at the same electric potential.

Matt Murray, SEW Eurodrive ‘s corporate trainer, believes that proper grounding tends to be more of a problem in the U.S. than in Europe and other parts of the world. This comes, in part, from the U.S.’s relatively reliable power grid and fairly clean power, compared to other countries. It seems that belief in grid reliability is causing some electric equipment installers to not focus first on proper EMI grounding practices, says Murray.

"European equipment and drives come with extra EMI measures-and higher cost-due to generally less grid reliability," says Murray. "OEMs don’t know where a system might end up being used when they’re designing it. This is the main reason for the increased measure of EMI protection included in a system. It includes filters, output chokes, and importantly, proper grounding for EMC."

Another part of the answer is that, currently, there is no binding EMC regulation for electric drives in the U.S. As a result, not all equipment installers are fully aware of EMC requirements.

To read the entire article, " Electromagnetic Interference: What Users of Drives Need to Know ," visit www.controleng.com/archive under Dec. 2009.

Also see: " Silence of the Drives " under June 2008
Frank J. Bartos, P.E, consulting editor, braunbart@sbcglobal.net

Separating different cable types is a prudent EMC measure. SEW Eurodrive recommends 10 cm (4 in.) spacing between cable groups I and II; 20 cm between groups II and III and between groups III and IV.