What’s Your Shield? Managing Harmonics and EMI

Power Systems and Harmonics

Our power systems continue to get smarter and more energy efficient. Which ultimately helps us control the process much better. Yet, they are also becoming increasingly complex. More energy efficient electronics – such as power supplies, LED lighting, and VFDs – have one major downfall. They produce harmonics (noise) as they do their work.

Harmonics aren’t new; they have always been a factor with these types of electronics. However, as the use of VFDs and LEDs become more common, harmonics are increasing in our electrical systems. Another problematic factor is found in the number of communication elements being added into plants, as these are more susceptible to interference from this noise. Equipment is running on Ethernet communications or low-voltage communication systems to pull back more information. This is great for operating facilities, but these signals are prone to having problems when harmonics are present.

Common issues resulting from harmonics include:

  • False signals on instrumentation. Example: Level transmitters giving false values.
  • Communication errors on equipment. Example: Ethernet communications dropping out randomly.
  • Damage to motors and bearings, and early motor failure. Example: Motors shorting out after a short lifetime.
  • Early failure of motor wiring on VFDs. Example: Motor wiring developing shorts in it with no apparent physical damage.

Managing Harmonics and EMI

Harmonics and Electromagnetic Interference (EMI) cause these issues. EMI is a type of magnetic field caused by the switching that is part of the operation of these electronics. You can’t completely eliminate harmonics and EMI unless you get rid of all the VFDs, LEDs, and electronics in a facility. Since that isn’t practical, harmonics and EMI must be managed through the design and installation of the electrical system.

Practical ways to do this are as follows:

  • Confine noise inside the cables from the harmonics producing loads.
  • Shield instrumentation/communications from noise producing loads.
  • Put distance between power wiring and that for instrumentation/communications.

Using a shielded VFD cable is an excellent way to keep harmonics confined inside the cables. These cables are specifically designed to keep the EMI inside the cable and to bring it back to the VFD, which is designed to drain the harmonics off to the ground. Shielded VFD cables hold up to VFD harmonics and protect motors from premature failure as well.

Make sure that instrumentation and communications are also properly shielded. This protects them from harmonics/noise coming from the outside and affecting the integrity of the signal.

Maintaining Distances

A final recommendation is to maintain distances. If you can keep your sensitive signals and electronics separated from power circuits that cause harmonics, then the noise will be less likely to be transferred between the power circuits and the instrumentation/communication circuits. Just putting them in different conduits is not always enough. Try to place them in different conduit racks or different underground ductbanks when running in parallel with each other. The greater the distance you can maintain, the less chance of interference you will encounter

An expert opinion is always valuable. If you are having issues that you can’t explain, early failures of equipment, or want to design a new system that properly addresses harmonics, we can help.


Brent Kooiman, Interstates Senior Project Engineer