I know you’ve probably heard harmonics get blamed for just about every electrical issue in the plant. If a bunch of techs are scratching their heads, someone’s bound to say, “It’s gotta be harmonics.” Let’s cut through the noise and talk about what harmonics really are, how they impact your equipment, and what we can do to keep them under control.
What Are Harmonics?
Harmonics, as defined by IEEE 519, are “a sinusoidal component of a periodic waveform or quantity having a frequency that is a multiple of the fundamental frequency.” In plain terms, they’re extra, unwanted frequencies riding on top of your power system’s fundamental frequency. Unlike transients and surges that come and go, harmonics are a steady distortion of the AC waveform, constantly stressing your electrical system.
When harmonics are present, they shorten the lifespan of motors, transformers, and other critical assets. You’ll see fuses blowing, circuit breakers tripping, and transformers running hotter than they should—all signs that something’s not right.
What Causes Harmonics?
Harmonics are the result of non-linear loads, which are equipment that doesn’t draw current smoothly from the power source.
Common Culprits Include:
- Computers and Power Supplies – Anything with an AC-to-DC conversion is a non-linear load.
- Variable Frequency Drives (VFDs) – Essential for motor control but known for producing harmonics.
- Uninterruptible Power Supplies (UPSs) – Provide backup power but can distort the current waveform.
- LED Lighting – Energy-efficient but contributes to harmonics.
Signs You Have a Harmonics Problem
If you’re seeing the following issues, harmonics might be to blame:
- Overheating transformers and motors – Extra frequencies force them to work harder than necessary, which causes excess heat.
- Unexpected tripping of circuit breakers and blown fuses – The extra harmonic current is unusable to your process but is current nonetheless that will trip your breakers and/or blow your fuses.
- Flickering lights and voltage instability – A clear sign of waveform distortion.
- Increased energy costs and power losses – Harmonics create inefficiencies that add up fast.
- Motor failure and torque fluctuations – Negative sequence harmonics cause motors to fight against themselves, leading to excessive heat and mechanical stress.
IEEE 519 sets standards for harmonic distortion in industrial facilities. If your Total Harmonic Distortion (THD) is above 5%, you might not only be damaging your equipment—you could also face fines from your utility provider.
How to Mitigate Harmonics
There are a few common ways to get harmonics under control:
- Install Line Reactors and DC Bus Chokes – Reactors and chokes smooth out current draw and reduce harmonic content in VFD applications.
- Use Multi-Pulse VFDs – Multi-pulse drive packages use phase-shifting transformers to cancel out harmonics before they cause issues. This is very common on large low voltage drives and almost all medium voltage drives.
- Deploy Harmonic Filters – Harmonic filters typically come in two types:
- Passive – Similar to a line reactor, passive harmonic filters have inductance but add capacitance as well. The capacitive component gives harmonics a low impedance path to go to instead of going to the drive. Passive harmonic filters are “tuned” to filter out harmonics better than line reactors and bus chokes.
- Active – Active filters inject counter-currents to cancel out harmonics in real time. This is the most expensive option but can be sized to reduce harmonics on multiple drives on a system. See this video video for our own experimentation/experience using an active harmonic filter at our test facility.
Need a Second Opinion?
Harmonics are a real issue, but they’re often a mystery. If you suspect drives are causing harmonics in your facility, give us a call, shoot us an email, or let’s chat via the feature on our site.