Power Line Harmonics

So you may have seen me yelling about power line harmonics… here’s what I was looking at earlier this morning. This is the power at a facility I was doing some work at earlier today. The same power has laid waste to two variable frequency drive units and an Omron 24v power supply used to run a Programmable Logic Controller (PLC).

I used the FFT mode on my Tektronix DPO 2012 oscilloscope to better detail what’s going on. The yellow trace is the waveform coming in (you can see it’s not a perfect sine wave). The red trace is a plot of the frequency vs. amplitude after a Fourier transform. This same kind of plot may already be familiar to you: it’s displayed on many audio players and stereo systems as a spectrum analyzer visualization.

If this were perfect clean 60 hz power, I would have only the tall peak at the left which is 60 hz, then it would roll off to the baseline. What I got, however, is this (and a realization I accidentally set my scope’s clock 12 hours off *before* DST kicked in…)

Harmonic Horrors


The FOUR other peaks are the harmonics. Initially I thought I was looking at a second, third, fourth, and fifth harmonic, which would put the last one at 300 hz– reading this again though, it’s scaled 500 hz/division so what I’m looking at is the odd order harmonics at up to 540 hz– a NINTH harmonic.


Now let’s look at that third harmonic, the most prominent. That one’s about 20dB down from the fundamental (the scale is 20dB per vertical division). That’s a voltage ratio of 0.1… so the third harmonic, if you were to isolate it, would be 12 volts at 180 hz. That just plain doesn’t belong!!!

A textbook square wave is the fundamental frequency plus all of its odd-order harmonics– 3, 5, 7, 9…. I suppose this makes sense, as if you smashed this AC waveform very very badly you would get a square wave.

Power line harmonics are an annoying effect of loads with a poor power factor. Want an example of a load with a poor power factor? You’re looking at it. The wall chargers for smartphones, power supplies for computers, and even LED, florescent, and compact florescent light bulbs are guilty due to the nature of their power supplies. Without getting too much into the theory I’ll say this: the way they work is that they draw power right at the peaks of the usual sine wave power. This is why the peaks in the above screenshot are getting smashed: that’s when EVERYTHING draws its juice.

Some *nicer* devices incorporate power factor correction circuitry to mitigate this. This usually takes the form of a low pass filter that acts on the amount of current drawn by the device, in an effort to keep it from just suddenly grabbing only the peak. Note that I say— “some”—

The harmonics tend to overstress everything by causing high currents to flow in wires, transformers, power supplies, etc– they are not only harsh to the equipment but they are a waste of energy.

And here, boy, do we ever have ’em.

The solution, ultimately, will probably be to have the local power company install a bank of capacitors for power factor correction on their poles.

Utillity PFC capacitors. Photo from a Cooper Power Systems press release.

Then, hopefully, the power will stop being quite so…….. hungry for electronic snacks……

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