Harmonics are Underappreciated

Only recently have I started to appreciate the influence of harmonics in electrical systems.

Here’s a power quality analysis of my refrigerator (this is what I do on Saturday nights). The true power “P” is 725.2W, the reactive power “Q” is 109.1VAR and lagging (unsurprisingly, due to all the motors inside a refrigerator), and the apparent power “S” is 739.1VA.

These figures may seem odd because they (seemingly) don’t add up. IAW the standard power calculation, S = √(P² + Q²), the answer should be approx 733.4VA. What the standard power calculation leaves out is harmonics. If you include the distortion power “D” produced by harmonic currents and voltages out to at least the 20th order you get the following power equation: S = √(P² + Q² + D²). Once you include distortion power, measured at 91.77VA by my analyzer, the math works out.

Now refrigerators are mostly linear loads, so they don’t produce a ton of harmonics. But the economic trend toward electrifying everything, including transportation, while probably welcome, well yield a massive increase in harmonic distortion, making standard power calculations laughably inaccurate.


Great post. I’ve always wondered about Power Quality at home and have been tempted to put a PQM on the panel from time to time just to see. Have you ever tried this with your AC system in the summer?

What size sensors are you using? If you are using a big Ro coil it may just look like garbage at low amps.