Electronics > Beginners

AC Line Waveform Distortion

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spiff72:
I took another stab at this tonight.  This is the cleanest looking FFT I could get and fill the screen with a reasonable Hz/div value (100).  In order to get this, I actually used "Memory" instead of "Trace" for the FFT, but to get a reasonable horizontal scale, I had to drop the memory depth to 12K points.  Timescale is 50ms/div.

I noticed that this one did NOT have the odd non-harmonic 700Hz peak.

I then reloaded the settings from yesterday (via a saved settings file), and this is the second image.

spiff72:
Well, I did a test over the weekend and plugged the scope into an outlet near my panel and set up my probe to test the line voltage.

I then shut off every breaker in the house except for one that feeds the circuit I was plugged into.  The result was that there was no difference in the signal. so I have concluded that one of the following two statements is true:

- The source of the distortion is outside the house (already present on the incoming feed)
- The source of the distortion is on the same circuit that I had powered up to check

I didn't get a chance, but I might re-run this test one more time to rule out a couple of additional doubts that came to mind after the test.

- I was using the "averaging" method of the scope during the test (under the acquire menu).  This could have possibly slowed the response time of the scope, but I doubt it.  I might repeat in high-res or normal mode.
- Plug scope into a different circuit to rule out that last circuit as the source of the distortion.
- I also want to re-capture the waveform and FFT plot for the "before" and "after" shutting off all breakers.  I did this, but lost track of which was which because the scope doesn't have a RTC to put a proper timestamp on the captured files.

At the end of the day, if the source is not in the house, do i have any leverage with the power company to request they clean up the power coming into my house?  Is it even anything to be concerned about?

Thanks again!

Ragnarok:
3rd harmonics is generally from unbalanced 3 phase load. The transformer in the street is usually 3 phase (with households tapping different Phase + Neutral)

5th/7th/11th/13th harmonics are generally from rectifiers. Rectifiers will generally have harmonics generated on the (6n +/- 1) fundamental, where N is based on the topology of the rectifier design. Usually these harmonics are probably stray and seeps through the network, possibly from industrial customers connected on the local grid.

soldar:
This is what mine looks like:

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