I'll try to address these questions with brief replies.
If the 3rd harmonic is at 180hz.
Is this the rate the power supplies are turning on/off "switching", causing this to signal to back to the supply?
This is more like the "First Odd Order Harmonic" in that Circuit on that System.
It is the beginning of the "Triple-N" scenarios found on the Common Grounded Conductor of a 4 Wire Wye Polyphase System.
A SMPS (Switch Mode Power Supply) "Switches" according to the desired output needed to drive the connected load(s).
It will switch on for a short period of time, then switch off (over and over) to create a certain output Power level (Voltage of "x" level, current of "x" amperes - combined to equal required power level).
Higher power level loads will switch on/off less rapidly than low power draws.
The resulting output looks more like a semi steady DC output, than a series of off and on pulses.
What gets reflected "outward" back into the AC power system, is mostly from the chopping of the Rectified AC input (what is seen at the output), along with the input Rectifier section.
Maybe someone could describe these effects, like as in Waveform Analysis per Fourier's Series and/or Fischer-Hinnen's method, without doing what I would do... make no sense what so ever!
Is it true harmonics are negligable"spelling" on single phase "180" supplies vs 3 phase"120" "polyphase" supplies?
Yes, the accumilative effects of "Triple-N" do not exist on 1Ø Systems - due to the fact there is only one Phase/Waveform dealt with.
Harmonics exist on 1Ø Systems too, they just don't have as extreme of issues - like the 3Ø 4 Wire Wye Systems do with the overloading of Commons (Grounded Conductors) in Balanced, fully-loaded 4 wire multiwire circuits.
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Of course, there is no "180° Phase Offset" in any 1Ø System - nor in any Polyphase (usable) System...
"180° Phase Offset" would result in a 2 Phase System with a Zero level output:
- Zero Volts,
- Zero Amperes,
- Zero Wattage,
- Zero Volt-Amperes,
- Zero VARs.
...not a very useful System at all!
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