090902-2013 EST
EEC:
At my web site
http://beta-a2.com/EE-photos.html
and at P5 --- Photo 2750A1 is shown a sine wave that was turned on at 90 deg. If without shifting the sine wave sideways but simply inverting the waveform you would have a waveform shifted 180 deg from the one shown.
Connect an oscilloscope common terminal to the neutral point and connect the channel 1 vertical probe to phase A and also synchronize the scope horizontal sweep to phase A. If a second channel is available, then connect this to phase B, and you will see the inverted sine wave on the second channel.
If only one channel is available use a separate probe to the sync input and maintain the sync from phase A. Move the vertical probe to phase B and you will see the inverted sine wave.
This results because the measurements are being made relative to the center tap (neutral) of the transformer, and the way the secondary windings are phased.
If you are working with full line voltage (120) and the center tap is grounded and connected to the EGC, then DO NOT connect the scope common to phase A or B. If you do, then you will get a big spark and something will burn up. If you run the test with a standard residential system vs a small isolated filament transformer, then do not connect the scope common to anything. Just let the scope use the EGC in its power cord to provide the common connection to the system neutral. Of course this does not allow you to use one of the hot lines as a reference.
If you use an isolated center tapped filament transformer as a test signal, then do not ground any output terminals, and connect the scope common to any point you want on the isolated secondary of the filament transformer.
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