"I see only 3 probes connected to what is supposed to be an RPC thought it does not look like an RPC."
Very good eyes sir. I'm not sure how this topic crawled out of the woodwork but
there's a lot of information in the original thread, linked at the top of this one.
Apparently you have not gone back and spooled through it - and I don't blame you as
it went on for some time.
The short version that answers the question above:
1) the scope probes (yes three of them) are connected to the power panel on my
ESM-59 hardinge lathe. It was the easiest way to hook all three probes on at once,
short of making up another twistlock cordset. The scope is indeed tied to all
three output lines of my converter.
2) the converter itself lives in a small crawl space in the back of the shop. It's
started with a small 1/8 hp motor via a belt, and consists solely of a 5 hp
allis chalmers motor. No capacitors, no contactors, no nothing. Just an idler motor.
3) the scope in question is pretty sophisticated. It has the ability to compute and
display waveforms that are mathmatically related to any of the four input channels.
Obviously I am using three of them for this measurment, the fourth is turned off.
The three waveforms you see on the scope screenshot (this particular scope allows
a direct jpg screenshot that can be downloaded via a zip drive, very handy) are
set up the following way, before the measurement is started:
4) Here the W's are the waveform definitions, and the V's are the actual input voltages:
W1 = V1 - V2
W2 = V2 - V3
W3 = V3 - V1
For a purely analog scope, you are absolutely correct, you basically need SIX
inputs with the ability to analog difference pairs. Tough animal to find!
This instrument (borrowed from the lab where I work, WITH permission from the boss!)
is, I think a 5 GHz digitizing sampling scope. So it can measure all the signals,
process them, and display the results in real time, keeping track of all the information.
Because of the speed and bandwith of the scope, it isn't even aware it's busy when
doing this sort of test!
So to recap:
a) voltages are measured against ground
b) displayed waveforms are computed from voltages.
c) only the differences between hot legs is displayed
d) no neutral information is shown on the screen, nor was the neutral voltage measured.
Not sure where this was going:
"Also the scope trace shown is out of a text book and not a photo!"
You will find no textbook with that photo in it sir. I downloaded that jpg
from the scope personally, and posted it on the site.
It is similar to the ones I did later, showing the power factor for this same converter,
using thes same scope:
Here I needed to use all four probes, to show voltage and current feeding the
incoming line to the converter.