RPC Theory

rwm said:
The point of the graph I posted was not that the generated leg voltage was wrong but that the phases were not 120 deg apart.
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As weird as it may sound, the angle offset in that plot is determined by where you put neutral. Zero in the graph is relative to neutral.The distance from neutral to any leg is its graphed voltage, and the angle determines its phase angle. If you look at my triangle, the angle from the neutral To L1 and neutral to L2 is 180 degrees apart, and they are 90 degrees from the neutral to generated leg line. But L1, L2 and the generated leg still form a equilateral triangle. Each leg is 120 degrees from a hypothetical center point of that triangle. That 3phase configuration is called high-leg delta.

If you use an oscilloscope with isolated probes, it would plot the three phases as 120 degrees apart. Isolated probes are necessary because most scopes reference the ground otherwise. Ground just happens to be at a weird and somewhat useless location. Doesn’t matter, our three phase motor doesnt care, any more than our car 12v system cares about earth ground.

In a very crude analogy electrical ground is like sea level. If you fall off a ladder, what matters is the height of the ladder, not how high above sea level the ladder is at. We could measure the height of the ladder by measuring how high above sea level the top and bottom of the ladder are, but that just makes more work with really long tape measures, and transits, and surveying techniques.
 
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rwm said:
I have been reading about RPCs ...
I guess my fundamental misunderstanding is how does the RPC take the single phase L1 and L2 lines and offset these by 120 deg?
You can see in this diagram that L1 is connected to T1 directly and L2 is connected to T2. By definition these phases are 180 deg apart?
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The difference between any two legs of a three-phase power is a simple sine wave, has no defined absolute phase.
The third leg as a rotary phase converter energizes it is what defines three difference signals with 120 degree
separation. It's misleading to consider ground as a phase-defining point, since a three-phase
motor (delta wired) hasn't any circuit ground, even if the frame has a safety ground.
 
It is good to know I can grab any single hot leg of a 480V supply with out fear of being shocked since there is no reference to ground. Just like my car battery. ;)
 
Assuming the graph shown in post #2 is plotting voltage, then it also is a non-standard voltage, or (more likely) the source has confused the difference between peak voltage and RMS voltage. It is showing the peak voltage of phase 1 & 2 as 120 (volts). But 120 is the RMS voltage in home electrical, the peak voltage is 120 * sqrt(2), approx 170 volts.
 
rwm said:
It is good to know I can grab any single hot leg of a 480V supply with out fear of being shocked since there is no reference to ground. Just like my car battery. ;)
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As a kid, I did grab an electric fence while standing on an old wooden chair to prove to myself I wouldn't get shocked. I didn't. But I'll leave the 480V test to you! :)
 
Back in the 80's I was contracted to build a delicatessen. Halfway through the job, the electrician got attacked by a junky with a razor knife. He had hundreds of stitches and one arm was in a cast, thus he couldn't do any physical work. I completed his work under his direction. One of the tasks was to splice in new larger cables from the panel to the mains. Since he wasn't licensed, we couldn't have the meter pulled and the splices had to be done live. I did it barehanded, standing on a wooden pallet and the entire meter wall covered by cardboard. I didn't even feel a tingle...
 
rwm said:
It is good to know I can grab any single hot leg of a 480V supply with out fear of being shocked since there is no reference to ground. Just like my car battery. ;)
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Well, it depends on your hairstyle. You’re going to act like a capacitor and inductor with a bit of resistance until things settle down. If you like curly hair it should be Ok…

I don’t understand why RPC’s are used instead of using VFD’s. I guess if everything you have is gearhead, then it’s probably cheaper.
 
StevSmar said:
Well, it depends on your hairstyle. You’re going to act like a capacitor and inductor with a bit of resistance until things settle down. If you like curly hair it should be Ok…

I don’t understand why RPC’s are used instead of using VFD’s. I guess if everything you have is gearhead, then it’s probably cheaper.
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RPCs are simpler, quicker, easier, cheaper, and don't befuddle folks like me who are 'technology challenged'...

Sorry, I know that VFDs have a lot of fans here... but that's my opinion...

-Bear
 
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