@rabler
You remember that I've already admitted to being electrically (and electronically) challenged, so excuse me.
IIRC, a very long time ago, I learned that single phase service is actually 120° (not 180°). Is that correct?
It made sense (at the time) related to pulling 1Ph off of 3Ph service and also generating 3Ph using an RPC.
It’s a vector problem. You really can’t define an angle without two phases, two different sine waves. You measure a voltage between 2 points.
voltage=Vrms*sqrt(2)*cos(angle+2*pi*f*t), where f = 60Hz for US power. Angle is the the angle at t=0, but what defines t=0? (who cares? no one) So unless you’re dealing with more than single phase, angle is arbitrary.
In a 3 phase center neutral configuration, you have angles of 0, 120, and 240 from neutral to the three legs. But running three phases plus a neutral is 4 wires on the power line. The power company doesn’t run a high voltage neutral. Less wire is easier. They use transformers between any two legs to pull off single phase. Those two legs are at 120 degrees relative to the third leg, but without the third leg that angle becames meaningless. (They actually try to rotate which two legs to balance the loads). This produces one sine wave (cos wave) but since t=0 is arbitrary it doesnt really have an angle. Your 3phase motor will run fine off this config.
Since the transformer isolates the low voltage side, Neutral is determined by where the ground rod is attached. In a pure three phase only system, you could ground one of the legs. (IIRC this is called a corner ground system). The vector triangle is the same shape, it just shifts where relative to the origin it is placed. As long as your triangle maintains that shape, your three phase motor runs fine. Rotation and translation of that triangle don’t impact the motor.
The big fallacy, common even among EE’s, is there is some magical definition of a zero voltage point called ground. That is typically choosen as the local electrical potential of the earth. But an experiment on my old campus in the early days of networking showed a difference of up to 90V between two such rods less than a mile apart, and don’t ask what it is if a lightning strike occurs nearby. This has some nasty implications for attempting to send low voltage Ethernet signals between remote buildings.
