[...] in star configuration two phases are connected together. But then why wouldn't the phases combine and enhance or cancel each other out at any given position? Where do those nice graphs of 3 sine waves 120º apart come from which my alternator is supposed to deliver (without a neutral!)?
OK, basics. Your alternator is a _generator_ right? Works by magnetic induction as do all generators of this kind. So what you will get is an induced voltage in each of the 3 coils. Forget for a moment that the coils are connected to anything, and just think of them standing free with both coil ends unconnected. So you have 6 wires coming out. Measuring at the ends of any one coil, you will see a sine wave whose amplitude is proportional to the induction and frequency to the rotating speed of the generator. Lets call it
Vcoil for later reference. This should be obvious. Assume you have 3 scopes (or 3 diff probes or whatever, so that you can see the coil voltages all at the same time. Further assume that your probe polarities are all the same (say probe ground on the anticlockwise end of each coil). You will now see those same sine waves, shifted 120 degrees to each other. Your scope may have a "ground wire" in the probe, but the generator does not. Every one of those coil ends is just a coil end.
Ok, leave the probes aside for a moment. If you now connect all coils into a loop (thus creating a delta connection) basically nothing changes. Remember that the voltages generated by the coils are at mutual 120 degree phase shift. If you know how to do the Kirchhoff voltage analysis across the loop, you will see that there is no net voltage, thus no idle current circulating in this loop. Or you can take my word for it
. If you now measure the voltage across one coil, you will see that nothing has happened - the voltage is the same
Vcoil as it was when the coils were unconnected. Please understand that the word "ground" is just a reference point. The real world doesn't need that for anything. Thus this circuit configuration is perfectly happy without one - it is just the voltages
between circuit nodes that are visible/measurable. You can call one of those "ground" if it floats your boat, but it doesn't change anything. (But it doesn't really make much sense either in this symmetric circuit).
OK, go back to where all the coils were separate. If you now connect all say anticlockwise coil ends together, you obviously have a star connection. The central connection point can be called the "ground" or neutral but you don't need to connect it to anything. Assume for a moment that it is invisible. Since one end of each coil is hanging in the air, there obviously won't be any circulating currents, but what about the voltages you will see between the loose ends? If you now connect a scope probe between any 2 free ends, what will you see? Assuming the generator is still the same and the speed is the same as before, you will obviously see the same frequency sine wave. Only this time the amplitude will be sqrt(3) *
Vcoil. Why - because there are now 2 coils in series between any 2 free ends, and the 120 degree phase shift in the induced voltages will give rise to this multiplication factor. It is almost trivially provable by drawing the voltage vectors on graph paper and measuring the length of the line connecting the free ends. Think of the Mercedes Benz logo with the 3 point star. Each arm of the star is a coil voltage. What you measure is the distance between the end points of the star. Distance = sqrt(3)*length of arm. Same thing.
My alternator is supposed to produce this and I *want* to measure it *without* a neutral because my alternator does not use a neutral to supposedly send these 3 sine waves to the rectifier. Sorry to be so dense but I just can't wrap my head around this. thank you
And produce it, it does. Your challenge is just that you cannot have one coil end act as the live wire in one measurement
and the ground in another, at the same time. That would be short circuiting the generator via your instrument. Like IanB points out, the only way to do this with a scope is to use differential probes. Their point being that the probe doesn't really have a ground. It just measures the voltage difference between the probe electrodes, exactly as the generator produces its voltages between coil ends. No ground anywhere in sight, unless you decide to call one point ground.
If this still doesn't light a bulb, the only additional thing i can recommend is to brush up about the basics of polyphase generators. Google should be your friend, as usual.
Re Escher: Gotta love the man. An artist after every engineer's heart.
I have at least 3 books of his art in the shelf and have visited several exhibitions displaying his original art, including many of the best know pieces. He wasn't much appreciated in his creative years but later disciplines such as topology, fractal math, chaos theory etc etc have found plenty of resonances with his art. For anyone interested, a recommendation: if you got the chance, do read Doug Hofstadter's "Gödel, Escher, Bach: an Eternal Golden Braid -
a metaphorical fugue on minds and machines". As the name says, it prominently features Escher's art, including "Relativity" in IanB's post.
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