Adding to Paul's excellent response, here's a little more info + some graphics to use as teaching aids (graphics found in the Technical Reference Area - either directly, or through the "Menu").
The easiest thing to visualize first off is how a "Wiggy" works when connected across two lines (phases) of a polyphase system - in this case, the polyphase system will be 3Ø.
If the "Black" lead of the Wiggy is connected to Line A (ØA), and the "Red" lead is connected to Line B (ØB), there will be a voltage reading. Same for connections between A & C + B & C. If the system is Grounded, there will typically be a reading from one Line (phase) to Ground, which is NEAR ½ the Line-to-Line voltage.
Keep these key points in mind, as they cover the basic idea overall.
Basic flow - part 1:
In the most simplest idea, a Polyphase Transformer (or Generator) has two, or more windings, connected in some fashion. Each "Winding" is a simple coil of wire, which has two ends (told you it's really a simple idea!).
Lets call the end to the left "X", and the other end "Y".
Induce an Alternating Current into this winding, and you have a Single Phase 2 wire source.
At a certain time, current flows out of the X side, and returns at the Y side. The flow reverses direction for a certain time, then the whole thing starts over again.
Keep this "Single Coil Flow" principle in mind also, which explains what's going on in a stand-alone coil winding on a polyphase system.
For reference, view this image:
BTW: Split coils are used here, but think of the windings as singluar units which are continuous.
As you can see, there is a Ground connection on one of the Secondary lines - yet this does not make that circuit conductor a "Neutral"! It is a Grounded Conductor, but it is not "Neutral". Remove the ground connection and the system functions normally! Grounding is a Safety Issue, not something to enhance normal operation (in a nutshell!).
On this system, voltage readings can be made from "A" to "B", and "B" to ground. Without the ground connection, only solid voltage reading will be found between "A" and "B".
This next image shows a single phase 3 wire system, which has a Grounded "Neutral" conductor:
Voltage readings may be taken from A-N, N-B and A-B. Also, with the system grounded, voltage to ground can solidly be measured from A to ground, and also B to ground. Remove the grounding connection, and there is no SOLID voltage reading to ground.
(PS: I refer to "Solid" readings as being something easilly readable, or readable from a low Impedance meter. High input Impedance meters can read voltages on ungrounded systems, but this is beyond the scope of this discussion).
Advanced flow - part 1:
Taking the principle ideas of the single phase coils shown above, here's how to apply them to a polyphase system.
Take three of the single phase 2 wire setups (like the one shown in the first image), and connect them together in a Triangle Fashion - so that "A" on one coil connects to "B" on another coil.
(PS - remove all the grounding connections!!!).
Here's a graphic image of the result:
Now you have a 3 phase 3 wire Delta system! Visualize currents flowing in one coil at a time, and you can see how the complete polyphase flow works!
Keep in mind that the Generating source will "Activate" a current flow in each coil, which occurs at 1/3 of the complete rotation of its Armature. Even though the time currents flo in a given coil is 120° ahead or behind another coil, the current flow characteristics in any coil at any time will always resemble the basic single phase model.
A 4 wire Delta will have one coil "Center Tapped", which is equal to replacing one of the coils from the first single phase example image, with one from the second single phase sample image.
Here's an image of a Wye system:
Same principles apply here!
Hope this is effective.
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