Three phase AC system

As it is well known fact that in a DC system, two terminal exist (positive and negative) and current or power flows from positive to
negative. so it can be say that one conductor carries current and other returns the same current and there must be close loop to current flow.
I m wondering what happens to three phase AC system. In a 3 - phase AC system, all three conductor carries current/ voltage/power with 120 deg apart assuming system is balanced. my question is how current is returned to the source. let's say source generates power and power flow from source to load but how about return power from load to source?
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Hello, and we've either got a "too little knowledge is dangerous" or language translation problem here. Current (electric charge flow) is returned to the source (generator) just like in any closed circuit, be it single or poly-phase AC or DC. You can analyze a 3-phase AC circuit with sinusoidal voltages/currents as the superposition of 3 single phase circuits. An understandable (you don't have to be an MSEE) and relevant book with many proctical examples is the Schaum's Outline Series "Electric Circuits" by J.A. Edminister. Sincerely,
John Wood (Code 5550) e-mail: snipped-for-privacy@itd.nrl.navy.mil Naval Research Laboratory 4555 Overlook Avenue, SW Washington, DC 20375-5337
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---------- Source generates a voltage as in the case of a single phase setup. However 3 voltages 120 degrees apart are generated. If there is a load, current flows and power is delivered to the load from the generator- it is not returned but converted in the load to mechanical, thermal, etc power and some will be lost in the wires as I^2R heating. In the balanced case the 3 line currents are 120 degrees apart and the sum of the currents is 0. That is the current in one phase will be returned by the other phases. Ia = -(Ib+Ic) etc. The loops are closed.
As J.B Woods says "Schaum's Outlines" provide a very good starting point.
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Don Kelly snipped-for-privacy@shawcross.ca
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Don Kelly wrote:

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Nothing! When you add up the instantaneous currents in the three conductors (four if there is a neutral), they wiukk add ti zero. No return is necessary.
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| As it is well known fact that in a DC system, two terminal exist | (positive and negative) and current or power flows from positive to | negative. so it can be say that one conductor carries current and | other returns the same current and there must be close loop to current | flow. | | I m wondering what happens to three phase AC system. In a 3 - phase | AC system, all three conductor carries current/ voltage/power with 120 | deg apart assuming system is balanced. my question is how current is | returned to the source. let's say source generates power and power | flow from source to load but how about return power from load to | source?
Regardless of how simple or bizarre a multi-phase AC system is made, if you have voltage between any two conductors, those two conductors will be the source and return for that current. What is source and what is return is a nebulous definition even for DC and then with AC it alternates, anyway. When there are loads on more than one pairing of wires, any wire that is in common between two separate load pairings will add the current phase vectors. That means if they are 180 opposite in phase, that common wire ends up with no current (e.g. a neutral in an Edison split single phase). And this applies by adding all phases. So with 3 equal three phase pairs on a wye/star system with the neutral as the common, the neutral ends up with no current. But you can get this effect even with no neutral. If the system is 240 delta, or 240/139 wye without the neutral used, and all loads are 240 volts, they can be connected to any pair of the 3 conductors. When the same amount of power is used on 240 delta or 240/139 wye, and in the case of the latter whether loads are line-to-line or line-to-neutral, you get the same amount of current on each current carrying conductor for the same balance of loads (sameness between wye and delta depends on the wye system balancing out the neutral to zero).
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The three wires take it in turns to be the source and the return.
But, as to who is the source and who is the return is continuously changing.
But, because we're dealing with sine waves with phase differences of 120 degrees, it can be difficult to see this, as you express above.
Sometimes, one of the wires will be a source for itself, but superimposed on its own current it will also be the return for somebody else.
So, congratulations on posing your question, You are not actually wrong in what you ask, but your picture is not big enough.
It's easier to grasp if you start out with 3 pairs of wires, giving six wires in turn.
Each pair will be 120 degrees out of phase with the next pair.
Now, it's AC so that you know that the source / return convention is changing all the time, 50 Hz here in Britland, 60 Hz over in Yankland.
Now, if you were to connect the source of one pair to the return of another pair and do this 3 times, you'll end up with the normal 3-wire circuit, and each wire will have the combined current of being its own source and also being the return of another pair.
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And part of the genius of three-phase power is that this works. When you connect 3 sets of windings in the alternator to 3 sets of windings in the motor using 6 wires, the 3 phase shifts can be anything you want and you would still have a (more or less) working system.
But when you convert to 3-wire transmission, you end up connecting the three alternator windings together in a triangle, and you could have destructive currents flowing around that triangle without reaching the load at all. Three sine waves of equal voltage and 120 degree phase shift between each pair have the special property that the sum of the three voltages is always zero, so this particular configuration gives no current flowing around the triangle. And it's completely symmetric, so the net current in the 3 wires is the same.
This same special property allows transformer manufacturers to build 3-phase transformers with 3 windings on 3 legs of the same core.
At the same time, the total power (proportional to the sum of squares of the 3 sine waves) is constant, which provides constant power transfer from alternator to motor - good for smooth motion.
With one phase, the power delivered cannot be constant, so this is not a reasonable system when large amounts of power are delivered. You *can* build a two-phase system with 90 degree phase shift that delivers constant power. But it either requires 4 wires, or 3 wires with one substantially larger than the other two. Three phase also gives constant power, but needs only 3 conductors of the same size. So for a small increase in the complexity of the alternator and motors, it's easier to distribute than 2-phase would be. You can get constant power and equal-sized conductors with 4 or more phases too, but those systems have more complex alternators, motors, and transmission than 3 phase has with no compensating benefit.
    Dave
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