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Could you please help me with the queries included in the attachment? I'm new to three phase system and studying it out of curiosity. Please keep it simple. Thanks.
Q1 The three wire connection works because there is a voltage between each of the three "line" connections A, B and C.
To represent the generators with + and - symbols seems a bit silly to me, there is nothing DC about it.
There are three AC sources, the voltages are the same but they are 120degrees out of phase with each other.
When connected in the star configuration, the line voltages are SQRT(3) x the phase voltages.
Q2 When connected in delta configuration, summing the voltages around the loop of three generators, you end up with zero which is why you are not shorting out the three generators.
Coming back to the idea of three lines and no neutral, for a star connected generator and a star connected load, if the load is balanced, there is no current flowing in the neutral anyway, so you dont need it. (This is not true for an unbalanced load, there will be a current in the neutral, and if the neutral is disconnected, the star point of the load will be at some voltage, not zero as with a balance load.)
You might also want to consider the Y configuration when the ground is removed. It then becomes a delta of a sort. The delta has no ground, that's all really.
AC sources are not the same as DC sources because they always have a phase. That phase is relative to some other phase if there is more than one AC source. You can connect AC sources differently than DC sources because the phase plays a big part in it too.
Consider a 100v DC source connected in series aiding with another 100v DC source. The total voltage is 200 volts DC.
Now consider a 100v AC source at 0 degrees connected in series with another 100v AC source at 180 degrees. The total voltage is 0 volts AC. It's not a short, but just that the total addition leads to no voltage at all.
Now a 100v AC source at 0 degrees added to a 100v AC source at 120 degrees, which are two arms in a three phase delta system. The result is a wave that is:
Vb=50 sin(wt) + 86.6 cos(wt)
Next, a 100v AC source at -120 degrees:
Vc=-50 sin(wt) - 86.6 cos(wt)
and then flipped by 180 degrees:
Vc=50 sin(wt) + 86.6 cos(wt)
Interesting? It should be because Vb=Vc and that's the voltage at one of the nodes in the Delta configuration, computed both from two of the generators in series and the third one flipped at the same node. Thus, both nodes are the same voltage and so when they are connected together there is no short circuit.
To try this yourself, all you have to do is break one of the connections in the Delta and analyze for the two voltages at the two loose ends. Call one node ground in the delta and compute the other two loose end voltages after breaking one of the other connections (see attachment). You should find that the voltage from 'a' to ground is the same as the voltage from 'b' to ground and so connecting them does no harm.
There are systems with some unbalance unlike this perfect situation, but that's another story.
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