Why is Three Phase More Efficient?

OK, I am talking about 100V phase to phase in the 3 phase instance. In the 2 phase case there will be three conductors with phase-to-phase voltages of

100, 100, and 141.4.

Please refer to my response to Don Kelly where we chew through the math.

Thanks in advance.

Reply to
BFoelsch
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| in article gJ%9d.291$ snipped-for-privacy@nnrp1.ozemail.com.au, John G at | snipped-for-privacy@ozemail.com.au wrote on 10/9/04 5:52 PM: | |> American 2 phase is not really it is just one phase with the centre |> earthed and called the neutral | | You are describing the Edison three-wire system. Two phase as used here | really refers to a four phase system with neutral where only two phases 90 | degrees apart are used.

The 90 degree system could have been wired with 3, 4, or 5 wires, depending on configuration. Going along with the use of "Y" for a three phase WYE system, you could use "L" and "X" for these systems, with the center of the X perhaps connected or perhaps not. Or the load connection could be "L" or a full square on an "X" system.

The confusion is whether to call this 2 phase or 4 phase.

If you had a 3 wire system with a neutral and 2 hot phases at 120 degrees apart, would you call that 1 phase, 2 phase, or 3 phase?

What if they were 90 degrees apart instead?

What if they were 105 degrees apart?

What if they were vectors of unequal length, such as 2 at 120 volts at

180 degrees from each other, plus another at 207.846 volts 90 degrees different from the other 2?

If I have a 3 phase system with 2 phases at 120 volts (-90 degrees and

+90 degrees) and 207.846 volts at 0 degrees, and then I remove 1 phase, is that now a 2 phase system or a 1 phase system?

A 120/240 system could be considered as 2 phases at 180 degrees. But you can't derive other phase angles from it. However, a 170L/120 system can have other phase angles derived from it.

But if I were going to use a 90 degree system, I'd use 495X/350.

Reply to
phil-news-nospam

----------- I agree on this- I did not read your numbers correctly. I was looking at the conclusion rather than the details- sorry.

However, I will still quibble as you are using 100V as the phase (to neutral)voltage in the 2 phase case but line to line voltage in the 3 phase case. Bugger factor of root(3). If you use a phase voltage (to neutral as in the 2 phase case) of 100V then the current is 66.7A on each wire. Now compare 3 conductors at 67A to 2 at 100 +1 at 141A. Off the top this still favours 3 phase.

Reply to
Don Kelly

---------- effectively an open delta with one corner grounded- so that would make it a

3 phase system. The neutral is then the 3rd phase conductor. You can still supply balanced 3 phase loads.

----------- This is the North American 2 phase system.

Reply to
Don Kelly

Not to belabor a point, but, when comparing two polyphase systems with three conductors, what is the significance of the term "neutral"? Why is the vertex formed by the two phases in the two phase example a neutral, while the corner of a corner grounded delta is not? Neither one carries the residue of an unbalanced load, which I suspect is a salient point in the definition of a "neutral."

In the current discussion, I would consider it more reasonable to compare the systems based on the "conductor to conductor" voltage. If we accept your argument about phase to neutral voltage, we wind up comparing a 3 phase system of 173.2 volts conductor to conductor with the 2 phase system of 100,

100, and 141.4 volts between conductors. My calculation assumed that the 3 phase system would be 100 volts between conductors, and the 2 phase would be 100, 100 and 141.4.

It would appear that the conductor-to-conductor voltage would be a better basis for comparison, regardless of any concept of neutral.

Note that my discussion at this point is pretty much philosophical. An old engineering reference text tabulates the current discussion, and indicates, to your point, that, depending on how the comparison is made, the 2 phase system can vary between about 2 percent more "copper efficient" than 3 phase all the way to 50 percent less efficient. The most efficient case is the one I present, using the 100 volt value as conductor to conductor on the 3 phase. However, if the 141.4 value necessitates extra construction, and the

3 phase is compared at 141.4 conductor-to-conductor, 3 phase wins by a factor of 2.

Many thanks for your time and interest in this discussion.

Reply to
BFoelsch

| Not to belabor a point, but, when comparing two polyphase systems with three | conductors, what is the significance of the term "neutral"? Why is the | vertex formed by the two phases in the two phase example a neutral, while | the corner of a corner grounded delta is not? Neither one carries the | residue of an unbalanced load, which I suspect is a salient point in the | definition of a "neutral."

I think of the grounded phase in a corner grounded system as a neutral. And why couldn't you use a 2 bus breaker panel with it (provided it has the voltage handling ability). Why wouldn't a 120 volt corner grounded delta system work through an ordinary single phase (Edison) panel? The difference between that and 120/240 volt systems is with the delta in this case, you just get 120 volts between phases instead of 240.

Reply to
phil-news-nospam

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