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Why only 3 phase AC current & not 2 phase or 4 phase
24-02-2013, 12:00 PM
Post: #1

Why only 3 phase AC current & not 2 phase or 4 phase

We have studied about 3 Phase alternating current or voltage in engineering, but why can't there be 2 phase or 4 phase ([even number] phase) AC is possible?
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24-02-2013, 01:32 PM
Post: #2

RE: Why only 3 phase AC current & not 2 phase or 4 phase

(24-02-2013 12:00 PM)gagan Wrote:  We have studied about 3 Phase alternating current or voltage in engineering, but why can't there be 2 phase or 4 phase ([even number] phase) AC is possible?

It is not that 2 or 4 Phase is not possible. It is very much possible but It is only that if we use 2 or 4 phase AC the neutral will have to carry a large amount of current for the same power and hence the I^2R loss will be very high and / or the conductor to be used for neutral will have to be very thick, The explanation and reasons are as under :

When there is 2 phase or 4 phase or for that matter any even number of phases then because they have to be evenly distributed in space, they will have have equal angle in them and as such a pair of phases in such system will always be in opposite direction to each other. For example in 2 phase system the two phases will have to lie in a plane 180 degrees opposing to each other (in the same line but opposite direction). So the current in the neutral will consist of return path for the current from both the phases. Now imagine that in 3 phase the angle between the phases will be 120 degrees and none of the phases come opposite to each other and vector addition of the current in neutral for all the three phases will be much less as the other two phases also partially form the return path for the one which is passing through the peak. This means the neutral alone does not have to carry the entire amount of return current and hence the conductor may be thinner and also the I^2R loss will be much less. However now the question may arise then why not 5 or 7 phase then? Absolutely right question. But it can be proved through equations that the amount of current does not decrease much by increasing the number of phases to 5 or 7 from 3 and hence there are only 3 phase AC currents and voltages.
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