Why we are using single phase & three phase

[SUNIL SAGAR]

Why we are using Three phase & single phase instead of using two phase , four phase, five phase.

 

[dlgoff]

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It's a trade-off between efficiency and cost.

A three-phase system is generally more economical than others because it uses less conductor material to transmit electric power than equivalent single-phase or two-phase systems at the same voltage.

http://en.wikipedia.org/wiki/Three-phase_power

The most obvious advantage of three-phase power transmission using three wires, as compared to single-phase power transmission over two wires, is that the power transmitted in the three-phase system is the voltage multiplied by the current in each wire times the square root of three (approximately 1.73). The power transmitted by the single-phase system is simply the voltage multiplied by the current. Thus the three-phase system transmits 73% more power but uses only 50% more wire.

http://en.wikipedia.org/wiki/Polyphase_system