Answer / kj

Three phase supply is the least number of phases that will
produce a rotating magnetic filed. This allows 3 phase
induction motors to start without special methods as with
single phase.
Extra phases have been used at times but it increases the
costs of all the power lines.
Where 3 phase can be 3 or 4 wire, 5 phase would be 5 or 6
wire and 7 phase would be 7 or 8 wire.

Answer / kumar

for single percentage is less than the 3 phase % of power improvement 57% after increasing 5 ph, 7 ph %power is increased only 7% that's why we use 3 ph

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