Answer / amol wazalwar

multiphase systems are very much possible but the problem is
the higher cost. we need a minimum of two phases for
production of a rotating magnetic field , this is one of the
reasons why the earliest generation was done in two phases.
But three phase provides us with the optimum compromise
between the power transfer capability and the the cost of
generation and transmission of electricity. i will give you
a brief mathematical derivation of why we use three phase
and not more than three phases.

let us assume a single phase system with the induced and
current I .
power output = E*I
assuming unity pf

for a two phase system , let us suppose that the induced
voltage is still E and current I
per phase voltage in a two phase system = 1/root(2) * E
(with two conductors in series and 90 deg electrically apart.
so power output = 2*1/root(2) * E *I
= root(2) *E*I
= 1.414 EI

for a three phase system , let the induced voltage still be
E and the current I
per phase voltage in a three phase system = 1/2 * E (with
three conductors in series and 120 deg electrically apart.
so power output = 3/2 * E *I = 1.5 EI

thus for a n phase system let the induced voltage still be E
and the current I
per phase voltage in a n phase system = piE/2n
so power output = n * piE/2n * I = 1.57 EI

thus from the above it can be seen that when we move from
single phase to two phase the power delivered increases by
42% then in three phase by 50% and in a n phase or higher
phases maximum by 57 %

thus 3 phase provides the optimum compromise between the
power delivered and the economics of laying n no of wires.

note * - well the last derivation i had given assumes a very
large no of phases. the problem can be visualized as trying
to fit n no of phases in a semicircle with dia as E. so for
a large no of phases emf per conductor can be taken as
length of the semicircle / no of phases
(pi *E/2)/ n
pi E/ 2n

Answer / sanjeev

as the first person said in case of three phase system we can get a rotating magnetic field where the phases are 120 degree phase apart. But if we use 5 phase we can get the phases apart from each other is 360/5=72 degree.
as we all know the sine wave is maximum at 90 degree.
So in case of 5 phase system we cannot get the sine wave at peak amplitude.
In case of three phase system one of the three phases always at peak position so we can get an uniform uninterrupted optimum service in case of three phase system.

Answer / umesh

in electrical field 3 phase supply have more advantages as
follow,
1. it more flexible compare other 2 or 4 phasr system
2.less transmission loss compare to same.
3.it can be convert in 3 phase 4 wire system it happens in
only 3 phase systems.

Answer / akki

Actually In 1-phase and 2-phase power, there are 120 moments per second when a sine wave is crossing zero volts. In 3-phase power, at any given moment one of the three phases is nearing a peak. High-power 3-phase motors (used in industrial applications) and things like 3-phase welding equipment therefore have even power output. Four phases would not significantly improve things but would add a fourth wire, so 3-phase is the natural settling point.

Answer / ashish

from generating alternator four wires comming, three are
phase(R,Y,V)& one is nutral which is grounded by the use of
nutral grounded transformer near to transformer used for
step up the generating voltge hence after generation we have
three phase so we use star delta transformer(step up) for
transmission & in delta three wire required & at
distributing end delda star transfomer(step down) is used so
four wire are required three phase & one nutral

Answer / vinai

Because Main generating Alternator has 3phase winding...
And phase angle is 120 degree for each...
So that's way we are supplying 3 phase only not a two
phase or four phase...
If possible to wound an alternator like 2(or)4 phase
then may be it's possible for supplying 2(or)4 phase...

Answer / ssreenu

we can supply the power in 2 phase or 4 phase or n phase
ideally but 3 phase system is more advantageous one as said
above.

Answer / abinash kumar agrawal

because all industries are demanding 3 phase power so how we
can generate 4 phase supply .because all the power get
wasted due to non acceptance.
i also like three phase power becuase three is my lucky number.

Answer / ashok

the power supplyed in electrical equipments is star or delta
.so star,delta having three or four phases.

Answer / ashok

the power supplyed in electrical equipments is star or delta
.so star,delta having three or four phases.

To avoid arcing what we connect in earthing.

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