Why do we use the 3phase and not the 4phase or other phase supply?

[Faraz Murtaza]

why do we use the 3phase and not the 4phase or other phase supply?

please answer...

 

[PhanthomJay]

do you mean 3 phase instead of 4 wire systems, or do you mean 3 phase instead of more than 3 live wires ? If the first, no need for the neutral if load is balanced amongst the live phase conductors as in AC transmission, although 4 wire is common at the lower distribution voltages main lines that directly supply customers. Three phase has become the norm for AC transmission, due primarily to economics. And Tesla. Some research was done awhile ago on 6 phase systems during the EMF issues of health concerns, but nothing was ever implemented of significance.

 

[azizlwl]

I think its due to Star-Delta transformation.
Transmission using delta , 3 wire(no neutral) and users star formation

 

[Faraz Murtaza]

do you mean 3 phase instead of 4 wire systems, or do you mean 3 phase instead of more than 3 live wires ? If the first, no need for the neutral if load is balanced amongst the live phase conductors as in AC transmission, although 4 wire is common at the lower distribution voltages main lines that directly supply customers. Three phase has become the norm for AC transmission, due primarily to economics. And Tesla. Some research was done awhile ago on 6 phase systems during the EMF issues of health concerns, but nothing was ever implemented of significance.

thanx for your supportive answer bt i think you did not get my question..ok let me elaborate it further that my question was...why do we use the 3 phase supply like in generation, transmission etc,,and why these devices are not supplied 2phase or 1phase supply??
please elaborate!

 

[Jobrag]

I think that the only answer to this is that Tesla decided to use three phase, and once the early infrastructure was laid down there was no point in changing to any other combination.
Why Tesla went for three phase I don't know but would guess that it's a compromise between fears that two phase power would cause synchronous motors to run less smoothly then three phase and the extra cost of cabling that would be needed by four phase.
As an after thought and I'm having real trouble visualising it, would a two phase motor run in the same direction every time you started it?

 

[PhanthomJay]

In a three phase AC system, the voltages between wires are time wise 120 degrees apart , and same with the current. With some vector algebra, the power delivered can be shown to be equal to (root 3)(V)(I) , where V is the voltage difference between any 2 phase conductors. For a 115 kV phase -to - phase 3 phase line, carrying 1000 amps in each conductor, this is about 200 MW of power assuming unity power factor and no line losses. Now with a single phase line (2 wire system with neutral return), you only get 115 MW out of it for the same current...it's economics...and other factors already mentioned.

 

[Jobrag]

In a three phase AC system, the voltages between wires are time wise 120 degrees apart , and same with the current. With some vector algebra, the power delivered can be shown to be equal to (root 3)(V)(I) , where V is the voltage difference between any 2 phase conductors. For a 115 kV phase -to - phase 3 phase line, carrying 1000 amps in each conductor, this is about 200 MW of power assuming unity power factor and no line losses. Now with a single phase line (2 wire system with neutral return), you only get 115 MW out of it for the same current...it's economics...and other factors already mentioned.

There are a couple of problems here by confusing V line and V phase (VL =VP/Root 3) in the first case V is VP in the second V is VL.

Power in a 3 phase system P= 3*VL*I (the 3 comes from the fact that that you have the same I on three conductors)

VL=VP/(root3) so P=(root3)*VP*I

BUT if you are taking power from a single phase P=VL*I

 

[PhanthomJay]

The terminology is confusing. For a three phase AC line, the voltage between any 2 phases, A to B, A to C, or B to C, is called the line voltage, V_ l, or what I refer to as the phase to phase voltage. The voltage between anyone phase and ground is called the phase voltage, V_p, or what I refer to as the phase to ground voltage, and where V_l = (root 3)V_p.
For a 3 phase 115 kV phase to phase line voltage, V_l = 115, V_p = 66, and Power = (root 3)(V_l)(I_l) or 3 (V_p)(I_l) if you wish, same result.

 

[Faraz Murtaza]

ok thanks phantomjay...