It is known that three phase lines from a transformer of a distribution line are kept at 120 degree phase angle with each other. What is the need for maintaining this phase differene ?
Regards
Nityananda Das
Do you mean why the voltages are kept displaced to each other by 120 degree with reference to a cycle ? This is to create a 'balanced' supply system that creates a steady (not pulsating) torque in a 3-phase AC induction motor stator winding.
Did you mean why the 3 conductors are physically kept at the three vertices of an imaginary equilateral triangle while the lines connect one point to another ? This is to create equal inductance in the lines (created due flux by its own current flow) such that the line reactances will be balanced.
Respected Biswas,
The balancing of Torque or equating inductance is not being clear to me.
Will you please recommend me any book for this topic ?
Regards
Nityananda Das
Any good text book on induction motor will show that while a 1-phase induction motor has a pulsating torque, a 3-phase induction motor has a steady instantaneous torque (assuming balanced conditions).
Any good text book on power transmission will explain that inductance per line is balanced if they are kept mutually at equal distance, since mutual coupling of flux is identical. If not kept so, their mutual positions have to be replaced after a fixed distance, which is known as "transposing" of the lines to get an end-to-end balanced inductance per phase.
I agree with all Sujit K. Biswas statements, except that the lines need not be placed in equilateral triangle, but can be placed in any configurations where the distance between each phase is equal.
Have a look at the following books which are thin but conceptual:
1) Electrical machinery by Chapman
2) Electrical Energy Systems Theory by Elgerd
There are many reasons to keep the three phase lines 120 angle apart. One may be that when there is 120 phase angle among the three phases the sum of voltage at any time instant is zero which have an advantage that when there is a balance load no current will flow in neutral wire.
Other reason may be that for three phase system 120 degree apart power delivered to load is constant means that power delivered is independent of time so (any) load connected is utilizing constant power like if induction motor it will utilize constant power and will produce constant torque for better performance.
All said by Sujit K. Biswas is correct!! There are many textbooks about transmission line inductance computation. You can find them on Internet
Because the phase angle circle = 360degrees it is divided by 3 so that every phase supplies power when the sine wave positivIty begins to change sign and orientation so that the 2nd phase takes over continuum of power supply doing same thing until positivity starts to wane when the 3rd phase takes over to do same thing. This allows additivity and efficiency of AC vs DC.