Methods of three-phase connection

A three-phase system has three windings (phases R, Y, B) whose voltages are equal in magnitude and 120° apart. The six ends of the windings can be connected in two ways.

1. Star (Y) connection

The similar ends (say the finishing ends) of the three windings are joined to a common point called the star or neutral point (N). The other three ends are connected to the three lines. A fourth wire may be taken from N (3-phase 4-wire system).

            R
            o  I_L
            |
           (Ph R)
            |
            N o------ neutral
          /   \
     (Ph B)   (Ph Y)
        /       \
       o         o
       B         Y

• Line voltage V_RY = V_R − V_Y (phasor difference).
• V_L = √3 V_ph, line voltage leads the phase voltage by 30°.
• I_L = I_ph (each line is in series with one phase).
• Gives two voltages: 400 V line-to-line and 230 V line-to-neutral (Nepal's LV supply), so it is used for distribution.

2. Delta (Δ) or mesh connection

The finishing end of one winding is joined to the starting end of the next, forming a closed triangle. Lines are taken from the three junctions. There is no neutral.

            R
            o
           / \
     Ph RB/   \Ph RY
         /     \
        o-------o
        B Ph YB  Y

• V_L = V_ph (each phase is directly between two lines).
• Line current I_R = I_RY − I_BR (phasor difference).
• I_L = √3 I_ph, line current lags the phase current by 30°.
• Used for motors and transmission where no neutral is needed.

Comparison

Power in both: P = √3 V_L I_L cos φ = 3 V_ph I_ph cos φ.