STAR OR WYE (Y) CONNECTION

In star or wye (Y) connections, the similar ends (either start or finish) of the three windings are connected to a common point called star or neutral point. The three line conductors are run from the remaining three free terminals called line conductors. Ordinarily, only three wires are carried to the external circuit giving three-phase, three-wire star-connected system. However, sometimes a fourth wire is carried from the star point to the external circuit, called neutral wire, giving three-phase, four-wire star-connected system.

As shown in Figure 8.4, the finish terminals a₂, b₂, and c₂ of the three windings are connected to form a star or neutral point. From the remaining three free terminals, three conductors are run, named R, Y, and B. The current flowing through each phase is called phase current I_phand current flowing through each line conductor is called line current I_L. Similarly, voltage across each phase is called phase voltage (E_ph) and voltage across two line conductors is called line voltage (E_L).

Fig. 8.4   (a) 3-phases connected in star (b) Star connected system

8.9.1  Relation between Phase Voltage and Line Voltage

The connections are shown in Figure 8.5(a). Since the system is balanced, three voltages E_NR, E_NY,and E_NBare equal in magnitude, but displaced from one another by 120° electrical. Their phasor diagrams are shown in Figure 8.5(b). The arrow heads on emfs and currents indicate the positive direction and not their actual direction at any instant.

Now, E_NR = E_NY = E_NB = E_ph(in magnitude)

Fig. 8.5  (a) Circuit representing phase and line voltages and currents in star connections (b) Phasor diagram of phase and line voltages in star connections

It may be seen that between any two lines, there are two phase voltages.

Tracing the loop NRYN, we get 

or

To find the vector sum of E_NYand −E_NR, reverse the vector E_NRand add it vectorially with E_NYas shown in Figure 8.5(b).

Therefore,

or

Similarly,

Hence, in star connections, line voltage =  phase voltage.

8.9.2  Relation between Phase Current and Line Current

From Figure 8.5(a), it is clear that same current flows through phase winding as well as the line conductor since line conductor is just connected in series with the phase winding.

 

I_R= I_NR; I_Y= I_NYand I_B= I_NB

 

where I_NR= I_NY= I_NB= I_ph(phase current) and I_R= I_Y= I_B= I_L (line current)

Hence, in star connections, line current = phase current.