Electrical Engineering ⇒ Topic : Four-Wire Star-Connected Unbalanced Load

Four-Wire Star-Connected Unbalanced Load

We can obtain this type of load in two ways. First, we may connect a 3-phase, 4-wire unbalanced load to a 3-phase, 4-wire supply as shown in Fig. (a). Note that star point N of the supply   is connected to the load star point N'. Secondly, we may connect single phase loads between any line and the neutral wire as shown in Fig. (b). This will also result in a 3-phase, 4-wire *unbalanced load because it is rarely possible that single phase loads on all the three phases have the same magnitude and power factor. Since the load is unbalanced, the line currents will be different in magnitude and displaced from one another by unequal angle. The current in the neutral wire will be the phasor sum of the three line currents i.e.

                                  Current in neutral wire, I_N = I_R+I_Y+I_B                                    ......... phasor sum

                                                (a)                                                                                                                      (b)

                                                                                                   figure 

The following points may be noted carefully :

(1) Since the neutral wire has negligible impedance, supply neutral N and load neutral N' will be at the same potential. It means that voltage across each impedance is equal to the phase voltage of the supply. However, current in each phase (or line) will be different due to unequal impedances.

(2) The amount of current flowing in the neutral wire will depend upon the magnitudes of line currents and their phase relations. In most circuits encountered in practice, the neutral current is equal to or smaller than any one of the line currents. The exceptions are those circuits having severe unbalance