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Distribution of Three Phase Alternating Current

If three separate coils with angular separation of 120o are connected in the armature of an A.C. dynamo, the voltages in the coils will have a phase difference of 120o. This is called 3-phase A.C. The three coils can be connected to three loads separately using six separate wires as shown in. Thus six heavy wires would be required in three separate single-phase systems. However, there are two methods of making connections by using only 4 or 3 wires.

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(1)    Star Connections. This method of transmission is used when all the phase are equally loaded. In case of balanced load, the neutral wire will be carrying three currents exactly similar but 120o out of phase with each other in a symmetrical 3-phase system. Their sum is zero. Hence neutral wire can be omitted and only three wires are required for transmission of 3-phase.
The emf between any line and the neutral gives the phase voltage Eph. The emf between two outer terminals is called line voltage VL. In the star connection, the line voltage is √3 times the phase voltage. The phase difference between them is 30o. The power consumed in the circuit is equal to the strength of phase current IL = Iph. The power consumed in the circuit is three times the power per phase.

Three phase four wire system. When the load is unbalanced, i.e., the different phases are unequally loaded, then 4-wire system is used instead of 3-wire system. The neutral or star point is connected to a wire called neutral line. Three lines are taken from the free ends of Q, R and S and are called the phase lines. The p.d. between phase lines is = √3 times the voltage between the phase and neutral point. For household supply, online one phase line and a neutral wire are connected. For power supply in factories, three phase wires and a neutral wire are connected to the factory.

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(2)    Delta Connection. Here the end of each winding is connected to the beginning of the next one, so that they form a closed triangle. In this type of connection, the line voltage of a generator is equal to its phase voltage. The line current is √3 times the phase current.
Output can be taken from QR, RS or QS. The windings may be delta connected only when the load on the phases is the same or almost the same. Otherwise the machinery may be damaged by strong currents in the closed circuit of the windings.

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