Three Phase Supply

THREE PHASE SUPPLY

A single phase ac voltage can be generated by rotating a turn made up of two conductors, in a magnetic field. Such as ac producing machine is called single turn alternator. But voltage produced by a single turn is very less and not sufficient for practical loads. So number of turns are increased in series to form one winding in a practical alternator. Such windings are called armature windings. OTOS ei tasteni yns is 29gstlov

To develop a polyphase system, the armature winding in an alternator is divided into number of phases required. In each section a separate ac voltage gets induced. A three phase voltage can be generated by using three windings. From these windings three separate AC voltages will be available having same magnitude and frequence but they will have a phase difference of 360°/3 = 120° with respect to each other. All those voltage with a phase difference of 120° will be available to supply a 30 load such a system is called as three phase system.

Advantage of three phase system:

1. The output of three phase machine is always greater than single phase machine.

2. Three phase motors are self starting.

3. It gives steady output.

4. Good power factor.

5. For converting machines like rectifiers, the dc output voltage becomes smoother if the number of phases are increased.

Generation of Three Phase Voltage  

The armature coils are divided into three groups such that they are displaced by an angle of 120° from each other as shown in Figure 1.50.

The phasor diagram of voltage is shown in Figure 1.51. As phasors rotate in anticlockwise direction, we can say that e_y lags e_R by 120° and e_B lags e_y by 120^o.

If we add three voltages vectorially it can be observed that the sum of the three voltages at any instant is zero.

The sequence in which the voltages in three phases reach their maximum positive values is called phase sequence. Generally the phase sequence is R-Y - B.