ET201: THREE PHASE SYSTEMS (PART 3)

1.1.2        Generation of three phase voltages (emf).

In a single phase AC system, alternating voltage may be generated by rotating a coil in magnetic field or by rotating a magnetic field within a stationary coil. The value of the voltage generated depends, in each case, upon the number of turns in the coil. Consider a case of a single phase alternator with one armature winding only. If the number of armature windings is increased, then it becomes polyphase alternator and it produces as many independent voltage waves as the number of windings or phases. These windings are displaced from one another by equal angles, the values of these angles being determined by the number of phases or windings. In fact, the word “polyphase” means poly(many) and phase(winding or circuit).

A three phase alternator, as the name shows, has three independent armature windings which are 120 electrical degrees apart. Hence, the voltages induced in the three windings are 120⁰ apart in time phase. In Figure 1.1.2(a) is shown a two pole, stationary-armature, rotating-field type three phase alternator. It has three armature coils RR’, YY’ and BB’ displaced 120⁰ apart from one another. Figure 1.1.2(b) is shown a pole, stationary-magnet, rotating coils type.

Figure 1.1.2. (a) Rotating-field, stationary coils   (b)Rotating- coils, stationary field