Electrical Engineering ⇒ Topic : Polyphase system

Polyphase System

A polyphase alternator has two or more separate but identical windings (called phases) displaced from each other by **equal electrical angle and acted upon by the common uniform field. Each winding or phase produces a single alternating voltage of the same magnitude and frequency. However, these voltages are displaced form one another by equal electrical angle.

(1) Fig. a (i) shows an elementary single-phase alternator. It has one winding or coil A rotating in anticlockwise direction with an angular velocity a) in the 2-pole field. The equation of the e.m.f. induced in the coil is given by

(2) Fig. a (ii) shows an elementary two-phase alternator. It has two identical windings or coils A and B displaced ***90 electrical degrees from each other and rotating in anticlockwise direction with an angular velocity a) in the 2-pole field. Here a₁ and b₁ are the start and a₂ and b₂ are the finish terminals of the two coils respectively. Note that the corresponding terminals al and b1 are 90 electrical degrees apart. Likewise terminals a₂ and b₂ are 90° apart. Since the two coils are identical and have the same angular velocity, e.m.f.s induced in them will be of the same magnitude and frequency. However, these e.m.f.s will have a phase difference of 90° as shown in the wave diagram in Fig. a (ii).

Note that e.m.f. in coil A leads that in coil B by 90°. The equations of the two e.m.f.s. are

                                                                            figure (a)

(3) Fig. (a) (iii) shows an elementary 3-phase alternator. It has three identical windings or coils A, B and C displaced 120 electrical degrees from each other and rotating in anticlockwise direction with an angular velocity co in the 2-pole field. Note that the corresponding terminals a₁, b₁ and c₁ are 120° apart. Likewise the terminals a₂, b₂ and c₂ are 120 electrical degrees apart. Since the three coils are identical and have the same velocity, the e.m.f.s induced in them will be of the same magnitude and frequency. However, the three e.m.f.s will be displaced from one another by 120°. Note that e.m.f. in coil B will be 120° behind that of coil A and the e.m.f. in coil C will be 240° behind that of coil A. This is shown in the wave diagram in Fig.a (iii). The equations of the three e.m.f.s can be represented as