PRODUCTION OF REVOLVING FIELD

Consider a stator on which three different windings represented by three concentric coils a₁a₂, b₁b₂, and c₁c₂, respectively, are placed 120° electrically apart.

Let a three-phase supply, as shown in Figure 12.5, is applied to the stator. Three-phase currents will flow through the three coils and produce their own magnetic fields. The positive half cycle of the alternating current (AC) is considered as inward flow of current in the start terminals and negative half cycle is considered as outward flow of current in the start terminals. The direction of flow of current is opposite in the finish terminals of the same coil.

Fig. 12.5  Wave diagram of 3-phase AC supply with instants t₁,t₂ and t₃

Let at any instant t₁, current in coil side a₁ be inward and in b₁ and c₁ outward, whereas the current in the other sides of the same coils is opposite, that is,in coil side a₂ is outward and b₂ and c₂ is inward. The resultant field and its direction (F_m) are marked in Figure 12.6.

At instant t₂, when θ is 60°, current in coil sides a₁ and b₁ is inward and in c₁ is outward, whereas the current in the opposite sides is opposite. The resultant field and its direction is shown in Figure 12.7, which is rotated through an angle θ = 60° from its previous position.

At instant t₃ when θ is 120°, current in coil side b₁ is inward and in c₁ and a₁ is outward. The resultant field and its direction is shown in Figure 12.8, which is rotated through an angle θ = 120° electrical from its first position.

Fig. 12.6  Position of resultant field at instant t₁

Fig. 12.7  Position of resultant field at instant t₂

Fig. 12.8  Position of resultant field at instant t₃

Thus, in one cycle, the resultant field completes one revolution. Hence, we conclude that when three-phase supply is given to a three-phase wound stator, a resultant field is produced which revolves at a constant speed, called synchronous speed (N_s = 120° f / P).

In this case, we have observed that when supply from phase 1, 2, and 3 is given to coil a₁a₂, b₁b₂, and c₁c₂, respectively, an anticlockwise rotating field is produced. If the supply to coil a₁a₂, b₁b_2, and c₁c₂ is given from phase 1, 3, and 2, respectively, the direction of rotating field is reversed. Therefore, to reverse the direction of rotation of rotating field, the connections of any two supply terminals are inter changed.