Winding Types

Winding Types

A three phase winding, in extremely simplified form, is shown in Fig. 8. The start and finish of all the coils in phase A are designated, respectively, as S_A and F_A. Phase A is shown as a solid line in the figure, phase B as a dashed line, and phase C as a dotted line. Note that each winding does not start and finish under the same pole. Further, note that the two coil sides of a given coil lie in identical magnetic conditions of opposite polarity. This implies that when seen from the coil terminals, the emfs produced in the two coil sides add up. If we assume that the poles on the rotor are moving to the left as shown, then the relative motion of the armature conductors is to the right. This implies that identical magnetic conditions will be seen by conductors of phase A, followed by phase C, followed by phase B. The induced emfs in phases A,C and B may be said to produce a phase sequence of ACBACBA.The time interval between two phases to achieve identical magnetic conditions would depend on the relative speed of motion, and on the spatial seperation of the phases. In Fig 8, the phases are so laid out that each phase is seperated from another by 120 electrical degrees (360^◦ being defined by the distance to achieve identical magnetic conditions).

As the distance between two adjacent corresponding points on the poles is 180 electrical degrees, we can see that the distance between the coil side at the start of A and that at the start of C must be 120 electrical degrees. Thus, the leading pole tip of a unit north pole moving to the left in Fig. 8 will induce identical voltages in corresponding coil sides

A, C, and B, respectively, 120 electrical degrees apart. Note that phase B lags phase A by240 electrical degrees or leads phase A by 120 electrical degrees.Fig. 8(b) is a representation that is frequently used to depict the windings of the three phases and the phase relationship

between them.

The winding depicted in Fig. 8 is an open winding since both ends of the windings have been brought out for suitable connections. It is a wave winding since it progresses from pole to pole. It is a concentrated winding because all the coils of one phase are concentrated in the same slot under one pole. It is a half-coil winding because there is only one-half of a coil (one coil side) in each slot. It is a full-pitch winding because the coil sides of one coil are 180^◦ electrical degrees apart i.e., they lie under identical magnetic conditions, but of opposite polarity under adjacent poles.Fig. 9, on the other hand shows the coils of a single phase,(A, in this case) distributed winding distributed over two slots under each pole.