Alternators:Two-Layer Winding

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Two-Layer Winding

This winding is either of wave-wound type or lap-wound type (this being much more common especially for high-speed turbo-generators). It is the simplest and, as said above, most commonly-used not only in synchronous machines but in induction motors as well.

Two important points regarding this winding should be noted :

(a) Ordinarily, the number of slots in stator (armature) is a multiple of the number of poles and the number of phases. Thus, the stator of a 4-pole, 3-phase alternator may have 12, 24, 36, 48 etc. slots all of which are seen to be multiple of 12 (i.e. 4 ´ 3).

(b) The number of stator slots is equal to the number of coils (which are all of the same shape).

In other words, each slot contains two coil sides, one at the bottom of the slot and the other at the top. The coils overlap each other, just like shingles on a roof top.

For the 4-pole, 24-slot stator machine shown in Fig. 37.12, the pole-pitch is 24/4 = 6. For maximum voltage, the coils should be full-pitched. It means that if one side of the coil is in slot No.1, the other side should be in slot No.7, the two slots 1 and 7 being one pole-pitch or 180° (electrical) apart. To make matters simple, coils have been labelled as 1, 2, 3 and 4 etc. In the developed diagram of Fig. 37.14, the coil number is the number of the slot in which the left-hand side of the coil is placed.

Each of the three phases has 24/3 = 8 coils, these being so selected as to give maximum voltage when connected in series.

When connected properly, coils

1, 7, 13 and 19 will add directly in

phase. Hence, we get 4 coils for this phase. To complete eight coils for this phase, the other four selected are 2, 8, 14 and 20 each of which is at an angular displacement of 30° (elect.) from the adjacent coils of the first. The coils 1 and 2 of this phase are said to constitute a polar group (which is defined as the group of coils/ phase/pole). Other polar groups for this phase are 7 and 8, 13 and

14, 19 and 20 etc. After the coils are placed in slots, the polar

image

are connected together with alternate poles reversed (Fig. 37.13) which shows winding for one phase only.

Now, phase Y is to be so placed as to be 120° (elect.) away from phase R. Hence, it is started from slot 5 i.e. 4 slots away (Fig. 37.14). It should be noted that angular displacement between slot No. 1 and 5 is 4 ´ 30 = 120° (elect). Starting from coil 5, each of the other eight coils of phase Y will be placed 4 slots to the right of corresponding coils for phase R. In the same way, B phase will start from coil 9. The complete wiring diagram for three phases is shown in Fig. 37.14. The terminals R2, Y2 and B2 may be connected together to form a neutral for Y-connection.

image

Wye and Delta Connections

For Y-connection, R1, Y1 and B1 are joined together to form the star-point. Then, ends R2, Y2 and B2 are connected to the terminals. For delta connection, R2 and Y1, Y2 and B1 B2 and R1 are connected together and terminal leads are brought out from their junctions as shown in Fig. 37.15 (a) and (b).

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