Simplex Wave Winding
Simplex Wave Winding*
From Fig. 26.31, it is clear that in lap winding, a conductor (or coil side) under one pole is connected at the back to a conductor which occupies an almost corresponding position under the next pole of opposite polarity (as conductors 3 and 12). Conductor No. 12 is then connected to conductor No. 5 under the original pole but which is a little removed from the initial conductor No. 3. If, instead of returning to the same N-pole, the conductor No. 12 were taken forward to the next N-pole, it would make no difference so far as the direction and magnitude of the e.m.f. induced in the circuit are concerned.As shown in Fig. 26.35, conductor AB is connected to CD lying under S-pole and then to EF under the next N-pole. In this way, the winding progresses, pass- ing successively under every N-pole and S-pole till it returns to a conductor A¢B¢ lying under the original pole. Because the winding progresses in one direction round the armature in a series of ‘waves’, it is known as wave winding.
If, after passing once round the armature, the winding falls in a slot to the left of its starting point (as A¢B¢ in Fig. 26.35) then the winding is said to be retrogressive. If, however, it falls one slot to the right, then it is progressive.
Assuming a 2-layer winding and supposing that conductor AB lies in the upper half of the slot, then going once round the armature, the winding ends at A¢B¢ which must be at the upper half of the slot at the left or right. Counting in terms of conductors, it means that AB and A¢B¢ differ by two conductors (although they differ by one slot).
It is clear that for YA to be an integer, there is a restriction on the value of Z. With Z = 32, this winding is impossible for a 4-pole machine (though lap winding is possible). Values of Z = 30 or 34 would be perfectly alright.
As shown in Fig. 26.36 and 26.37, conductor No. 5 is taken to conductor No. 5 + 7 = 12 at the back and is joined to commutator segment 5 at the front. Next, the conductor No. 12 is joined to commutator segment 5 + 7 = 12 ( ∵ YC = 7) to which is joined conductor No. 12 + 7 = 19. Continuing this way, we come back to conductor No. 5 from where we started. Hence, the winding closes upon itself.
Brush Position
Location of brush position in wave-winding is slightly difficult. In Fig. 26.36 conductors are supposed to be moving from left to right over the poles. By applying Fleming’s Right-hand rule, the directions of the induced e.m.fs in various armature conductors can be found. The directions shown in the figure have been found in this manner. In the lower part of Fig. 26.36 is shown the equivalent ring or spiral diagram which is very helpful in understanding the formation of various parallel paths in the armature. It is seen that the winding is electrically divided into two portions. One portion consists of conductors lying between points N and L and the other of conductors lying between N and M. In the first portion, the general trend of the induced e.m.fs. is from left to right whereas in the second
portion it is from right to left. Hence, in general, there are only two parallel paths through the winding, so that two brushes are required, one positive and one negative.
From the equivalent ring diagram, it is seen that point N is the separating point of the e.m.fs. induced in the two portions of the winding. Hence, this fixes the position of the negative brush. But as it is at the back and not at the commutator end of the armature, the negative brush has two alternative positions i.e. either at point P or Q. These points on the equivalent diagram correspond to commutator segments No. 3 and 11.
Now, we will find the position of the positive brush. It is found that there are two meeting points of the induced e.m.fs. i.e. points L and M but both these points are at the back or non-commu- tator end of the armature. These two points are separated by one loop only, namely, the loop com- posed of conductors 2 and 9, hence the middle point R of this loop fixes the position of the positive brush, which should be placed in touch with commutator segment No. 7. We find that for one position of the +ve brush, there are two alternative positions for the -ve brush.
Taking the +ve brush at point R and negative brush at point P, the winding is seen to be divided into the following two paths.
In path 1 (Fig. 26.36) it is found that e.m.f. in conductor 9 is in opposition to the general trend of e.m.fs. in the other conductors comprising this path. Similarly, in path 2, the e.m.f. in conductor 2 is in position to the direction of e.m.fs. in the path as a whole. However, this will make no difference because these conductors lie almost in the interpolar gap and, therefore e.m.fs. in these conductors are negligible.
Again, take the case of conductors 2 and 9 situated between points L and M. Since the armature conductors are in continuous motion over the pole faces, their positions as shown in the figure are only instantaneous. Keeping in this mind, it is obvious that conductor 2 is about to move from the influence of S-pole to that of the next N-pole. Hence, the e.m.f. in it is at the point of reversing. However, conductor 9 has already passed the position of reversal, hence its e.m.f. will not reverse, rather it will increase in magnitude gradually. It means that in a very short interval, point M will
become the meeting point of the e.m.fs. But as it lies at the back of the armature, there are two alternative positions for the +ve brush i.e. either point R which has already been considered or point S which corresponds to commutator segment 14. This is the second alternative position of the positive brush. Arguing in the same way, it can be shown that after another short interval of time, the alternative position of the positive brush will shift from segment 14 to segment 15. Therefore, if one positive brush is in the contact with segment 7, then the second positive brush if used, should be in touch with both segments 14 and 15.
It may be noted that if brushes are placed in both alternative positions for both positive and negative (i.e. if in all, 4 brushes are used, two +ve and two -ve), then the effect is merely to short- circuit the loop lying between brushes of the same polarity. This is shown in Fig. 26.40 where it will also be noted that irrespective of whether only two or four brushes are used, the number of parallel paths through the armature winding is still two.
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