Short Pitched Coils (Chorded)
Short Pitched Coils – So far it was assumed that the stator coils are full-pitched (a span of iv rad elect). Coils may have a span of less than the full-pitch. This arrangement offers certain advantages. Consider that the coil-span is less than the full-pitch by an elect. angle Osp (short-pitching angle) as shown in Fig. 5.20(a). With reference to Fig. 5.15, Eq. (5.5) for the flux linking the coil now modifies as under:
The flux linking the coil and therefore the coil emf reduces by a multiplicative factor of
The meaning of the pitch factor can be arrived at from another angle. In Fig. 5.20(a) with positive direction of coil-side emfs marked in opposite direction, the coil emf is the phasor sum of coil-side emfs, i.e.
In the case of a full-pitch coil Ea, and Ea are in phase (they are ,r rad apart but their positive directions are marked oppositely) so that
as shown in Fig. 5.20(b).
In a coil short-pitched by Osp, Ka, will lead (or lag) Ka by the angle Osp as shown in Fig. 5.20(c). From the geometry of the phasor diagram
Hence the reduction of the coil emf due to short-pitching is governed by the factor
Because of short-pitching the expression of Eq. (5.15) for the phase voltage modifies to
Thus short-pitching helps in elimination of any particular harmonic or in reduction of the harmonic content of the induced voltage in general. It is easily seen from Fig. 5.20 that short-pitched coils have shorter end connections. Thus there is a saving in copper per coil but part of this saving is wiped off by the fact that more series coils/ phase would now be needed to generate a specified phase voltage. A designer has to weigh these factors in arriving at a decision on the angle of short-pitching.