Pitch Factor is also known as Coil span factor. To improve the wave formation of the induced e.m.f. generally the short pitch or long pitch windings are used instead of full pitch windings. In case of full pitch winding the e.m.f. induced in both coil sides is equal and inphase, but when fractional pitch winding is used the e.m.f. induced in both the coil sides is not in phase. Hence the resultant can only be given by the vector sum which is always less than the arithmetic sum of the e.m.f. of both coil sides as shown in Fig. 17.8.
Now the ratio of the vector sum to the arithmetic sum of the induced e.m.f. per coil is known as the coil span factor or pitch factor. It is always less than unity so,
Fig. 17.8. Coil span factor
kd = Vector sum of the voltage in two sides of a coil / Arithmetic sum of the voltages in two sides of a coil
Let the coil sides are shortened by one slot, the angle of displacement between e.m.f.’s of two coil sides.
n = Number of slots/pole/phase
kc = Resultant e. m.f. / 2 x e.m.f,/coil side
If the pitch is given in fractions then also the kc can be calculated