Fig. 1,538.—Three phase, 10 pole, 30 slot winding in two ranges. In this winding perfect symmetry occurs after every four poles. Accordingly in the case of an odd number of pairs of pole, one of the coils must necessarily be askew going from the inner to the outer range as at M.
Each of the three individual windings must be concentrated into narrow belts so as to leave sufficient space for the other windings between them. This limits the breadth or space occupied by the winding of any one phase to one-third of the pole pitch.
Fig. 1,539.—Three phase 10 pole 30 slot winding in three ranges. The coils of each phase are alike, those of the A phase being all in the straight out range, those in the B phase, in a bent up range, and those in the C phase in a bent down range. This arrangement has the disadvantage, that by reason of the third range, the field magnet cannot be withdrawn. This treatment of the coil ends is more clearly shown in fig. 1,540.
Ques. How are three phase coil ends treated?
Ans. They may be arranged in two ranges, as in fig. 1,538, or in three ranges, as in fig. 1,539.
Ques. What kind of coil must be used for three phase windings in order that the ends may be arranged in only two ranges?
Ans. Hemitropic or half coils; that is, the number of coil per phase must be equal to one-half the number of pole.
