Subject : Electrical Machines II (AC Machines)
Unit : Alternators
Distribution Factor
Distribution or Breadth Factor or Winding Factor or Spread Factor:
- In each phase, coils are not concentrated or bunched in one slot, but are distributed in a number of slots to form polar groups under each pole. These coils/phase are displaced from each other by a certain angle.
- The result is that the e.m.fs. induced in coil sides constituting a polar group are not in phase with each other but differ by an angle equal to angular displacement of the slots.
- In Fig. shows the end connections of a 3-phase single-layer winding for a 4-pole alternator. It has a total of 36 slots i.e. 9 slots/pole. There are 3 slots /phase / pole. For example, coils 1, 2 and 3 belong to R phase. Now, these three coils which constitute one polar group are not bunched in one slot but in three different slots.
- Angular displacement between any two adjacent slots = 180°/9 = 20° (electrical) If the three coils were bunched in one slot, then total e.m.f. induced in the three sides of the coil would be the arithmetic sum of the three e.m.f.s. i.e. = 3 E_{S}, where E_{S} is the e.m.f. induced in one coil side [Fig.1 (a)].
- Since the coils are distributed, the individual e.m.fs. have a phase difference of 20° with each other. Their vector sum as seen from Fig. 1 (b) is