Alternators:Distribution or Breadth Factor or Winding Factor or Spread Factor

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Distribution or Breadth Factor or Winding Factor or Spread Factor

It will be seen that 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. 37.19 are shown 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

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slots/pole. Obviously, 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° (elect.)

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 ES, where ES is the e.m.f. induced in one coil side [Fig.37.20 (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. 35.20 (b) is

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Equation of Induced E.M.F.

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If the alternator is star-connected (as is usually the case) then the line voltage is 3 times the phase voltage (as found from the above formula).

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