E.M.F. Equation of an Alternator : Part4 Considering full pitch, concentrated winding. E ph = 4.44 f Φ T ph Volts. But due to short pitch, distributed winding used in practice, this will reduce by factors and . So generlised expression for e.m.f. equation can be written as For full pitch coil, K c = 1. For concentrated winding K d = 1. Note : For short pitch and distributed winding K c and K d are always less than unity. Example 1 : An armature of a three phase alternators has 120 slots. The alternators has 8 poles. Calculate its distribution factor. Solution : n = Slots/Pole = 120/8 = 15 m = slots/pole/phase = n/3 = 15/3 = 5 β = 180 o /n = 180 o /5 = 12 o = 0.957 Example 2 : In a 4 pole, 3 phase alternator, armature has 36 slots. It is using an armature winding which is short pitched by one slot. Calculate its coil span factor. Solution : n = Slots/pole = 36/4 = 9 β = 180 o / = 20 o Now coil is shorted by 1 slot i.e. by 20 o to full pitch distance. . . . α = Angle of short pitch = 20 o . . . K c = cos ( α /2) = cos (10) = 0.9848
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