Tuesday, February 21, 2012

Generalized Expression for E.M.F. Equation of an Alternator

E.M.F. Equation of an Alternator : Part4 
       Considering full pitch, concentrated winding.
                   Eph   = 4.44 f Φ Tph   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, Kc = 1.
       For concentrated winding K= 1.
Note : For short pitch and distributed winding Kc and Kare 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 = 180o/n = 180o/5 = 12o

               = 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 = 180o/ = 20o
       Now coil is shorted by 1 slot i.e. by 20o to full pitch distance.
...     α = Angle of short pitch = 20o
...      Kc = cos (α/2)  = cos (10)

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